# Eneloops: 5000 cycles and still going...



## Power Me Up (May 2, 2015)

*This initial post was originally written up after doing 2700 cycles - updated information as more cycles were completed has been added to the bottom of this post.*

As a follow on to my previous cycle testing of Eneloops (and other NiMH cells) I decided to run a modified test on a pair of 4th Generation Eneloops – one made in China and one from Japan.

Previous cycle testing results showed significantly less cycles in my testing compared to the results claimed by Sanyo/Pansonic, so I decided to run a test that was somewhat closer to the IEC standard testing method that they used.

The key differences between the testing methods were that my testing was done at higher rates, were full cycles and with rest periods after both charging and discharging. For this test, I kept the high charge and discharge rates, but removed the rest periods and limited both the maximum charge level and depth of discharge. Charging was set to terminate at a fixed 1.44V and discharging stopped at 1.1V

My aim with this test was to see if it is possible to achieve a similar number of cycles as claimed by the manufacturer. My hope was that limiting the depth of discharge and maximum charge level it would reduce the amount of wear and tear with each cycle. The results from this have exceeded my expectations by a huge margin.







After 2700 partial cycles, the Japanese cell is holding up extremely well. Partial cycle capacity has dropped somewhat but has plateaued at around 900 mAh. The full discharge cycle is still showing a very respectable capacity of around 1,730 mAh. Cumulative discharge capacity is now up to 2,870 Ah – equivalent to a total of about 1,450 full cycles which I think is quite good considering how well it is still performing – internal resistance is currently running at around 35 milliohms – less than the resistance of the Chinese made Eneloop when it was brand new! 

The Chinese made Eneloop isn’t fairing anywhere near as well as the Japanese made Eneloop. Fast discharges are now down to around only 250 mAh and is continuing to slowly drop off. Slow discharge cycles are still showing a reasonable capacity of around 1,400 mAh. The capacity of this cell is actually significantly higher than this - after 2100 cycles, I put it through a series of 3 full charge and discharge cycles and got a final capacity of around 1,830 mAh. What is happening with the normal slow discharge is that the 1.44V charging cut off is now so low for this cell that even with a 600 mAh top off, it’s still not getting a full charge. It appears that the 3 full charge/discharge cycles have temporarily improved the capacity of this cell, but they may have also triggered an increase in the capacity decline at fast discharge rates – slow discharges have plateaued however. Internal resistance for this cell has more than doubled from about 50 milliohms up to around 110 milliohms.  Cumulative discharge capacity is now up to 1,930 Ah which works out to be equivalent to about 1,000 full cycles. The partial discharge capacity of this cell is now so low that I’d consider it to be nearly useless for most applications. No doubt, if I was to raise the charging cut off, the useable capacity would be significantly higher, but I would expect that this would only hasten the overall decline of the cell. Despite putting up a poor effort compared to its Japanese made equivalent, it’s still performing considerably better than I’ve seen with the other testing that I’ve done.

An interesting observation with this test is that after the first 49 partial discharges, the first full discharge showed slight but noticeable voltage depression (aka memory effect) on both the Chinese and Japanese Eneloops. Below is a graph of the 50th discharge for the Chinese Eneloop – the Japanese Eneloop discharge curve looked much the same.






After 2100 cycles, the voltage depression had changed a little and become more noticeable – again, the Japanese Eneloop looked quite similar, so I’m only showing the curve for the Chinese Eneloop:






After this, I ran the Chinese cell through a series of 3 full charge/discharge cycles. The voltage depression was pretty much gone by the first of these cycles and was pretty much undetectable in the following cycles. Capacity improved slightly with the 2nd cycle at 1830 mAh but remained constant for the 3rd cycle. 






After 2100 cycles, I also ran the Japanese Eneloop through a single full charge/discharge cycle. Voltage depression wasn’t obvious on this cycle and capacity came out to 1770 mAh which was actually lower than the Chinese Eneloop. It should be kept in mind however that the Japanese Eneloop started with slightly lower capacity and has been through a lot more effective cycles.






Looking at the first chart, it appears that the full discharge done every 50th cycle helps with the fast discharge capacity – most likely due to erasing most of the voltage depression. It would be interesting to see what would happen if no slow discharges were being performed – it may be the voltage depression would get to the point where there was virtually no useable capacity available before the cell voltage dropped below 1.1V Conversely, it would be interesting to see what would happen if a full slow discharge was done more often – perhaps every 20 cycles. My best guess would be that the fast discharge capacity would be slightly improved, but it would cause greater wear and tear on the cells.

It would be useful to know which change I made for this test was responsible for the greatly increased cycle life – with this knowledge, it would hopefully make it possible to achieve greater cycle life with real world usage. Unfortunately, there isn’t enough information from the testing that I’ve done so far to be able to pick a definite reason, but I think that it can be narrowed down to one or more of the following:



Partial Charging 
Partial Discharging 
No rest periods. 


It is well known that overcharging NiMH cells is bad for them. Although the chemistry is quite robust and won’t “vent with flames” like Lithium cells can when overcharged, overcharging is still not good for them and can lead to accumulated damage such as reduced capacity and increased internal resistance. Despite being quite robust, continuous overcharging at low rates can damage Eneloops quite quickly as demonstrated by testing done by others. It may be that charging a NiMH cell to 100% without actually overcharging it still does minor damage which can mount up over time, so limiting the maximum charge level to something like 80 or 90% may be beneficial for the long term health of the cell.
Reverse charging a NiMH cell won’t kill it entirely, but it can cause reduced capacity and increased internal resistance relatively quickly. It may be that even discharging down to 0.9V which is normally considered to be safe may still cause minor damage to the cell and that keeping a reserve of 10 or 20% may also be beneficial for the long term health of the cell.

Having no rest periods would at first seem counter intuitive if you’re wanting to extend the cycle life of a rechargeable cell – after all, the rest period allows the cell a chance to cool off which would seem to be good for the cell rather than running hot all of the time. I suspect that allowing the cell to cool off is actually not good for it, since it allows for thermal stress during the cooling and subsequent heating cycles and that may do damage to the internal structure of the cell. It may also be that sitting at 0% or 100% charge level for even brief periods of time may not be ideal for the cell. Lead Acid batteries in particular don’t like to be left in a discharged state for longer than necessary – if they’re left discharged for too long, their performance can be severely degraded. The same may apply to NiMH cells, but to a lesser extent.

