# When to charge 18650?



## ekincam (Nov 10, 2011)

I have a few AW 18650 2.9AH that I use for a XML light that runs at 1.4A. I did not keep track of how much time I used the light this week as my evening commute hasn't been exactly consistent due to errands. 

Open voltage shows 3.606V on my Radioshack DMM. Is it time to recharge or can I continue to use this battery?

The battery label is marked 3.7V.


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## RCM (Nov 10, 2011)

Best to recharge at 3.6 volts, they don't have any capacity left then, and you may damage the cell if you discharge to 3.5 volts. 
This only applies to li-ion, 
4.2V is 100%
4.1V is around 90%
4.0V is around 80%
3.9V is around 60%
3.8V is around 40%
3.7V is around 20%
3.6V is empty
going to 3.5 volts will damage the cell.


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## jasonck08 (Nov 10, 2011)

The cell you mentioned is not your typical LiCo 18650. but rather is a hybrid cathode cell. At 3.6v it still has about 20-30% capacity left. But it would be a fine time to recharge it now....


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## bestsystem (Nov 10, 2011)

RCM said:


> Best to recharge at 3.6 volts, they don't have any capacity left then, and you may damage the cell if you discharge to 3.5 volts.
> This only applies to li-ion,
> 4.2V is 100%
> 4.1V is around 90%
> ...



Ae you sure that the damaging voltage is at <3.5V??Isn't tat too high?:wave:

my HP laptop with samsung 2600 ICR cells is happy to go down to 2.9-3.0 V:shakehead


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## CKOD (Nov 10, 2011)

bestsystem said:


> Ae you sure that the damaging voltage is at <3.5V??Isn't tat too high?:wave:
> 
> my HP laptop with samsung 2600 ICR cells is happy to go down to 2.9-3.0 V:shakehead



What gets repeated on here keeps creeping up every few months. I'm waiting to see it at 3.7v+ eventually


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## Norm (Nov 10, 2011)

RCM said:


> going to 3.5 volts will damage the cell.



The default discharge Voltage for Li-ion using my CBA II is 2.8V although I find that discharging past 3.2V gives almost no extra capacity, these are the readings under load.

As far as charging my EDC, I charge about once a fortnight depending on usage, for best cell life it is better to charge frequently rather than charge a deeply discharged cell.

Norm


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## 45/70 (Nov 10, 2011)

It is important to note that there is a big difference between the lowest recommended voltage under load, vs. the lowest voltage OC. For storage, it's best not to let the voltage go below about 3.5 Volts, preferably 3.6. This is why a 40% level of charge is reommened when storing LiCo cells. It allows for any self discharge that may occur, as the voltage drops faster at the low end than when the cell is more fully charged.

Under load, things are quite different. A lot depends on the current load that is being applied to the cell, but the minimum recommended voltage under load, is generally between 2.50 and 2.75 Volts, for LiCo cells, dependiong on the specific cell chemistry.

Dave


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## Poppy (Dec 30, 2012)

I just bought a few EagleTac 3400 mAh batteries. Stated on the outside of the 18650 the overdischarge protection is at 2.5-2.7v I'm guessing that it would be OK to run the batteries down to that level. I'm a noobe, and am pretty clueless.

I've read that for long storage you should run them down to 40% capacity first. What is considered long storage? 

I pretty much want to have my gear "ready to go" whenever I want to use it. Is there going to be a big difference in keeping the batteries topped off, even when they are not being used for weeks or months at a time?

I noted that someone made a note on the side of his charger to recharge at 3.9 volts. I have also read that these batteries don't have a memory and that it is better to keep them charged. So I am a little confused. If I bothered to pull it out to check its voltage, and it was not full, let's say down to 4V or 80% capacity, why not top it off?


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## Bullzeyebill (Dec 30, 2012)

Some good reading re protection circuits and inherent dangers.

Bill


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## Verndog (Dec 30, 2012)

RCM said:


> Best to recharge at 3.6 volts, they don't have any capacity left then, and you may damage the cell if you discharge to 3.5 volts.
> This only applies to li-ion,
> 4.2V is 100%
> 4.1V is around 90%
> ...



3.5 volts is not even close to cell damage. You should easily be able to go to 3.0 volts (under load), and overdischarge protection doesn't usually even kick in until 2.9-2.5 volts.

This is right from Sony on their Li-Ion cells, they measure capacity by discharging to 3.0 volts which is obviously safe.



