# AA fast chargers - Delta V?



## tr098a (Feb 2, 2008)

I'm looking for an AA fast charger, i've read about -delta V, but can anyone explain what +delta V and 0delta V are?
And why a charger would have all 3 features? 
Specifically i am looking at these two chargers
http://www.vapextech.com.hk/ProdManagerA/ChinaIn/prodmain.asp?productid=505
VTE4000
-dV cut-off function. 

http://www.vapextech.com.hk/ProdManagerA/ChinaIn/prodmain.asp?productid=504
VTE2000
+V, -V, 0V cut-off function

I presume having all 3 provides redundancy and is therefore better?

Thanks


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## turbodog (Feb 2, 2008)

Delta V is the change in voltage over time. Negative delta v is when the cell's voltage actually drops when it's fully charged. 0 delta v is when it simply stops climbing in voltage when charged.

The end of charge (eoc) voltage behavior is different for different cells. nicad cells have a negative v at full charge. nimh have either a flat plateau or a very small negative v at full charge.

Some chargers assume you're using a specific type of cell and only monitor 0/- v. Some nicer chargers will use the dat gained from the cell during charge to determine the exact cell type (chemistry --> nicd/nimh/li-ion/etc) and then adjust accordingly. I would call this final type a +/-/0 v type.

For simple, safe nimh aa cells just get whatever's convenient. The cells are cheap and don't post much of a safety issue if overcharged.

Li-ion or lithium polymer will burn down your house if improperly charged so you want a GOOD charger with them. And never leave them unattended.


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## SilverFox (Feb 3, 2008)

Hello Tr098a,

You may find this article informative.

Tom

Edit: It appears this article is no longer available. See post 51 for a link that is available.


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## tr098a (Feb 3, 2008)

SilverFox said:


> Hello Tr098a,
> 
> You may find this article informative.
> 
> Tom


Very interesting article, but how come if -*ΔV *actually overcharges nimh cells, why do nearly all of the chargers i've looked at on the web use it? Because I'm looking at the cheap ones? 



turbodog said:


> For simple, safe nimh aa cells just get whatever's convenient. The cells are cheap and don't post much of a safety issue if overcharged.



Yeh, it's for nimh cells, so i'll probably get the 30min one (it has a fan).


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## Mr Happy (Feb 3, 2008)

tr098a said:


> Very interesting article, but how come if -*ΔV *actually overcharges nimh cells, why do nearly all of the chargers i've looked at on the web use it? Because I'm looking at the cheap ones?



It's for practical reasons. If you want to charge NiMH cells quickly, you have to stuff a lot of current into them for a short time, but then stop when the cells are nearly charged (before you cook them). The difficulty is telling when to stop. One way is to look at the voltage, but the voltage is not the same for every cell, so the charger doesn't know what voltage to look for. Another way is to measure the temperature of the cell, but it is very difficult to reliably measure the internal temperature of a cell in a consumer charger, especially one that handles different sizes of cell in the same cradle.

So what it comes down to is the best of the worst options, and -dV is what is left. The alternative is a slow timed charge, but people don't always want to wait 12 hours or more.


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## ridgerunner (Feb 4, 2008)

tr098a said:


> Very interesting article, but how come if -*?V *actually overcharges nimh cells, why do nearly all of the chargers I've looked at on the web use it?



+1
Yeah, Why?

After reading SilverFox's excellent reference application note on the ST6210 charger chip above: (From Nickel-Cadmium To Nickel-Hydride Fast Battery Charger - by: J. NICOLAI, L. WUIDART), I now want a charger that stops on the inflection method! Give me a smoothed first derivative curve of V = f(t) and a charger that stops the fast charge _before_ the battery starts heating up and damaging itself! (I wish that I had read this one month ago... I'm now saying: "D'oh!" for my four recent charger purchases: MH-C9000, Lacrosse BC-900, MH-C800S and MH-C401FS.) Are there any decent (kinder, gentler) NiMH chargers out there (inexpensive or otherwise) that use the ST6210 IC and its three termination methods?

Or is it just that folks are more interested in cramming every last mA into every charge (even though that extra last bit of charge may be damaging to the cells)? Or was the article just plain wrong and charging NiMH to the -dV/dt point is not actually harmful? (I find it difficult to believe that Maha would choose a default method that would be in any way harmful to the battery cells!)

Hey SilverFox, what's your take on this?


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## Mr Happy (Feb 4, 2008)

If you have a recent Maha C9000 and use it to charge Eneloops, the charging circuit terminates when the cell voltage reaches 1.47 V, which is before the peak voltage and before the cells get warm. Eneloops have their -dV at about 1.51 V, and I've never seen an Eneloop reach this point in the C9000 other than break-in mode. (In fact, I have not yet seen the C9000 terminate on -dV, though I am hoping to see it with some cheapo NiMH cells I bought from Harbor Freight to play with. These particular cells don't get higher than 1.41 V on break-in mode, so hopefully I will get to see the elusive -dV with them.)