I have already started running more cycle life tests to try to determine which of the above factors plays a role in increasing cycle life – for one test, I’m fully discharging the cells to 0.9V, but only charging them back up to 1.44V. In the other test, I’m fully charging the cells (using inflection termination) but only discharging them to 1.1V. In both cases, the tests are being performed without rest periods. I suspect that stopping discharges earlier is possibly the biggest factor, although the other factors also probably have a role as well. It will be interesting to see how these results turn out to see if my suspicion is correct.

Even though it’s too early to say what is helping the cycle life with this test, it is at least possible to rule out some common misconceptions of things that are bad for Japanese Eneloops (and probably NiMH cells in general) :



Short or no rest periods. 
Charging (and discharging) at high (1C) rates. 
Allowing cells to reach 40 degrees C for extended periods during charging and discharging. 


Another interesting observation from this test is that the coulombic efficiency is generally 99.5% or higher – for both Eneloops – most likely due to the charge termination being done at 1.44V – well before any overcharge reactions start to occur. Note that coulombic efficiency isn’t the same as energy efficiency which was well below 100%

When will this test finish? At this stage, I really don’t know! At the current rate of degradation, I wouldn’t be surprised if the Japanese Eneloop manages to exceed 5000 partial cycles and still keep going! I’ll aiming to keep this page updated at fairly regular intervals!

*Update 27/05/2015:*

These cells have now completed a total of 3200 cycles each. Not much has changed - their capacities have actually leveled off quite a bit. The Chinese Eneloop showed increased capacity for quite a few cycles, but that was due to a bad connection which reduced the maximum discharge current for those cycles.

Total cumulative capacity for the Chinese Eneloop so far is now about 2,100 Ah - equivalent to over 1000 full cycles. The Japanese Eneloop has done a total of about 3,300 Ah - equivalent to around 1,650 full cycles!


*Update 15/08/2015:*

These cells have now completed a total of 5000 cycles each. For the most part, the trend has continued on pretty much as I expected. After 4200 cycles, I ran another slow charge/discharge cycle on both cells and that seems to have unlocked a significant amount of capacity for the Chinese cell during regular slow discharges.

Total cumulative capacity for the Chinese Eneloop is now about 2,400 Ah - equivalent to about 1,200 full cycles. The Japanese Eneloop has now done a total of about 4,700 Ah - equivalent to 2,350 full cycles - significantly more than the advertised rating of 2,100 cycles (which is measured with only partial discharges, so this goes well beyond the official rating, and it still isn't dead yet!) I suspect that this is the most cycled Eneloop ever - if anyone is aware of any cycle testing which has given more cycles on an Eneloop (or any NiMH cell) please let me know about it!


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## MidnightDistortions (May 2, 2015)

*Re: Eneloops: 2700 cycles and still going...*

Awesome!! Thanks for these tests, i didn't think that rest periods could be damaging to the cells.


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## SilverFox (May 2, 2015)

*Re: Eneloops: 2700 cycles and still going...*

Hello Power Me Up,

Interesting test. Thanks for sharing.

I have noticed that cells when used everyday and subjected to partial cycles seem to go forever as far as cycles goes. However, if they are allowed to sit for a few days unused, they show extreme signs of wear. Often they can be brought back by once again putting them into frequent use.

I found a work around for this. At work we will often use our lights all week then let them sit during the weekend. I started seeing issues the first couple of days of the new week and then they would go away. My solution was to keep the cells in "hot" storage over the weekend. The idea was to not let the chemicals inside the cells cool off. This actually worked very well. I had an insulated box and using light bulbs was able to hold the temperature at around 90 F.

If you want to "stress" test your cells and see what impact the cycle testing has on them, let them cool off for a couple of days then do a charge discharge to compare the voltage under load under "cooler" conditions.

Tom


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## Kurt_Woloch (May 2, 2015)

*Re: Eneloops: 2700 cycles and still going...*

Thank you very much for these tests! I know why I normally try to avoid running my batteries completely down. ;-) Except for testing purposes, that is...

Some more comments:

The IEC calls for a discharge of 60% of the original capacity in average... for Eneloops, this would be 1.9 or 2.0 Ah, so 60% would be 1.14 or 1.2 Ah. But they do it at a slower rate, I think of only 0.2C. In contrast, with your high charging and discharging current, you actually arrive at using a lower percentage of the capacity, at least after the first 600-700 cycles. That's, I think, why the degradation of the cells actually seems to slow down more and more... because actually a lower and lower percentage of their capacity gets used. That's also why they do more cycles for you than rated, I think. Maybe it's also because these cycles are done very quickly, and they would degrade more with time.

I think if you lowered the current to 1A and kept the voltage thresholds the same, you would actually get less cycles because then a bigger percentage of the cell capacity gets used with each cycle.

You think the Chinese made Eneloop is now unusable for most purposes? Well, maybe the purposes you have. I actually have a lot of semi-low-drain devices like portable cassette players (at 150 mA) or a wireless microphone (at 200 mA)... even my new Yamaha keyboard only draws 200-300 mA. For these devices, I think the Chinese Eneloops would still work well... at least too well to discard them in my calculation scheme. What also could work is to lower the charging current to 400 mA and slightly raising the charging end voltage. How much headroom is there between the 1.44 V end point and the inflection point on the Chinese Eneloops?

I've expected the voltage depression. What happens here, I think, is that the unused part of the charge continues to self-discharge and raise in internal resistance, so as you go to use it, you get a higher internal resistance than normal. At least that's how I think about it currently. And it self-discharges faster than if the cell wasn't in use because of the higher temperature.

And the myth that a high discharge or charge rate is bad for the cells may have to do with the gradually increasing internal resistance... the higher the discharge rate is, the less cycles it will take until the cell has degraded so much that it won't be able to supply the load anymore. On the other hand... maybe this isn't even true since a higher load also causes a bigger percentage of the cell capacity remaining unused, rendering the discharge slightly more shallow. Maybe the people who call high rates bad for cells didn't take this into account...