> *The capacity in the fully-charged state prior to storage
> is taken to be 100%, and the discharge capacity after
> storage is determined by first discharging the battery to
> the cutoff voltage, then fully recharging at 4.2V and 700
> ...


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## Verndog (Dec 30, 2012)

Back to the original question. I believe you are asking for how much you have left checking resting voltage with a voltmeter. I ran a discharge test on my Eagletac 3100 (Panasonic cell with hobby charger) to 3.0v cuttoff @ .7amps discharge then pulled off charger and let sit 1 hour and checked resting voltage. I got 3.32v @ full discharge and it stabilized there. After the cell hit 3.2v under the 700ma load, it dropped off very sharply to the 3.0v cuttoff. So with my cells I estimate from mah being discharged that at the 3.6v resting with voltmeter the OP is asking you are somewhere around 40% left.


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## southernknives (Jun 8, 2013)

*At what voltage should a 18650 be recharged?*

I read somewhere that lithium ions should not be deep cycled.
The article said that they will last longer if charged before they are completely dead.
What is the ideal voltage to put them back on the charger at?
I am new to the 18650 batteries. I have been out of the flashlight loop for a while, and the technology has left me behind!


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## SemiMan (Jun 8, 2013)

*Re: At what voltage should a 18650 be recharged?*



southernknives said:


> I read somewhere that lithium ions should not be deep cycled.
> The article said that they will last longer if charged before they are completely dead.
> What is the ideal voltage to put them back on the charger at?
> I am new to the 18650 batteries. I have been out of the flashlight loop for a while, and the technology has left me behind!




Most of us, even if we 100% discharge our cells, are unlikely to impact cell life dramatically unless we are using our lights 100% every day. It all comes down to how you use them. I have 5 year old cells for my bike light that are regularly discharged till the protection cells kick in. They still have most of the original capacity, but I do keep them stored cold. It all comes down to what your requirements are.

If you cycle your batteries every day, then you may not want to cycle them past 70% (or even fully charge them). If you use your batteries once a week, then don't worry about fully discharging them as you are still going to get 10 years of cycling. You are likely to get a bigger hit from full charge capacity depletion.

Semiman


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## Knight_Light (Jun 8, 2013)

*Re: At what voltage should a 18650 be recharged?*



southernknives said:


> I read somewhere that lithium ions should not be deep cycled.
> The article said that they will last longer if charged before they are completely dead.
> What is the ideal voltage to put them back on the charger at?
> I am new to the 18650 batteries. I have been out of the flashlight loop for a while, and the technology has left me behind!


Most people don’t use their batteries often enough for any of this to matter long-term. Consider the following. Good quality batteries nowadays can be bought for $20. Let’s assume 300 charge discharge cycles per battery. Most people don’t even charge their batteries once a week. Do you see where I’m going with this.

My advice would be to get high quality batteries and a good-quality charger. Try not to charge unattended. And use the batteries in whatever way is most convenient to you. But if I had to pick a voltage as a cutoff from when to start charging it would probably be 3.7 V.

In the meantime here are a few basic facts to get you up to speed on lithium’s.



Charge between .6C to 1C. *.6C is considered the ideal* .
Lithium-ion is a very clean system and does not need formatting when new. Formatting makes little difference because the maximum capacity is available right from the beginning. Nor does a full discharge improve the capacity once faded. In most cases, a low capacity signals the end of life.
Lithium-based chemistries should be stored at around a 40 percent state-of-charge (SoC).
The SoC of Li-ion is roughly 50 percent at 3.80V/cell and 40 percent at 3.75V/cell.
Allow Li-ion to rest 90 minutes after charge or discharge before taking the voltage reading to get equilibrium.
All batteries must have the ability to relax after charged, even when kept on float or trickle charge.
Discard Li-ion if the voltage has stayed below 2.00/V/cell for more than a week.


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## SemiMan (Jun 9, 2013)

*Re: At what voltage should a 18650 be recharged?*



Knight_Light said:


> Lithium-based chemistries should be stored at around a 40 percent state-of-charge (SoC).



Many sites recommend storing LiFeP04 at 50% SOC, but I really wonder if this is not a holdover from the Lithium Ion days. The storage failure mechanisms of the two chemistries are similar, but LiFeP04 with its much lower voltages should not experience the same charged loss of capacity of other lithium technologies.

Thoughts?