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## ridgerunner (Feb 4, 2008)

*Five Methods?*

The article describes the preferred fast charging method (to prevent any damage due to even the slightest overcharging) for NiMH batteries to be the: *1.)* "Inflexion Point Detection" method, where the derivative of the Voltage-as-a-function-of-Time curve is smoothed and used to pick the inflexion point. (Because right after this inflexion point is where the (damaging) temperature really starts to rise.) The SGS-THOMSON ST6210 IC incorporates three additional back-up charge termination techniques: *2.)* -dV/dt, *3.)* Temperature High Limit Exceeded, and *4.)* Timer Limit Exceeded.

So did I hear you right that the MH-C9000 uses a _fifth_ method of charge termination? i.e. *5.)* Voltage High Limit Exceeded?

willchueh where are you? Please tell me that my shiny new smart chargers are indeed smart (and gentle)!


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## Mr Happy (Feb 5, 2008)

That is a fascinating article, and the inflection point detection method does look interesting, though I suspect there may be some practical difficulties that the authors do not mention. I feel that good engineers would be aware of possible snags in a method and would mention drawbacks as well as benefits, so that others can be fully informed. 

To detect the inflection point there is a need to take first and second numerical derivatives. Taking numerical derivatives is always difficult, especially with noisy signals, and the authors do mention that several kinds of smoothing are required. Another point is that if you don't start from an empty cell there might not be an inflection point to find, and then you have to catch the -dV or high temperature signal as a fall back.

This sparks my curiosity though, and I wonder if ultra-fast chargers like the Energizer 15 minute one use such a method...?

Regarding the C9000, I have not seen the high voltage termination officially documented, but I have observed it over many trials in such a consistent way that I am sure it exists. I believe in the past there were some worries about missed terminations with the first version of the C9000, and this high voltage test might be a safeguard that was added to help prevent them in the later firmware update.

I can say however, that I can charge Eneloops at a 1 amp rate and the cells really don't get significantly warm. I also know from discharge measurements that the cells are about 100 mAh short of a maximum charge if I remove them immediately they are done, but that is not a huge bother to me.


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## ridgerunner (Feb 5, 2008)

So with the MH-C9000, if you leave the Eneloops on after the "Done" state is reached, my understanding is that (for both AAs and AAAs), a fixed "Top-off" charge of 100mA is provided for 2 hours, after which a 10mA trickle charge mode is entered forever. It seems that for AAs, this does an adequate job of topping off 2000mAh Eneloops in a relatively gentle manner. Yes? (But this extra 200mAh "top-off" does seem a bit much for AAAs?)

I wonder why Maha went with the -dV/dT termination method knowing that some (many?) NiMH batteries will not terminate properly, particularly at slow charge rates (<.33C which are actually better for battery life)? Instead, they recommend to always charge at a fast rate (so the -dV/dt will happen and be detectable) and then apparently use an over voltage back-up method (V > 1.47) to safe guard the process? (in addition to an over-temp cut-off.) Seems to me a kludge and certainly not optimal. (Note that according to SilverFox back in March of '06, the cutoff voltage for the MH-C808M was 1.6 Volts) Note that my La Crosse BC-900 charger gets away with charging at a slow (0.1C) 200mA rate by default and always seems to terminate properly (although it also supposedly uses the -dV/dt method). Go figr'.

After reading the (lengthy) MH-C808M and MH-C9000 threads here at CPF (with lots of helpful input from Maha's engineer - willchueh), and after actually using several of them myself hands on for a month or so, I feel (somewhat) knowledgeable about the Maha (and LaCrosse) chargers. But after reading SilverFox's recommended article, I am not terribly impressed with either brand! Does anyone know of any other brand chargers that utilize the "Inflexion Point Detection" method as described in the article?


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## tr098a (Feb 5, 2008)

Mr Happy said:


> This sparks my curiosity though, and I wonder if ultra-fast chargers like the Energizer 15 minute one use such a method...?



According to this datasheet http://data.energizer.com/PDFs/ch15mn.pdf
it uses 


> *Shutoff Mechanism:* Delta V Detection
> Temperature Detection
> Timer Control


All chargers i've looked at use -DeltaV, apart from that one i posted 
http://www.vapextech.com.hk/ProdManagerA/ChinaIn/prodmain.asp?productid=504

If -deltaV method overcharges the battery before stopping, and using the inflection point undercharges it, surely stopping when the deltaV is 0 is the perfect charge? Or is this impractical to easily detect? Is that what this charger does? ^


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## SilverFox (Feb 5, 2008)

Hello Ridgerunner,

Battery manufacturers recommend two charging rates. 0.1C for 16 hours, or in the range of 0.5 - 1.0C using a charge termination method.

You may be able to get away with slower charging rates at first, but as your battery ages the possibility of a missed termination increase. NiMh chemistry is very sensitive to over charging, so you end up with reduced cycle life.

The BC-900, C9000, Vanson Speedy Box, all of the other consumer chargers, and all of the hobby chargers miss primary termination when you charge at low rates. Fortunately, there are secondary termination methods that limit the amount of overcharge to the cell.

Your BC-900 does not get away with slow charging, it just cooks your cells, at a low simmer...  , until it hits a secondary termination.

I am not aware of any charger manufacturer that advertises using inflexion point detection as the primary termination method. The closest thing we have had was the Ray O Vac IC3 charger. It terminated the charge on a build up of pressure inside the cell, which is directly comparable to the inflexion point.