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## Power Me Up (May 2, 2015)

*Re: Eneloops: 2700 cycles and still going...*



SilverFox said:


> I have noticed that cells when used everyday and subjected to partial cycles seem to go forever as far as cycles goes. However, if they are allowed to sit for a few days unused, they show extreme signs of wear. Often they can be brought back by once again putting them into frequent use.
> 
> I found a work around for this. At work we will often use our lights all week then let them sit during the weekend. I started seeing issues the first couple of days of the new week and then they would go away. My solution was to keep the cells in "hot" storage over the weekend. The idea was to not let the chemicals inside the cells cool off. This actually worked very well. I had an insulated box and using light bulbs was able to hold the temperature at around 90 F.



That's quite interesting!

What discharge rates were you using them at? 

How much of a difference did you see between letting them cool down and keeping them warm?

Did you notice a difference between new and used cells? i.e. did new cells see much of a difference by keeping them warm over the weekend?



> If you want to "stress" test your cells and see what impact the cycle testing has on them, let them cool off for a couple of days then do a charge discharge to compare the voltage under load under "cooler" conditions.



Since the main aim of this test is to see how many cycles I can squeeze out of them, I don't think I'll put them through any unnecessary stress in case it does shorten their life. Might be interesting to run a test like this later with some new cells though...


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## Power Me Up (May 2, 2015)

*Re: Eneloops: 2700 cycles and still going...*



Kurt_Woloch said:


> The IEC calls for a discharge of 60% of the original capacity in average... for Eneloops, this would be 1.9 or 2.0 Ah, so 60% would be 1.14 or 1.2 Ah. But they do it at a slower rate, I think of only 0.2C. In contrast, with your high charging and discharging current, you actually arrive at using a lower percentage of the capacity, at least after the first 600-700 cycles. That's, I think, why the degradation of the cells actually seems to slow down more and more... because actually a lower and lower percentage of their capacity gets used. That's also why they do more cycles for you than rated, I think. Maybe it's also because these cycles are done very quickly, and they would degrade more with time.



You may be right there - will be interesting to see how the test continues to progress and see if the capacities do stabilize even further.



> I think if you lowered the current to 1A and kept the voltage thresholds the same, you would actually get less cycles because then a bigger percentage of the cell capacity gets used with each cycle.



That is a possibility - something which requires further testing!

I've actually already got another pair of cells doing a similar test but at 500 mA. Even though it was started at the same time as this test, it's only up to 550 cycles so far because it takes so much longer to do each cycle at only 500 mA!

It's a bit early to say how that test is going to turn out, but so far, despite the more complete cycles, the results are looking quite similar to this test.



> You think the Chinese made Eneloop is now unusable for most purposes? Well, maybe the purposes you have. I actually have a lot of semi-low-drain devices like portable cassette players (at 150 mA) or a wireless microphone (at 200 mA)... even my new Yamaha keyboard only draws 200-300 mA. For these devices, I think the Chinese Eneloops would still work well... at least too well to discard them in my calculation scheme. What also could work is to lower the charging current to 400 mA and slightly raising the charging end voltage. How much headroom is there between the 1.44 V end point and the inflection point on the Chinese Eneloops?



Sorry - I didn't word that bit very well. The capacity wouldn't be useful for most applications, but the cell itself would still be useful for all but the most demanding applications - if that's any clearer. I.e. adjusting the charging/discharging cutoff would yield much better capacities even if the charge and discharge rates were kept at 2 A. I've got a number of cells that I'm still keeping in service despite having higher IR and lower capacities.

I'd estimate that the Chinese Eneloop has about another 1000 mAh before it would hit the inflection point...



> I've expected the voltage depression. What happens here, I think, is that the unused part of the charge continues to self-discharge and raise in internal resistance, so as you go to use it, you get a higher internal resistance than normal. At least that's how I think about it currently. And it self-discharges faster than if the cell wasn't in use because of the higher temperature.



I don't think that it's self discharge as such - the capacity is still there, it's just that the voltage sags under load for that part of the capacity.

From what I've read, voltage depression is actually due to growth in the crystal size on one of the electrodes - the larger crystal size reduces the available surface area for the chemical reaction to take place - effectively increasing the internal resistance of the cell.



> And the myth that a high discharge or charge rate is bad for the cells may have to do with the gradually increasing internal resistance... the higher the discharge rate is, the less cycles it will take until the cell has degraded so much that it won't be able to supply the load anymore. On the other hand... maybe this isn't even true since a higher load also causes a bigger percentage of the cell capacity remaining unused, rendering the discharge slightly more shallow. Maybe the people who call high rates bad for cells didn't take this into account...



I suspect that it's just an intuitive thing that you would expect higher charge rates to be harder on the cells. IMHO, higher charging rates probably are harder on the cells, but the difference just isn't noticeable until you get to charge rates that are significantly higher than 1C. i.e. I personally wouldn't recommend or use 15 minute chargers...


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## Power Me Up (May 2, 2015)

*Re: Eneloops: 2700 cycles and still going...*



SilverFox said:


> If you want to "stress" test your cells and see what impact the cycle testing has on them, let them cool off for a couple of days then do a charge discharge to compare the voltage under load under "cooler" conditions.



One further comment to this is that because of the lower effective capacity of the Chinese Eneloop, it is now completing the 50 cycles in about half the time of the Japanese Eneloop - that means that it's spending about a day or so fully discharged and idle while the Japanese Eneloop catches up. It has been that way from about cycle 2000 onward, so that might explain why the Chinese Eneloop has taken a bit of a dive towards the end.

The Chinese Eneloop now completes 2 cycles in the time that it takes for the Japanese Eneloop to discharge once - because the charger can only charge 1 cell at a time, the Chinese Eneloop then has to sit idle waiting for the Japanese Eneloop to finish charging before it can start charging again. Due to this waiting period, the cell obviously has a chance to cool off a little and it does affect the capacity - just checking the most recent capacity results, the difference is around 20% or so - i.e. around 250 mAh when starting cool versus about 300 mAh starting from warm.

This up and down capacity explains why the last part of the Chinese Eneloop line looks more "fuzzy"


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## SilverFox (May 2, 2015)

*Re: Eneloops: 2700 cycles and still going...*

Hello Power Me Up,

We were using 25 of the StreamLight Survivor LED lights. The draw in the light was around 1.3 amps [EDIT I screwed up on this. The actual draw was in the 500 - 600 mA range. ENDEDIT]. The cells were Panasonic 2400 mAh and were charged individually at 1.0 amp. We were using 300 cells and had 15 eight channel chargers.