Semiman


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## Knight_Light (Jun 9, 2013)

*Re: At what voltage should a 18650 be recharged?*



SemiMan said:


> Many sites recommend storing LiFeP04 at 50% SOC, but I really wonder if this is not a holdover from the Lithium Ion days. The storage failure mechanisms of the two chemistries are similar, but LiFeP04 with its much lower voltages should not experience the same charged loss of capacity of other lithium technologies.
> 
> Thoughts?
> 
> Semiman


 
That is one of the reasons I mentioned the level for 50% SOC. In reality however I don’t think for most people under real-world conditions it would make much of a difference. Whether it was 40% or 50%. I think the main point is not to be fully charged nor fully discharged when doing long-term storage.


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## kenwahoo (Jul 19, 2013)

*Re: At what voltage should a 18650 be recharged?*

I am using Samsung ICR 2600mAh ones and Samsung datasheet states that cutoff is at 2.75 volts which is pretty low and well below that generally recommended. If I have read the advice given in Battery University correctly, charging 18650s when they are only a little depleted extends the life in the sense that the battery can with stand more charges. With this in mind why would you let the battery discharge completely before recharging?


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## SemiMan (Jul 19, 2013)

Knight_Light said:


> That is one of the reasons I mentioned the level for 50% SOC. In reality however I don’t think for most people under real-world conditions it would make much of a difference. Whether it was 40% or 50%. I think the main point is not to be fully charged nor fully discharged when doing long-term storage.



My point is that storing a 100% charged lifep04 is not nearly as big an issue as storing a 100% charged lithium cobalt.

Semiman

Sent from my Nexus 4 using Tapatalk 4 Beta


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## SemiMan (Jul 19, 2013)

kenwahoo said:


> I am using Samsung ICR 2600mAh ones and Samsung datasheet states that cutoff is at 2.75 volts which is pretty low and well below that generally recommended. If I have read the advice given in Battery University correctly, charging 18650s when they are only a little depleted extends the life in the sense that the battery can with stand more charges. With this in mind why would you let the battery discharge completely before recharging?



Convenience mainly but since it is bad to keep them stored charged it can be easier and longer life to just run them dead then charge them especially if that takes several uses.

Sent from my Nexus 4 using Tapatalk 4 Beta


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## kawaii433 (Jul 19, 2013)

*Re: At what voltage should a 18650 be recharged?*



Knight_Light said:


> Most people don’t use their batteries often enough for any of this to matter long-term. Consider the following. Good quality batteries nowadays can be bought for $20. Let’s assume 300 charge discharge cycles per battery. Most people don’t even charge their batteries once a week. Do you see where I’m going with this.
> 
> My advice would be to get high quality batteries and a good-quality charger. Try not to charge unattended. And use the batteries in whatever way is most convenient to you. But if I had to pick a voltage as a cutoff from when to start charging it would probably be 3.7 V.
> 
> ...



Great information on this thread. Thank you. Will it matter if you charge the battery at 3.8 vs 3.5 as far as maximum charges (battery lifespan)? For example, if I charge my batteries at 3.8V can I extend the ~500 maximum charges for my 18650? Thanks in advance.


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## reppans (Jul 19, 2013)

kawaii433 said:


> Great information on this thread. Thank you. Will it matter if you charge the battery at 3.8 vs 3.5 as far as maximum charges (battery lifespan)? For example, if I charge my batteries at 3.8V can I extend the ~500 maximum charges for my 18650? Thanks in advance.



Here's a military study on different charging habits with 18650s. While lower depths of discharge do extend cycle life, higher average states of charge reduce capacity. So you need to find your right balance of usage and reserve power. 







As many mentioned here, I don't find it worthwhile to manage flashlight batts too closely (cheap and easily replaced), but I certainly do manage non-removable batts in my smartphones and tablets - I actually use a light timer to manage daily 70/30/70 cycles and minimize time resting @ 70% (ie timer set to charge between 5-7am).


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## Poppy (Jul 19, 2013)

*Re: At what voltage should a 18650 be recharged?*



kenwahoo said:


> I am using Samsung ICR 2600mAh ones and Samsung datasheet states that cutoff is at 2.75 volts which is pretty low and well below that generally recommended. If I have read the advice given in Battery University correctly, charging 18650s when they are only a little depleted extends the life in the sense that the battery can with stand more charges. With this in mind why would you let the battery discharge completely before recharging?