Tom


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## SilverFox (Feb 5, 2008)

Hello Tr098a,

Using a low value for -dV termination (in the 0 - 3 mV per cell range) often results in false terminations and under charged cells. This works well at 1C charging rates, but at slower rates it is not unusual to see fluctuations in the voltage that will trigger termination before the cell if fully charged.

Tom


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## altis (Feb 5, 2008)

I agree with Mr Happy - relying on the differential of a noisy signal could be problematic. Note, I say _could be_. In practice, this may not be an issue.

Looking at the charge curve, it strikes me that there is plenty of information available much earlier in the charge cycle. With all the mathematical power of a microcontroller it should be possible to do some sort of curve fitting. That way you'd be averaging over the whole charge period and thus side-stepping any noise issues. Obviously, the algorithm would have to cope when the user tried to charge an already full cell but that's a problem whatever sort of charge termination you use.


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## Mr Happy (Feb 7, 2008)

The article linked to by SilverFox describing an inflection method for charge termination on NiMH cells made me curious enough to do an experiment of my own to see how it might work out.

For the test I used a "Chicago Electric" cell of 2000 mAh label capacity, which I measured at 1700mAh actual capacity using break-in mode on the Powerex MH-C9000 charger. This cell is from a set obtained at $3.49 for 4 at Harbor Freight, so I didn't feel too upset about torturing it in the pursuit of science.

The test itself consisted of charging the cell at a rate of 1600 mA (0.95C) and taking regular readings until the C9000 terminated the charge and showed "Done".

Since I lack sophisticated test equipment I had to improvise a bit. I measured time, voltage, and charge by reading the C9000 display, and I measured temperature using a thermocouple sensor attached to the wall of the cell and covered by a layer of paper and aluminium foil for insulation purposes. The C9000 only displays down to 10 mV resolution where 1 mV would have been ideal, but I was able to get a reasonable trend line by smoothing the data using a rolling average.

The first chart (below) shows the trend of voltage and temperature. Charging terminated after 82 minutes at which point the cell temperature had reached 51°C and the total charge input was 1971 mAh. After an 8 hour rest for the cell to cool down, a discharge test at 500 mA give a measurement of 1586 mA. This gives an estimated charging efficiency of 1586 / 1971 = 80%. We can also note that even at the -dV cutoff point, the cell was left apparently 100 mAh short of a full charge (unless the self-discharge over 8 hours was abnormally high, which needs another test not yet done).







The second chart shows the slope of the voltage curve (smoothed, to show the trend). It can be seen there are two inflection points, and it is the second one we are interested in. This occurs at a charge of about 1600 mAh, and this is where the charge would terminate if using the inflection method. Comparing with the first chart, the temperature at this point was 41°C, so ten degrees cooler than the maximum reached at -dV. If we assume the same charging efficiency of 80% applies, the actual stored charge would have been 0.8 x 1600 = 1280 mAh, now over 400 mAh or 25% short of a full charge.






Conclusions? Well, this is only one test on one cell of course, and it might not be the best quality cell either. On the other hand, real chargers cannot pick and choose which cells people will try to charge, so they need to be able to deal with whatever is thrown at them. Given that, it does seem that the inflection algorithm can be implemented and does provide a termination signal that can be detected.

With this particular cell the temperature rises throughout the charging process and therefore the cell still gets quite warm by the time the inflection point is reached. It also seems that the cell will not be fully charged at this point, so any algorithm of this nature would need to follow up with a significant top-off charge to ensure full charging.

In the future, if time and enthusiasm permit, I might repeat the experiment with an Eneloop. I do know, however, that the C9000 will not reach the -dV signal when charging an Eneloop, so it might actually terminate before the inflection point is reached. I suspect the low internal resistance of an Eneloop will lead to lower temperatures during the early part of the charge cycle. I also suspect the charging efficiency will be higher than 80%.


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## altis (Feb 7, 2008)

Nice work Mr Happy - and well presented too. I'm impressed!

I'm looking forward to the Eneloop results too.


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## uk_caver (Feb 7, 2008)

Even at intermediate charge rates (0.1-0.5C), with adequate signal smoothing (which isn't hard with a microcontroller), from a naive viewpoint it would seem like it shouldn't be hard to find the 0deltaV point even if a -deltaV signal is not present to a useful extent.

Is it possible that at intermediate rates, even in a ideal clean-signal world, the 0deltaV point is actually *past* the point where the cells are overcharged?

At lower charge rates, is a given amount of overcharge more or less damaging to NiMH cells?


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## uk_caver (Feb 7, 2008)

[duplicate deleted]


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## Mr Happy (Feb 7, 2008)

uk_caver said:


> Even at intermediate charge rates (0.1-0.5C), with adequate signal smoothing (which isn't hard with a microcontroller), from a naive viewpoint it would seem like it shouldn't be hard to find the 0deltaV point even if a -deltaV signal is not present to a useful extent.


I have asked the same question in the past. I was shown a charging profile by someone here (I would have to search for the thread), which showed that a 0 dV point was not reached at all. The voltage retained a small positive slope for hours after the charge was essentially complete. So in practice you would have to test for a +dV point of a small enough value while avoiding the similarly small +dV slope that can occur during the middle of the charge.



> Is it possible that at intermediate rates, even in a ideal clean-signal world, the 0deltaV point is actually *past* the point where the cells are overcharged?