"Normal" run time was around 3 hours. Cool cells were only going barely over 2 hours before dimming down. They seemed to return to "normal" run time after a couple of charge/discharge cycles.

The cells were purchased for the project and I don't have a comparison of old and new cells. We ran approximately 1500 cycles on these cells and they were still going at the end of the project. I was pleased with their performance and our ability to manage their use. I provided basic instructions on NiMh use and care and since we were all dependent on these lights there was very little abuse.

At the end of the project we distributed the lights to those that had been using them and recycled the cells figuring they were basically used up. Those who were higher on the "pecking order" got the chargers.

Tom


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## Grijon (May 2, 2015)

*Re: Eneloops: 2700 cycles and still going...*

Thank you for the testing, Power Me Up - and the write-up! lovecpf


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## Power Me Up (May 3, 2015)

*Re: Eneloops: 2700 cycles and still going...*



SilverFox said:


> Hello Power Me Up,
> 
> We were using 25 of the StreamLight Survivor LED lights. The draw in the light was around 1.3 amps. The cells were Panasonic 2400 mAh and were charged individually at 1.0 amp. We were using 300 cells and had 15 eight channel chargers.
> 
> "Normal" run time was around 3 hours. Cool cells were only going barely over 2 hours before dimming down. They seemed to return to "normal" run time after a couple of charge/discharge cycles.



Are you sure about them drawing 1.3 A? If it ran for 3 hours drawing that current, the batteries would have to provide 3.9 Ah!

Were they the same as the following model:
http://www.streamlight.com/static/document/fact_sheet/221.pdf

According to that fact sheet, it could be run from either 4 alkaline AA cells, or from a rechargeable battery of 4 sub C NiCads with 1800 mAh capacity. Rated run time for the rechargeable option is 3.5 hours - the run time chart looks like output was pretty even for a bit over 3 hours, so that would equate to a current draw of about 600 mA. You should have been able to get about 4 hours out of new 2400 mAh cells - assuming that they met their capacity rating...

It wouldn't surprise me if they started off at 4 hours, but once they got a decent amount of wear and tear on them, they could only manage 3 hours when warm and only 2 when cool.

When they were new, I'd be very surprised if there was much performance difference between warm and cool - The internal resistance on new cells tends be quite low, so warming them up won't give much improvement, but cells that are starting to degrade do show significant performance improvements when they are warmed up.



> The cells were purchased for the project and I don't have a comparison of old and new cells. We ran approximately 1500 cycles on these cells and they were still going at the end of the project. I was pleased with their performance and our ability to manage their use. I provided basic instructions on NiMh use and care and since we were all dependent on these lights there was very little abuse.



1500 cycle is quite impressive for 2400 mAh cell! Were they being routinely charged before they ran completely flat? What would you estimate was the average depth of discharge?

BTW, which model of charger were you using? Was it the Maha C800S perhaps?



> At the end of the project we distributed the lights to those that had been using them and recycled the cells figuring they were basically used up. Those who were higher on the "pecking order" got the chargers.



How many chargers did you manage to snag? ;-)

I'm guessing that you never did a capacity test on any of these cells before they were recycled?

I'm curious to know what the nature of the project was - if you can tell us?


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## Power Me Up (May 3, 2015)

*Re: Eneloops: 2700 cycles and still going...*



Grijon said:


> Thank you for the testing, Power Me Up - and the write-up! lovecpf



No problem - glad to hear that it's appreciated!


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## kreisl (May 3, 2015)

*Re: Eneloops: 2700 cycles and still going...*

Thank you PMU for your various valuable interesting contributions to the battery/charger community!!

Your tests confirm that Eneloop Japan are higher quality than Eneloop Chinese, i only buy Eneloop Japan from now on.


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## TinderBox (UK) (May 3, 2015)

*Re: Eneloops: 2700 cycles and still going...*

I have a lidl tronic rapid charger and if you put in an partly charged battery, it sometimes does a partial discharge maybe a couple of %, it`s called an refresh function, and then it starts to charge.

It can also do something called pre-charging no idea what that is for.

John.


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## SilverFox (May 3, 2015)

*Re: Eneloops: 2700 cycles and still going...*

Hello Power Me Up,

Oops... I was thinking the battery arrangement was 2S2P where it actually has all the cells in series. Thanks for finding that specification sheet. It was a while back and I was going from memory.

You are correct. The draw was closer to the 500 - 600 mA range. We used the alkaline holder and filled it with NiMh cells. We each carried a set of alkaline cells with us as a back up.

"Normal" run time was an estimate. The light has high and low settings. We used the high setting during specific times and the light was on low the rest of the time. Cells were changed at roughly 4 hour intervals which avoided over discharge. 

I did an "audit" of a sample of the cells to make sure they were still working. A change in the weather saw some cold weather degradation and the idea of keeping the cells warm was put into play. The "cold snap" ended and we stopped keeping the cells warm but I noticed that the cells weren't performing as well as they had before, so I fired up the warmer and observed the performance come back into line. Based upon this observation we kept them warm from then on.

Actually we used the C808 because we had a few people that had Mag lights with NiMh C and D cells. I used a C9000 for analysis. I ended up with the C9000 and a light and a charger. I traded the light for another charger so I came away with 3 chargers.  

The project involved measuring and documenting deformation, settlement, and corrosion in large diameter buried pipe.

Tom


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## Kurt_Woloch (May 3, 2015)

*Re: Eneloops: 2700 cycles and still going...*

I was thinking about how the problem with the Chinese Eneloops (especially charging) could be solved...

what would happen if you charged NiMh cells like you would charge LiIons? That is, first charging with a pretty high amperage (maybe 0.7 C or so) until a certain voltage is reached, then lowering the charge current to keep the voltage at this point (this would be roughly 1.4 to 1.45 volts I think)... then maybe end the charge after a certain time or if the current starts to rise again (due to warming and internal resistance lowering), whichever comes first.