Its my understanding that five 10% charges is the same as one 50% charge when you are counting charge cycles.
Further, there are four things that diminish the life of the battery.
1 time... the older it gets the less capacity (we can't control time)
2. percent of charge (the higher the average percent of charge, the quicker it loses capacity)
3. temperature (cells will age faster- the warmer they are, and slower if kept cooler, but not frozen)
4. charge cycles... (for the most part, this is not something the average user will be concerned about. The cells are likely to die of old age before it runs through 200-300 full charge cycles)

The two things that we can control to manage the life of our batteries is percent of charge, and temperature.

Here's my strategy, regarding 18650 batteries, FWIW:
I have more batteries than flashlights, and more lights than I need on any average day. So I keep a bunch of my batteries in storage, in my refrigerator, but I want them to be USEABLE immediately if I need them. Storing dead batteries is IMO similar to having a 2D cell incandescent light in the drawer with dead batteries. I'll store them between 3.9 and 4.0 volts. 
The couple of lights that I use regularly, when I get around to checking their voltage, I'll typically top them off.

Certainly there can be an argument made for storing them at 3.7 volts, and charging them when needed. That of course would require an off-grid power source in the event of a power failure.


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## SemiMan (Jul 19, 2013)

Poppy said:


> Its my understanding that five 10% charges is the same as one 50% charge when you are counting charge cycles.
> Further, there are four things that diminish the life of the battery.
> 1 time... the older it gets the less capacity (we can't control time)
> 2. percent of charge (the higher the average percent of charge, the quicker it loses capacity)
> ...



5 - 10% cycles are not the same as 1-50% cycle. You may get 20- 50k cycles at 10% versus 1k cycles at 50%.

I put lithium in the freezer. Consumer freezers are not cold enough to damage the cells. Condensation ... Distilled water for a bit when reheating is not an issue. I do that with spare camera batteries all the time.

Sent from my Nexus 4 using Tapatalk 4 Beta


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## Poppy (Jul 19, 2013)

SemiMan said:


> 5 - 10% cycles are not the same as 1-50% cycle. You may get 20- 50k cycles at 10% versus 1k cycles at 50%.
> 
> I put lithium in the freezer. Consumer freezers are not cold enough to damage the cells. Condensation ... Distilled water for a bit when reheating is not an issue. I do that with spare camera batteries all the time.



Whatever the actual numbers are, I don't know, but the point I wanted to clarify is that each time you top off a battery doesn't necessarily count as a FULL charge cycle, but that partial charges count as a partial charge cycle. When I first got mine, I was concerned that I get a total of 300 charges IE I get to charge it 300 times, period. I was delighted to learn that partial charges count as partial charge cycles.


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## SemiMan (Jul 19, 2013)

Poppy said:


> Whatever the actual numbers are, I don't know, but the point I wanted to clarify is that each time you top off a battery doesn't necessarily count as a FULL charge cycle, but that partial charges count as a partial charge cycle. When I first got mine, I was concerned that I get a total of 300 charges IE I get to charge it 300 times, period. I was delighted to learn that partial charges count as partial charge cycles.



I think that is generally understood by most people.

Semiman


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## Poppy (Jul 19, 2013)

SemiMan said:


> I think that is generally understood by most people.
> 
> Semiman



If your pool of MOST PEOPLE is the general populace, I'd say that MOST people have never even heard of 18650 batteries.
If your pool of Most People is the group who have multiple 18650 batteries and have had them for a while, then I'd agree.
If your pool of MOST People is the group who are new enough to 18650 batteries that they have to ask, WHEN to Charge them, (as this thread is titled). I respectfully disagree.


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## kawaii433 (Jul 19, 2013)

Poppy said:


> If your pool of MOST PEOPLE is the general populace, I'd say that MOST people have never even heard of 18650 batteries.
> If your pool of Most People is the group who have multiple 18650 batteries and have had them for a while, then I'd agree.
> If your pool of MOST People is the group who are new enough to 18650 batteries that they have to ask, WHEN to Charge them, (as this thread is titled). I respectfully disagree.



It's a good thread for us noobs so yeah... I fall into the dreaded pool of Most people who are new enough to ask _when _to charge 18650s and thank you again for all the responses. They are helpful and couldn't find good answers anywhere else. It is great to know that partial charges count as partial charge cycles. I had thought that 300 charges meant 300 charges. Period. I really appreciate all the other responses because that's how we learn. Thanks again.


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