Perhaps yes, you would have to conclude this considering my comment above.



> At lower charge rates, is a given amount of overcharge more or less damaging to NiMH cells?


This I am not sure about. I think there are some irreversible chemical/physical changes that slowly occur in a cell during overcharge even at low temperatures, and of course excessive temperature is bad too. For many people, unless they are likely to reach the 500 cycle life expectancy of a cell, the shortening of cell life due to slight overcharging may not be noticed very much.

I was very interested to see in my test that the cell was not even fully charged at the -dV point, and never reached an overcharge state at all.


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## uk_caver (Feb 7, 2008)

I guess if at slow charge rates, there is a mid-charge slope that isn't any steeper than the slope at full charge, then unless slope measurement can be safely combined with absolute voltage readings, it's a problem.

However, there do seem to be quite a few smart chargers around which charge at significantly less than 0.5C, and which seem to terminate somewhere near the point of battery warming. Unless they're using temperature, they must presumably be using some kind of voltage measurement.

When it comes to smart chargers, do they all measure the off-charge voltage, or do some measure the voltage under charge?
Is there much difference in accuracy/curve shape between the two methods?


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## SilverFox (Feb 7, 2008)

Hello Mr Happy,

Excellent work.

I think it would be interesting to see what happens with the same cell charged at around 400 mA...

Also, you may find that you do get a -dV termination when charging Eneloop cells at 1C.

Tom


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## altis (Feb 8, 2008)

Perhaps the Energizer 15-minute charger uses the inflection point to reduce its charge current - as could be suggested by this post:

https://www.candlepowerforums.com/posts/2306504#post1719120


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## Mr Happy (Feb 9, 2008)

Well, curiosity got the better of me again, so following on from my previous experiment with a cheap brand of NiMH cell I did the same experiment with an Eneloop. This is a cell with a previously tested actual capacity of about 1900 mAh.

The test protocol was the same as before: charge at 1600 mA and record voltage, charge, and temperature for later analysis.

So here is the result of experiment 1, using the automatic charging mode of the MH-C9000:







The first point of interest is that the cell didn't come close to the 0 ΔV point before the C9000 stopped charging. This was apparently due to the maximum voltage safety termination that the C9000 applies at 1.47 V. The second point of interest is that the cell remained quite cool at a maximum 34°C throughout the charge. This contrasts significantly with the previous cheap cell which got steadily hotter throughout the charge.

Charging stopped after 72 minutes with a supplied charge of 1754 mAh. A subsequent discharge at 500 mA give a measurement of 1763 mAh, so the charging efficiency was essentially 100%. Note how this compares with the 80% measured for the cheap cell.

Since the C9000 kindly stopped charging before the inflection point or the −ΔV point was reached, it became necessary to perform experiment 2. This was the same as experiment 1, charging at 1600 mA, but with manual charge termination instead of automatic:






(I have removed the experimental data points from the chart for clarity and have just shown the trend lines.)

This time the voltage reached a maximum of 1.54 V and the cell temperature started climbing rapidly. I pulled the plug when the temperature reached 40°C (poor Eneloop).

With this new test it is possible to look at the slope of the voltage curve and find the inflection point, which occurred at just a little over 1800 mAh:






Finally, we can examine the end of charge conditions in more detail:






The C9000 would have terminated at 1.47 V and 1720 mAh, working out at 1720/1900 = 90% of a full charge and an end point temperature of 34°C. The inflection point algorithm would have terminated just past 1800 mAh, this resulting in 95% of a full charge and much the same temperature of 34°C. Lastly the −ΔV signal would have been detected at about 1960 mAh and 38°C. Presumably this would have resulted in a 100% complete charge. I'm discharging the cell at the moment to see what charge it actually retained. (Edit: the discharge test at 500 mA showed 1834 mAh.)

I think the conclusion from testing the Eneloop is that it clearly demonstrates a much better level of performance than the cheap cell. It works at a higher voltage (the high charging voltage is reflected in higher discharge voltages), it accepts charge more efficiently, and it remains much cooler during charging.

Secondly, there is the interesting result that using the C9000 to charge Eneloops gives an early, low temperature charge termination that approximates what the inflection test would do. It means that to get a 100% charge you have to leave the cells on the charger for two hours of top-off charging (100 mA x 2 hours = 200 mAh, 1700 mAh + 200 mAh = 1900 mAh). On the other hand, the early termination will increase the cycle life of the cells.


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## shadowjk (Feb 9, 2008)

I would have thought that a higher voltage during charging meant higher internal resistance?
After looking at voltages of AA I've charged at 1000mA rate on BC900, I found the 1.47V cutoff a bit odd...


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## Mr Happy (Feb 9, 2008)

shadowjk said:


> I would have thought that a higher voltage during charging meant higher internal resistance?


If you were comparing the same cell at different charging rates this would likely be true. However when comparing two cells of different provenance, the difference in voltage is almost entirely due to differences in cell construction and chemistry. Not all NiMH cells are the same, and there are very observable differences in operating voltage between different brands and cell types.



> After looking at voltages of AA I've charged at 1000mA rate on BC900, I found the 1.47V cutoff a bit odd...