I'd also be curious how a discharge and maybe also charge curve for a 2A cycle (from 1.1 to 1.44 volts and back) looks like on the Japanese and Chinese Eneloops the way they are worn out now. Do you have the means to track such a cycle voltage-wise? Maybe by the looks of that curve one could estimate if they could use a lower or higher charge or discharge endpoint (not for this test though which should of course stay the way it is) or if a big portion of their actual remaining capacity somehow gets wasted.


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## RI Chevy (May 3, 2015)

*Re: Eneloops: 2700 cycles and still going...*

Nice test. Thank you for doing this for all of us. :thumbsup:


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## Power Me Up (May 3, 2015)

*Re: Eneloops: 2700 cycles and still going...*



SilverFox said:


> Oops... I was thinking the battery arrangement was 2S2P where it actually has all the cells in series. Thanks for finding that specification sheet. It was a while back and I was going from memory.
> 
> You are correct. The draw was closer to the 500 - 600 mA range. We used the alkaline holder and filled it with NiMh cells. We each carried a set of alkaline cells with us as a back up.



No worries. It doesn't actually mention cell arrangement on the spec sheet, so it could be 2S2P, but either way, the individual cells would still be seeing a load of around 600 mA.



> "Normal" run time was an estimate. The light has high and low settings. We used the high setting during specific times and the light was on low the rest of the time. Cells were changed at roughly 4 hour intervals which avoided over discharge.



Having those cells last for so many cycles when not being fully discharged every use is inline with my thoughts on regular full discharges being bad for cell longevity. Did you do periodic full discharges?



> I did an "audit" of a sample of the cells to make sure they were still working. A change in the weather saw some cold weather degradation and the idea of keeping the cells warm was put into play. The "cold snap" ended and we stopped keeping the cells warm but I noticed that the cells weren't performing as well as they had before, so I fired up the warmer and observed the performance come back into line. Based upon this observation we kept them warm from then on.



With your "audit" did you do discharge testing in the C9000, or were you just doing runtime tests on the lights that you were using? If you did do testing with the C9000, do you remember what capacities you were seeing?



> Actually we used the C808 because we had a few people that had Mag lights with NiMh C and D cells. I used a C9000 for analysis. I ended up with the C9000 and a light and a charger. I traded the light for another charger so I came away with 3 chargers.



Ah - very nice! I was thinking that it might have been the C800S since you mentioned charging at 1 amp rates - The C801D and C808M default to 2A rates, so you must have trained everyone to use the soft charging mode?



> The project involved measuring and documenting deformation, settlement, and corrosion in large diameter buried pipe.



I was having trouble imagining what sort of project would require such heavy use of lights, but only on weekdays, so that answers my curiosity!


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## Power Me Up (May 3, 2015)

*Re: Eneloops: 2700 cycles and still going...*



Kurt_Woloch said:


> I was thinking about how the problem with the Chinese Eneloops (especially charging) could be solved...
> 
> what would happen if you charged NiMh cells like you would charge LiIons? That is, first charging with a pretty high amperage (maybe 0.7 C or so) until a certain voltage is reached, then lowering the charge current to keep the voltage at this point (this would be roughly 1.4 to 1.45 volts I think)... then maybe end the charge after a certain time or if the current starts to rise again (due to warming and internal resistance lowering), whichever comes first.



In principle, I think that it should be possible to use the same type of Constant Current/Constant Voltage charging as used for Lithium rechargeables. I suspect that the main reason why it isn't used for NiMH is because it's quicker and easier to use constant current with -dV termination.

It would be interesting to do some testing and see if it really is feasible to charge NiMH with a CC/CV method!



> I'd also be curious how a discharge and maybe also charge curve for a 2A cycle (from 1.1 to 1.44 volts and back) looks like on the Japanese and Chinese Eneloops the way they are worn out now. Do you have the means to track such a cycle voltage-wise? Maybe by the looks of that curve one could estimate if they could use a lower or higher charge or discharge endpoint (not for this test though which should of course stay the way it is) or if a big portion of their actual remaining capacity somehow gets wasted.



Yes, all of the data is logged. Total data for this test alone is 1.58 GB so far.

Here are the voltage curves that you asked for:

Chinese Eneloop Charge:





Chinese Eneloop Discharge:





Japanese Eneloop Charge:





Japanese Eneloop Discharge:


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## SilverFox (May 3, 2015)

*Re: Eneloops: 2700 cycles and still going...*

Hello Power Me Up,

In general I store NiMh cells discharged. With a typical 2400 mAh AA cell I do a discharge at 1000 mA and store the cell after the discharge. About once a month I subject the stored cells to a charge/discharge cycle and then put them back into storage. My NiMh cells are lasting around 10 years and remain reasonably vibrant to the end. I retire them when they fall below 80% of their initial capacity.

This project didn't give me enough time to properly break in the cells prior to use, so I just tested a few at a time on the C9000 to keep track of how they were doing. The Break In function takes too long so I just charged and discharged them to get a capacity. They came in at roughly 2200 mAh under this testing.

Most of the time everyone remembered to hit soft charging but there were times when people forgot. Occasionally charging at 2 amps didn't seem to be a problem for these cells.

Tom


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## Power Me Up (May 4, 2015)

*Re: Eneloops: 2700 cycles and still going...*



SilverFox said:


> Hello Power Me Up,
> 
> In general I store NiMh cells discharged. With a typical 2400 mAh AA cell I do a discharge at 1000 mA and store the cell after the discharge. About once a month I subject the stored cells to a charge/discharge cycle and then put them back into storage. My NiMh cells are lasting around 10 years and remain reasonably vibrant to the end. I retire them when they fall below 80% of their initial capacity.



That would seem to be a good indicator that letting cells rest in a fully discharged state isn't bad for them in itself.

It may also seem to indicate that discharging fully isn't actually hard on the cells, but I suspect that repeated full discharges are likely much harder on cells than just leaving them in a discharged state - might be worth running some testing to see if that is the case!



> This project didn't give me enough time to properly break in the cells prior to use, so I just tested a few at a time on the C9000 to keep track of how they were doing. The Break In function takes too long so I just charged and discharged them to get a capacity. They came in at roughly 2200 mAh under this testing.



Was that capacity when they were new, or after they had been used for a while?



> Most of the time everyone remembered to hit soft charging but there were times when people forgot. Occasionally charging at 2 amps didn't seem to be a problem for these cells.



I suspect that it may not have mattered if they had always been charged at the full 2 amps...

One thing with the C808M (and also the C801D) is that it doesn't do a full charge. From memory, it stops its charge at 1.47V - the same as the C9000 but then doesn't apply a top off, so in a lot of ways, your usage reflected the way that I ran this test...


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## ChibiM (May 4, 2015)

*Re: Eneloops: 2700 cycles and still going...*

interesting stuff. read it, and will follow! 
Waiting for the other results.. 

just a little disappointment is that the charger only charges 1 cell at a time.. (as can be seen, that (can) influence the results of the test)


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## SilverFox (May 4, 2015)

*Re: Eneloops: 2700 cycles and still going...*

Hello Power Me Up,

The cells were labeled 2400 mAh. The initial test showed a capacity of around 2200 mAh. I planned on replacing the cell when it dropped below 1750 mAh. None of the cells were replaced during the project.

Tom


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## Mr Floppy (May 5, 2015)

*Re: Eneloops: 2700 cycles and still going...*



SilverFox said:


> The cells were labeled 2400 mAh. The initial test showed a capacity of around 2200 mAh.



These were regular NiMH? Green and white? I had a heap of these.


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## SilverFox (May 5, 2015)

*Re: Eneloops: 2700 cycles and still going...*

Hello Mr Floppy,

Those are the ones.

Tom


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## Kurt_Woloch (May 7, 2015)

*Re: Eneloops: 2700 cycles and still going...*

Thank you for the additional curves! I can see that the "knee" is missing in the curves for the Chinese Eneloop cells both for charge and discharge. With "knee" I mean the flattest section of the curve after which the curve gets steeper again. With the Japanese Eneloops, that section is still there, but with the Chinese ones, I guess it would lay somewhere after the cutoff point. Therefore, I would deem this method of charging and discharging not the optimum one for the Chinese cells. As for discharging, of course in real world you can't do much about the minimum voltage and power drain of a device, but as the charger manufacturer, you can do something about the charger behavior. In this case, the charging endpoint should either be higher (somewhere after the knee) or the current lower (so that the cutoff voltage gets reached after the knee). Of course, not for this particular test which should stay the way it is now.

I normally deem a battery unsuitable for a specific application if the shut-off point gets reached before the "knee".



Power Me Up said:


> In principle, I think that it should be possible to use the same type of Constant Current/Constant Voltage charging as used for Lithium rechargeables. I suspect that the main reason why it isn't used for NiMH is because it's quicker and easier to use constant current with -dV termination.
> 
> It would be interesting to do some testing and see if it really is feasible to charge NiMH with a CC/CV method!



Well, I think the main reason why it's not being used for NiMH's is because contrary to Lithium cells, it's not dangerous to overcharge NiMH cells, it just shortens their life. With the chargers normally being built as cost-effective as possible (read: lowest possible cost) it doesn't make as much sense as with Lithium cells, hence it isn't done.

You said you had additional tests running with a higher charge end point and with a lower discharge endpoint. How are these tests going? How many cycles have been completed there and can you already see some degradation?`Maybe enough degradation to make some assumptions about which method yields the higher capacity and more cycles or which method causes what kind of degradation? My assumption would be that deeply discharging cells leads to more capacity reduction while highly charging them leads to more rise of internal resistance.


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## Power Me Up (May 7, 2015)

*Re: Eneloops: 2700 cycles and still going...*



Kurt_Woloch said:


> Thank you for the additional curves! I can see that the "knee" is missing in the curves for the Chinese Eneloop cells both for charge and discharge. With "knee" I mean the flattest section of the curve after which the curve gets steeper again. With the Japanese Eneloops, that section is still there, but with the Chinese ones, I guess it would lay somewhere after the cutoff point. Therefore, I would deem this method of charging and discharging not the optimum one for the Chinese cells. As for discharging, of course in real world you can't do much about the minimum voltage and power drain of a device, but as the charger manufacturer, you can do something about the charger behavior. In this case, the charging endpoint should either be higher (somewhere after the knee) or the current lower (so that the cutoff voltage gets reached after the knee). Of course, not for this particular test which should stay the way it is now.
> 
> I normally deem a battery unsuitable for a specific application if the shut-off point gets reached before the "knee".



I agree - with a higher charge cutoff, the Chinese Eneloop would provide significantly more capacity - to the point where it would be usable. Whether raising the charge cutoff point would cause increased degradation though is open to speculation at this point however.



> Well, I think the main reason why it's not being used for NiMH's is because contrary to Lithium cells, it's not dangerous to overcharge NiMH cells, it just shortens their life. With the chargers normally being built as cost-effective as possible (read: lowest possible cost) it doesn't make as much sense as with Lithium cells, hence it isn't done.



Yes - I agree that cost is likely a part of it and that it's done because it's not dangerous to do so.

For a multi-chemistry charger that already does CC/CV charging for Lithiums, it shouldn't be hard to do the same for NiMH as well, so cost shouldn't be a factor for those chargers. I'd suspect that because -dV has been the standard for so long, it's just what everyone expects...



> You said you had additional tests running with a higher charge end point and with a lower discharge endpoint. How are these tests going? How many cycles have been completed there and can you already see some degradation?`Maybe enough degradation to make some assumptions about which method yields the higher capacity and more cycles or which method causes what kind of degradation? My assumption would be that deeply discharging cells leads to more capacity reduction while highly charging them leads to more rise of internal resistance.



It's still a bit early to say much about the results from those tests so far: 

The 1.44V/0.9V test is up to 400 cycles so far - the Chinese cell in this test has lost over 100 mAh over the last 100 cycles, so it's looking like it's going to confirm that the 0.9V cutoff is indeed not good for the cells. The Japanese cell is holding up a lot better so far though.

The Inflection/1.1V test is only up to 250 cycles so far, so it's really too early to say much about that test so far other than they're not showing much signs of degradation - but that's to be expected at this point anyway.


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## Power Me Up (May 7, 2015)

*Re: Eneloops: 2700 cycles and still going...*



> My assumption would be that deeply discharging cells leads to more capacity reduction while highly charging them leads to more rise of internal resistance.