You are right about this. A limit of 1.47 V is fine for cells of older vintage like the first one I tested, where the zero dV/dt point usually occurs at somewhat lower voltages. But for newer and higher quality cells, the zero dV/dt point seems to occur at voltages above 1.47 V. Among cells I've tested this is true of the Eneloop, the Uniross Hybrio and the Duracell 1700. It's not all bad though, since the cutoff occurs pretty close to a full charge and it certainly avoids the cells getting hot at all.


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## wptski (Feb 9, 2008)

So where are the inflexion point chargers? Nothing mentioned in their paper about percentage of charge. Looks be very close to the dT/dt method of termination.


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## Mr Happy (Feb 9, 2008)

wptski said:


> So where are the inflexion point chargers? Nothing mentioned in their paper about percentage of charge. Looks be very close to the dT/dt method of termination.


Yes, this is a good question.

I think it may come down to engineering conservatism and perceived market value. To make a consumer charger using the inflexion point method would require extra design, prototyping, testing and risk analysis. This would have to be weighed against the potential market benefit from such a design -- would it reduce the price the charger could be sold at, or would the average consumer recognize a benefit worth paying more for?

Now for a specialist niche product such as the Maha C9000, maybe there is a possible market. We can perhaps hope that someone out there reads this thread and considers some of the questions raised here.


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## Mr Happy (Feb 10, 2008)

SilverFox said:


> I think it would be interesting to see what happens with the same cell charged at around 400 mA...


I agree it would be interesting to do, but at that current it would take 4-5 hours for charging to complete. I just can't see me watching it and writing down measurements for that length of time, unfortunately. Maybe if I ever have access to automatic data logging equipment...


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## NiOOH (Feb 10, 2008)

Excellent work Mr Happy. This exactly matches my results with recent Maha C9000 chargers. The C9000 terminates on maxV of 1.47 V/cell. Towards the end of the top-off stage the voltage may climb to 1.48-1.49 V on some cells.
I wrote about this some months ago, but noone here believed it.


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## SilverFox (Feb 10, 2008)

Hello NiOOH,

I remember you bringing this up, and I think I gave you a bad time about it. It looks like your observations were correct. 

I have seen termination voltages above and below 1.47 volts, but there does seem to be a lot of charges that end right at 1.47 volts.

Tom


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## MrAl (Feb 10, 2008)

Hi there,


A few quick points:

First, that article Tom linked is very interesting and informative. I did find
that their terminology is a little strange though when it came to what they
were calling "Inflexion Point", which is really the calculus "Inflection Point".
If you ignore the rather strange spelling, they are both the same.
And just to clear things up a bit, the inflection point detection when it
comes to charging cells like NiMH is very much the same as zero voltage
change (or zero slope) detection. It's the point where the slope either
changes sign or is just about to change sign, or viewed another way,
it's the point where the voltage has been rising and is now about to start
decreasing. The difference between true zero voltage and inflection point
detection methods is that the true zero voltage detection does not require
the voltage to actually start to fall, whereas the inflection point might, or
might pick up the voltage starting to rise again, which could also trigger
an inflection point detection. It largely depends on how the manufacture
pre-filters the signal first though. There are many ways to do this and this
changes the overall response to some degree too. It also depends greatly
on the manufacturers algorithm, which can be better or worse.

Second, the difference in pre-filtering also affects the minus delta 
detection too, and doing this wrong could lead to overcharging also, 
which is then blamed on the minus delta technique itself. The minus
delta technique isnt as bad as it's made out to be on some web sites,
and the proof is in the monitoring of the voltage *and* temperature
of several cells of different manufacturers and of different cycle ages.
What happens is that as the voltage rises, the temperature does too,
and since it's the temperature (times the time) that does the most damage
the shorter the time the cell is subjected to a higher than normal 
temperature the longer the cell will last, but for short temperature
vs time increases not that much extra damage will be done to the cell.
This means that a small time addition where the temperature is higher,
although we want to avoid this, wont do that much extra damage to a cell.
I does matter however how the relatively noisy signal is filtered, and
that makes a big difference, and every voltage measurement has to be
pre-filtered even if that filtering is indirect (part of the algorithm).

Another thing (#3) to think about is that when comparing zero voltage or
inflection point detection to minus delta V detection one has to also take
into consideration the effect on cell capacity after a typical charge cycle.
Many cells capacities are defined by using the minus delta V technique,
so avoiding that method voids the cell capacity rating. Of course it's up
to the individual here to make the choice between highest cell capacity
and shorter charge time. For example, with my Li-ion cells i almost
always undercharge them just a little (10 to 20 percent) so i get longer
cell life.


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## Mr Happy (Feb 10, 2008)

Hi MrAl,

Thanks for your comments.

Actually "inflexion" is an alternative spelling of "inflection" and both are the same word. In the past "inflexion" was the preferred spelling (and language purists would argue it still is), however in modern times "inflection" has become the norm.

If you examine my chart titled 'End of Charge Detail', you can see the inflection point where the purple arrow is, and the zero voltage slope where the red arrow is. The inflection point comes before the zero slope, and also before the temperature has started on its rapid rise.

An interesting point for me is that the particular cell I tested does not seem able to hold a charge any greater than 1830 mAh, so the inflection point essentially marks the point of 100% full charge. Continuing to the zero slope point does not add any more charge to the cell, and in fact is just overcharging it while heating it up. One might as well stop at the inflection point and consider the charge complete.