I actually suspect that it's the other way around - I think that deep discharges cause damage to the electrodes and that leads to increased internal resistance. Overcharging can cause venting which reduces the amount of available electrolyte and that results in reduced capacity.

Things are complicated a bit though because increased internal resistance leads to an apparent reduction in capacity - especially at high discharge rates. Electrolyte loss may also cause an increase in internal resistance as well...


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## eva2000 (May 17, 2015)

*Re: Eneloops: 2700 cycles and still going...*

nice info and testing from a fellow Brisbanite 

sad to see alot of Aussie stock of Eneloops are now China based not Japan. Got my first 2x8 AA Eneloop made in China recently. Rest of mine are Japanese ones.


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## MidnightDistortions (May 17, 2015)

*Re: Eneloops: 2700 cycles and still going...*



Power Me Up said:


> I actually suspect that it's the other way around - I think that deep discharges cause damage to the electrodes and that leads to increased internal resistance. Overcharging can cause venting which reduces the amount of available electrolyte and that results in reduced capacity.
> 
> Things are complicated a bit though because increased internal resistance leads to an apparent reduction in capacity - especially at high discharge rates. Electrolyte loss may also cause an increase in internal resistance as well...



That sounds right although i had some older Energizers vent (i can tell by the rust around the positive terminals) though all 4 cells are this way. I'm unsure if they were overcharged or overdischarged (cell reversal) or why all 4 would have the same problem unless they are beyond crap cells. The only problem they had besides being around 60% were that they would die within a few days. I suppose they are near death and the charger uses a voltage termination (like at 1.46 volts) or so (Duracell CEF14N) but it's all for science :bow:.


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## SilverFox (May 17, 2015)

*Re: Eneloops: 2700 cycles and still going...*

Hello MidnightDistortions,

Keep in mind that the seals used in cells are "less than perfect." One of the issues with aged cells is that chargers frequently miss the end of charge termination signal and subject the cell to some degree of overcharge. This can result in a slight leak from the seal and that shows up as corrosion.

Cells that are recycled when they fall below 80% of their initial capacity do not have this problem.

Tom


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## MidnightDistortions (May 17, 2015)

*Re: Eneloops: 2700 cycles and still going...*



SilverFox said:


> Hello MidnightDistortions,
> 
> Keep in mind that the seals used in cells are "less than perfect." One of the issues with aged cells is that chargers frequently miss the end of charge termination signal and subject the cell to some degree of overcharge. This can result in a slight leak from the seal and that shows up as corrosion.
> 
> ...



Yeah, the cells were probably overcharged. They did dip down in capacity but appear to not have high-IR. They however do self discharge quickly which is most likely due to aging/poor manufacturing.


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## Power Me Up (May 26, 2015)

*Re: Eneloops: 3200 cycles and still going...*

These Eneloops have now completed 3200 cycles, so I've updated the chart and included an update in the first post.


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## Power Me Up (Aug 15, 2015)

This test has now reached 5000 cycles, so I've updated the chart and added a new note to the bottom of the first post.


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## WalkIntoTheLight (Aug 15, 2015)

Power Me Up said:


> This test has now reached 5000 cycles, so I've updated the chart and added a new note to the bottom of the first post.



Very cool, but your chart still shows 3200 cycles. ?


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## HB 88 (Aug 15, 2015)

*Re: Eneloops: 3200 cycles and still going...*

Impressive testing, thank you. Also, I did not know about the Eneloop cells being manufactured in China. The majority of my NIMH batteries are 3-6 years old , mostly Eneloop AA and Imedion AA. I will need to pay particular attention when purchasing replacement cells.


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## Power Me Up (Aug 15, 2015)

WalkIntoTheLight said:


> Very cool, but your chart still shows 3200 cycles. ?



It's showing correctly for me. Try hitting reload in your browser and see if that fixes the problem for you.


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## RI Chevy (Aug 15, 2015)

I gotta ask. Are you really counting? I use my eneloops daily, but don't have the discipline to actually count to that many charge cycles. I just use them.


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## Power Me Up (Aug 15, 2015)

RI Chevy said:


> I gotta ask. Are you really counting? I use my eneloops daily, but don't have the discipline to actually count to that many charge cycles. I just use them.



Are you asking if I'm counting just for this test, or also with general use?

For this test, the data is logged to an SD card which I save onto my computer after every 50 cycles. I've written a program to extract the data and put it into a file which I can then import into Excel, so it's easy for me to have an accurate count of the number of cycles that these cells have gone through.

For general charging/testing, I do save the data as well - because it's easy to do and may come in handy for future analysis.


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## RI Chevy (Aug 15, 2015)

Wow! Ok. Very good data there. [emoji106]


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## Grijon (Aug 17, 2015)

Power Me Up said:


> *Update 15/08/2015:*
> 
> These cells have now completed a total of 5000 cycles each. For the most part, the trend has continued on pretty much as I expected. After 4200 cycles, I ran another slow charge/discharge cycle on both cells and that seems to have unlocked a significant amount of capacity for the Chinese cell during regular slow discharges.
> 
> Total cumulative capacity for the Chinese Eneloop is now about 2,400 Ah - equivalent to about 1,200 full cycles. The Japanese Eneloop has now done a total of about 4,700 Ah - equivalent to 2,350 full cycles - significantly more than the advertised rating of 2,100 cycles (which is measured with only partial discharges, so this goes well beyond the official rating, and it still isn't dead yet!) I suspect that this is the most cycled Eneloop ever - if anyone is aware of any cycle testing which has given more cycles on an Eneloop (or any NiMH cell) please let me know about it!



Incredible stuff, Power Me Up; thank you!


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## bob_ninja (Aug 20, 2015)

I am looking at the chart lines labeled as "Slow" that spend most of the time between 1400 and 1800. I not clear on what method is this, charge rate, etc. Can you clarify please?

_>>> After 4200 cycles, I ran another slow charge/discharge cycle on both cells and that seems to have unlocked a significant amount of capacity for the Chinese cell during regular slow discharges._

If the test is "slow" then how is this "slow charge/discharge" different? Clearly Chinese Eneloop recovered from 1400 to 1700+ so benefited from it a lot. Still I am not clear on how is this slow cycle different from the standard "slow" test?