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## wptski (Feb 10, 2008)

If the inflexion/inflection point is a 100% charge, it contradicts everything else that many of us ever read. I agree that this point occurs before the zero voltage and even before dT/dt.

This document was dated in 1994, so that means that they are refering to early Ni-MH cells. There were a few chargers that used dT/dt termination but they never worked well, so it would seem like using temperature input would be tricky.


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## Mr Happy (Feb 10, 2008)

wptski said:


> If the inflexion/inflection point is a 100% charge, it contradicts everything else that many of us ever read.



I'm only saying that it appears to be so for the Eneloop cell on test for this experiment. Eneloops are a new technology and they seem to perform better than previous generations of NiMH cell.

Here is the evidence for my statement:

When the C9000 terminated automatically at 1.47 V the charge input to the cell was 1754 mAh and a discharge test at 500 mA gave 1763 mAh, so the charge acceptance up to this point was 100%. In the second test when I terminated charging manually, the charge input to the cell at the −ΔV signal was about 1970 mAh and a discharge test at 500 mA gave 1834 mAh. This is an average charge acceptance over the whole period of 93%. If we assume the charge acceptance had again been 100% up to the 1700 mAh point, then the charge acceptance over the last part from 1700 mAh to 1970 mAh was 50% at best.

My observation is that the 100% cell capacity is about 1830 mAh, and the inflection point occurred at 1810 mAh. Making the presumption that the charge acceptance remained high up to this point, the cell was essentially fully charged at that time and further high rate charging was of little value.

I believe that if you want your cells to be fully 100% charged to the max, then the best approach is to stop the high rate charging before the rapid temperature rise begins and move to a lower top-off rate to complete the charge. The inflection point does look like a good predictor of the rapid temperature rise since it captured this point for a cheap Chinese cell of unknown manufacture and also for a modern high performing Eneloop.


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## eluminator (Feb 10, 2008)

I guess the C. Crane NiMH and NiCad charger uses "inflection". The Saitek manual says it uses Negative Delta V and Delta V Square over Delta t Square. Whatever it uses, it works. You can charge anything from one AAA cell to 4 D cells and it always gets it right. You can charge NiMH or NiCad. The only restriction is that all cells charged together should be the same. I think the negative delta V is only used to switch from top-off to trickle.

I've had a couple of these chargers for 5 or 6 years and it's the only charger I use. I like the fact that it analyzes the cells, and I always do that before I charge them. The LCD display shows what's going on when charging or discharging on analyzing. Actually it's a good idea to test the cells before charging in case the cells have different voltages. Because it charges cells in parallel, the voltage of each cell should be similar.

This charger has "soft start" so it will charge cells no matter how much they are discharged. They claim it also prevents the generation of damaging heat. The charger also has "negative pulse" which they claim prevents crystallisation and reduces gas bubbles which increases charging efficienty.

The charger has four phases. Soft start, fast charge, top off, and trickle. None of these phases are timed and I don't think they depend on absolute voltage either. They are all based on feedback from the battery. When I charge different size cells or cells with a different initial charge, the duration of each phase (except trickle) is different and the amount of charging current during the various phases is different also. You can't fool this charger. No matter what I put in it, it handles the situation. Before it starts charging, it analyzes the cells for ten seconds and then starts the charge.

Here's the manual for the Saitek. It's no longer available as far as I can tell but the C. Crane is functionally equivalent except it doesn't charge 9 volt batteries.
http://www.hypercon.net/~blisscomm/Pictures/CPF/Saitek_CCrane_charger/Saitek_SmartCharger.pdf


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## NiOOH (Feb 11, 2008)

SilverFox said:


> Hello NiOOH,
> 
> I remember you bringing this up, and I think I gave you a bad time about it. It looks like your observations were correct.
> 
> ...


 
Hello Tom.
Yes, I remember it also. In fact, it is possible to observe cut-off voltages belov 1.47 V. I have a set of cheap, low capacity NiMH cells that came with a multiband receiver. The celles are labeled at 1300 mAh. When charged at 1000 mA the voltage peaks at 1.45-1.46 V. In this case the -dV kicks off and the charger terminates at voltages lower than 1.47 V. On all other cases the termination occurs shortly after the display shows 1.47 V. During the top-off, the voltage could rise above 1.47, but I haven't seen terminating the fast charge above 1.47.


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## Mr Happy (Feb 11, 2008)

NiOOH said:


> Yes, I remember it also. In fact, it is possible to observe cut-off voltages belov 1.47 V. I have a set of cheap, low capacity NiMH cells that came with a multiband receiver. The celles are labeled at 1300 mAh. When charged at 1000 mA the voltage peaks at 1.45-1.46 V. In this case the -dV kicks off and the charger terminates at voltages lower than 1.47 V. On all other cases the termination occurs shortly after the display shows 1.47 V. During the top-off, the voltage could rise above 1.47, but I haven't seen terminating the fast charge above 1.47.


This is exactly the same as I observe. The cheap Chinese cells in my test above terminated on the -dV signal below 1.47 volts. However if you watch the charger with cells like the Eneloops, Done appears in the display as soon as the voltage rolls over 1.47. Following this the voltage may creep up to 1.48 or 1.49 during top off, but this is always after Done has appeared and never before.