Assuming the slow charge/discharge rate scenario (so usual simple electronics) does your finding imply higher risk of voltage depression for Chinese cells? Does it mean that Chinese cells require periodic break-in cycle or refresh cycle (like on Maha C-9000)?

How would you interpret it?

Here is my interpretation. Jump in if I got it wrong:

For low current applications (wireless mouse, keyboard, flashlight, etc.) Chinese cells perform fine and need periodic refresh cycle on advanced charger to restore voltage. For higher current applications (high power flashlights, etc.) Japanese cells are more reliable and perform better with steady capacity/voltage.

thanks for all the hard work


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## Min Min (Aug 20, 2015)

This info is gold Power Me Up, cheers mate!


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## markr6 (Aug 20, 2015)

Unbelievable! Makes me glad I'm mostly sticking to Li-Ion 18650 lights. Here I was thinking all Eneloops were the same; just regular vs Pro. Too bad


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## Power Me Up (Aug 20, 2015)

bob_ninja said:


> I am looking at the chart lines labeled as "Slow" that spend most of the time between 1400 and 1800. I not clear on what method is this, charge rate, etc. Can you clarify please?



Slow refers to every 50th cycle - the cell is charged normally as per the other cycles, but then given a top off charge, a rest and then discharged at 400 mA.



> _>>> After 4200 cycles, I ran another slow charge/discharge cycle on both cells and that seems to have unlocked a significant amount of capacity for the Chinese cell during regular slow discharges._
> 
> If the test is "slow" then how is this "slow charge/discharge" different? Clearly Chinese Eneloop recovered from 1400 to 1700+ so benefited from it a lot. Still I am not clear on how is this slow cycle different from the standard "slow" test?



For the 4200th cycle, the cells were charged at 1A until the inflection point, given a 200 mAh top off, rested for 60 minutes and then discharged at 400 mA. With the Chinese cell in particular, the cells weren't being fully charged even on every 50th cycle that included a top off, so that's why the capacity is significantly higher for that cycle. As for why it seems to have made a long term difference for the Chinese cell this time and not when I did the same for cycle 2100, I honestly don't know!




> Assuming the slow charge/discharge rate scenario (so usual simple electronics) does your finding imply higher risk of voltage depression for Chinese cells? Does it mean that Chinese cells require periodic break-in cycle or refresh cycle (like on Maha C-9000)?
> 
> How would you interpret it?



If you have got a device that has a high voltage cut off, I think that my data indicates a periodic full cycle would be beneficial for both types of cells - with the Chinese cell probably benefiting a bit more. If you have a device that discharges down to 0.9V, I don't think that a full cycle will provide much benefit since the cells are already being fully cycled anyway.



> Here is my interpretation. Jump in if I got it wrong:
> 
> For low current applications (wireless mouse, keyboard, flashlight, etc.) Chinese cells perform fine and need periodic refresh cycle on advanced charger to restore voltage. For higher current applications (high power flashlights, etc.) Japanese cells are more reliable and perform better with steady capacity/voltage.



That seems like a fairly reasonable assessment. The Chinese cells start with higher internal resistance and their resistance increases quite a bit quicker than the Japanese cells, and that's not good for high drain applications. Even in low drain applications, I would expect you to get more cycles out of Japanese cells, but it would probably take so many years to get to that point that the difference would become immaterial.



> thanks for all the hard work



No worries - I'm glad that my work is appreciated by so many people!


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## WalkIntoTheLight (Aug 21, 2015)

markr6 said:


> Unbelievable! Makes me glad I'm mostly sticking to Li-Ion 18650 lights. Here I was thinking all Eneloops were the same; just regular vs Pro. Too bad



Yeah, it's too bad that some regions in the world are getting Chinese Eneloops, but if you live in NA, you're safe for now.

Even so, it looks like even poor Chinese Eneloops have way more cycles in them than most (all?) 18650's. And with Japanese Eneloops (like we have in NA), the regular ones will keep working well after you've gone though your 5th set of 18650's.


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## BringerOfLight (Aug 21, 2015)

WalkIntoTheLight said:


> Even so, it looks like even poor Chinese Eneloops have way more cycles in them than most (all?) 18650's.


I wouldn't interpret the results that optimistically. Internal resistance is going up very rapidly and later discharges are extremely shallow due to the early low-voltage cutoff.

Some LiIon chemistries retain 70% capacity for thousands of cycles after the initial degradation.


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## Power Me Up (Aug 21, 2015)

BringerOfLight said:


> I wouldn't interpret the results that optimistically. Internal resistance is going up very rapidly and later discharges are extremely shallow due to the early low-voltage cutoff.



I agree that these results shouldn't be considered that optimistically - this is essentially a best case scenario and you'll generally get far fewer cycles out of Eneloops...

With that said, internal resistance on these cells is still not bad - the Chinese cell is around 150 milliohms and the Japanese cell is still down at around 38 milliohms!


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## WalkIntoTheLight (Aug 21, 2015)

Power Me Up said:


> With that said, internal resistance on these cells is still not bad - the Chinese cell is around 150 milliohms



That's about the same resistance as a fresh alkaline. I thought from your graphs it looked worse.


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## Power Me Up (Aug 21, 2015)

WalkIntoTheLight said:


> That's about the same resistance as a fresh alkaline. I thought from your graphs it looked worse.



What is killing the capacity for the Chinese cell is the low charge cutoff and high discharge cutoff - together combined with the 2A charge/discharge rate. Adjusting any of those parameters could increase the measured capacity - but may well cause it to start dying more quickly.


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## samgab (Jan 7, 2017)

Excellent thread, just the sort of info I was looking for. Late to the party, but it's still great data. Good for those who insist on using really slow charge rates.... On quality Japanese eneloops, it's really not necessary as this testing shows. If you stick to 0.5C (1A), with proper termination, they'll last a long long time. :thumbsup:


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## Kurt_Woloch (Jan 8, 2017)

I'm actually curious... has the test concluded at 5000 cycles, or has it been continued?


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## samgab (Jan 8, 2017)

I'd like to see a test like this, but only say 1000 cycles, with the same type of cell (Japanese eneloop standard AA), charged and discharged at 500mA, 1000mA, and 2000mA, to see how higher charge and discharge rates decrease cycle life over time. There isn't a lot of actual test data available on this, just a lot of rumours and conjecture.


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