I have version number 0G0D01. It is possible that the first firmware version of the C9000 is different in this regard to the later one.


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## bob_ninja (Feb 11, 2008)

Wow, very nice work Mr. Al
Thank you very much 

I remember the termination problems in early C9K versions and debates about difficulties in identifying a reliable end of charge signal, especially for older cells. I am only speculating here: Could this max voltage 1.47V end of charge trigger be the latest firmware update resulting from early problems? Perhaps they added this max voltage end of charge signal as a safer alternative to other methods to stop those runaway charges that kept pumping many Ah into cells. Just wondering.

The interesting outcome of stopping at 1.47V is that the old problem of cooking cells if now gone and C9K is actually cooler than BC900. Funny reversal.

After the inflection point temperature starts to rise much faster, so it is probably the best point to stop. However, there is only about 100 mAh between it and the 1.47V "signal", or 5.5% difference. The important point is that the remaining 200 mAh delivered by top off charge is done at a much lower rate (100 mA instead of 1600 mA in your example). Thus it still squeezes those last few mAh without heat buildup. I'd say that is a damn well way to ensure virtually 100% full charge without much complexity.

Also, I am not sure what is the point of trying to squeeze in every last mAh anyway as SD is highest at 100% SOC, even for LSDs. The car batteries are charged only up to 80% SOC for longevity. In this instance, the original 1.47V charge termination resulted in 94% SOC which is pretty close.

Once again I am impressed by this charger. Great stuff.


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## Mr Happy (Feb 11, 2008)

bob_ninja said:


> Wow, very nice work Mr. Al


But...but...it was me that did the testing... :mecry:



> Thank you very much


You are welcome. It was fascinating to look at the detail of exactly what happens with this charger.


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## wptski (Feb 11, 2008)

Somewhere in the early C9000 threads is a post of mine with a temperature graph of two cells. One is charging inserted in the unit and the other is connected outside the unit. If I remember correctly, the cell outside the unit runs >20F cooler, so the built in PS is generating much of the cell's heat. So how to you enter that into the equation? Some slots run warmer than others too.


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## Mr Happy (Feb 11, 2008)

I did wonder about heat from the C9000 itself. I tried to minimize that by testing one cell in isolation. 

During the charging process the C9000 did not feel very warm to the touch, but I know that with four cells at once at high charge rates it does tend to get a bit warm. In the test with the Chicago Electric cell that got hot, the heat was definitely generated inside the cell.


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## wptski (Feb 11, 2008)

So this method is for charging a single cell at a time?


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## Mr Happy (Feb 11, 2008)

As with other methods like -dV it applies to a single charging channel of a possibly multi-channel charger. I wouldn't know how effective it might be when charging two or more cells in series such as in a battery pack.

What I was saying above was that to minimize the heating effect of the charger and adjacent cells on the temperature measurements, I just tested one cell by itself.


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## NiOOH (Feb 12, 2008)

bob_ninja said:


> I remember the termination problems in early C9K versions and debates about difficulties in identifying a reliable end of charge signal, especially for older cells. I am only speculating here: Could this max voltage 1.47V end of charge trigger be the latest firmware update resulting from early problems? Perhaps they added this max voltage end of charge signal as a safer alternative to other methods to stop those runaway charges that kept pumping many Ah into cells. Just wondering.
> 
> The interesting outcome of stopping at 1.47V is that the old problem of cooking cells if now gone and C9K is actually cooler than BC900. Funny reversal.
> 
> ...


 
This is exactly what I think too. IMO, MAHA engineers pressed with time, did this quick and safe fix.. Unfortunately, my BC900 broke so I cannot directly compare charge completeness between it and c9000. For someone with these two chargers, can you make the experiment lkike charging the same cell at the same rate on both chargers and discharge on the MAHA to get the available capacity


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## bob_ninja (Feb 12, 2008)

Mr Happy said:


> But...but...it was me that did the testing... :mecry:
> 
> 
> You are welcome. It was fascinating to look at the detail of exactly what happens with this charger.




Ohh, shoot I got mixed up. Sorry ment thank you Happy Face


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## bob_ninja (Feb 12, 2008)

NiOOH said:


> This is exactly what I think too. IMO, MAHA engineers pressed with time, did this quick and safe fix.. Unfortunately, my BC900 broke so I cannot directly compare charge completeness between it and c9000. For someone with these two chargers, can you make the experiment lkike charging the same cell at the same rate on both chargers and discharge on the MAHA to get the available capacity



Do you think that there would be much of a difference? Do you think it is worth an experiment?

I thought BC900 also charged bit below 100%

I have both so could try it I suppose. Using Eneloops? Using like 0.5C, so 1000 mA charge and 500 mA discharge? That sort of thing?

I don't like using more than 1A charge rate on BC900, so ....


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## wptski (Feb 12, 2008)

Discounting topoff or trickle charge. If one thinks a charger terminates at peak or zero Delta, it would charge to a less degree of fullness then -DeltaV. This inflexion method would be even less.


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## NiOOH (Feb 12, 2008)

bob_ninja said:


> Do you think that there would be much of a difference? Do you think it is worth an experiment?
> 
> I thought BC900 also charged bit below 100%
> 
> ...


 
I think that would be great:twothumbs thanks in advance. Just make sure that you always discharge on the same charger, I'd guess it ought to be the C9000. It would be interesting to see the results Maha C9000 shortly after done vs BC900 vs Maha C9000+ 2 hours top off.


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## bob_ninja (Feb 17, 2008)

C9000 0G0E01, BC-900, Eneloop HR-3UT6

Charge 1A, discharge 0.5A, min 2 hour rest between each cycle
Used C9000 for discharge for both tests

C9000

Charge:
57 min 868 mAh 1.4V
88 min 1335 mAh 1.41V
113 min 1712 mAh 1.45V
C1 116 min 1763 mAh 1.46V
C2 115 min 1748 mAh 1.46V
left C1 for 2 more hours, removed C2 when done without delay

Discharge:
C1 1812 mAh
C2 1732 mAh

Both cells were very close (such as same voltage) so same number for both.
I tried using a thermometer for my kid, so it would display error for any temperatures that are too far from normal for humans. It displayed errors, so temperature was below 34C, fairly cool.

BC900

Charge:
93 min 1540 mAh 1.42V 34.3C
102 min 1700 mAh 1.45V/.44V 35C/34.4C
109 min 1810 mAh 1.48V/.46V 35.3C/34.5C
120 min 2 Ah 1.52V HI/38.4C

C1 122 min 2.03Ah 1.53V
C2 125 min 2.08Ah 1.53V

left C2 for 2 hour trickle charge at 59mA, removed C1 without when done
(got mixed up so removed the wrong cell, still they are close)

Discharge (on C9000):
C1 1811 mAh
C2 1868 mAh (trickle charge)

This time cells did register human-like temps. Just before the end C1 registered a HIGH temp error, so likely around 40C. Overall temperatures were higher but still fine.

BC900 shows trickle charge rate. Interesting that its "trickle charge" rate is 59mA, close to C9000's "topoff charge" rate of 100 mA.

C9000 stopped close to 1.47V (I probably missed it so registered 1.46V)
While it stopped short of nominal 2Ah capacity, it is interesting that C2 pumped out almost exactly what was put in (in 1748 out 1732). Seems it is very efficient in putting energy in and just the right amount.

On the other hand BC900 kept going longer. When C9000 stopped (around 115 min) at the point BC900 was still going and cell temperature started rising faster. While BC900 pumped in about 250 mAh more, it only got out about 50-80 mAh more.

Anyway I should probably stop rambling now 
Bottom line is C9000 is more conservative and (for my own taste  stops right around the point where cell temperature starts to rise, so gives up the last 5%-10% or thereabouts. BC900 kept going to a higher voltage so got bit more in.


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## unattended (Jan 25, 2012)

wptski said:


> So where are the inflexion point chargers?



you have to pay for a hobby charger to get that - and even there the inflexion method is pretty rare.
i bought the "pulsar 2+", they have the even more expensive pulsar 3 and there are one
or two other manufacturers with inflexion chargers.


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## Hedles (Apr 10, 2015)

ridgerunner said:


> +1
> Yeah, Why?
> 
> After reading SilverFox's excellent reference application note on the ST6210 charger chip above: (From Nickel-Cadmium To Nickel-Hydride Fast Battery Charger - by: J. NICOLAI, L. WUIDART), I now want a charger that stops on the inflection method! Give me a smoothed first derivative curve of V = f(t) and a charger that stops the fast charge _before_ the battery starts heating up and damaging itself! (I wish that I had read this one month ago... I'm now saying: "D'oh!" for my four recent charger purchases: MH-C9000, Lacrosse BC-900, MH-C800S and MH-C401FS.) Are there any decent (kinder, gentler) NiMH chargers out there (inexpensive or otherwise) that use the ST6210 IC and its three termination methods?
> ...




The article linked above and in the referenced post by SilverFox, is no longer available at the linked address but it can be downloaded from http://application-notes.digchip.com/005/5-10683.pdf. Perhaps a moderator can change the link in the original posts if judged appropriate. This post would then be obsolete and could be removed.


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## SilverFox (Apr 10, 2015)

Hello Hedles,

Welcome to CPF.

I edited my post to look to your post for the current information. Thanks for finding this.

Tom


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## Power Me Up (Apr 11, 2015)

As far as I'm aware, the only charger that uses Inflection termination is the UltraSmartCharger - I'm happy to be corrected on this though.

Disclaimer: I wrote the firmware for the UltraSmartCharger!


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## Benediction (Dec 11, 2016)

Power Me Up said:


> As far as I'm aware, the only charger that uses Inflection termination is the UltraSmartCharger - I'm happy to be corrected on this though.
> 
> Disclaimer: I wrote the firmware for the UltraSmartCharger!



I agree, but there might be a new competitor on the block. Since it appears you can't get any more UltraSmartChargers (design Paul Allen!, coding by PowerMeUp) I will mention the Panasonic BQ-CC55 charger - their name for Inflection appears to be “peak sensing technology” -


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## uk_caver (Dec 12, 2016)

If so, it seems a poor description, as 'peak sensing' rather suggests 0dV, or a sensitive -dV.


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