What voltage is it safe to run a li-ion down to before needing to recharge it?

It just occurred to me that I don't really know the answer to the question above.
Can someone chime in please?

Technically 2.9V and maybe a bit lower for some cells but discharging beyond 3.6 or 7 gives very little capacity and will probably shorten cell life. For high load devices, I run them down to about 3.5 or 3.6 and low load devices 3.3 give or take.

easilyled said:

It just occurred to me that I don't really know the answer to the question above.
Can someone chime in please?

Click to expand...

Depends on the cell and the discharge current. With a 2.75v cutoff cell, the voltage will generally rebound to about 3.3v or higher (depending on discharge current). With a 2.5v cutoff cell, the voltage should rebound to 3v or higher. Any lower and you've most likely over-discharged the cell.

BIGLOU said:

I use this chart and depending on how much I use of my flashlight I play it by ear and when I get a chance I check the voltage. I dont let my batterries go lower than 3.8V. Also some drop-ins will flash/flicker to let you know the voltage is low and some batteries have overdischarge protection.

LiIon Battery Charge Status

4.2V – 100%
4.1V – 87%
4.0V – 75%
3.9V – 55%
3.8V – 30%
3.5V – 0%

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This chart that floats around CPF is reasonably accurate, except for the 3.5v figure. For example the Panasonic NCR18650 / NCR18650A has about 15-20% capacity left even at 3.5v. Other cells, would have around 5-10% or so left at 3.5v. I'd say generally ~3.3v would be considered 0% for most cells.

Also when discharged down to 0% or pcb cut off. Charge them up as soon as possible. But you probably already know this.

jasonck08 said:

This chart that floats around CPF is reasonably accurate, except for the 3.5v figure.

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You bring up a good point, Jason. Also, it was mentioned long ago by, I believe SilverFox and probably others as well, that estimating the remaining capacity of traditional LiCo cells is a bit harder at voltages of ~3.80 Volts and below. This is due to minor differences in the actual chemical composition and physical construction of different manufacturer's cells.

Also, I've found that particular chart to be on the low side. I think this one is a bit more accurate, most of the time.

4.2V – 100%
4.1V – 90%
4.0V – 80%
3.9V – 60%
3.8V – 40%
3.7V – 15–20%
3.6V – basically discharged

Even this one seems conservitive sometimes, however I think it's pretty close with older/used, conventional LiCo cells.

When you get into other chemistries, it gets even more complicated. Even LiMn/IMR cells are quite different. They start out about the same, but as the OC voltage gets lower, they're pretty far off. I estimate my AW 18650 cells, for example, to be at about a 60% SOC at 3.75 Volts OC, but are still able to maintain higher voltage under load than LiCo cells with a higher OC voltage. And when you get into LiFe/IFR it's even worse. The discharge curve for these cells is so flat, that it's really difficult to estimate remaining capacity, at all.

Dave

The knee in the discharge curve varies by battery type and discharge current - it may be 3.6 volts or it may be 3.0 volts.

Rather than being dependant on generalisations, this thread http://www.candlepowerforums.com/vb/showthread.php?308451 shows actual discharge curves so you can decide for your situation when you have gleaned most of the capacity out of the battery.

moderator007 said:

Also when discharged down to 0% or pcb cut off. Charge them up as soon as possible. But you probably already know this.

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I've seen nothing to suggest that LiIons suffer from being stored at low charge levels - unlike Lead Acid which sulphate whenever they're below 100 % charge.

In fact for long term storage, manufacturers recommend charge no higher than 40% for LiIons.

45/70 said:

You bring up a good point, Jason. Also, it was mentioned long ago by, I believe SilverFox and probably others as well, that estimating the remaining capacity of traditional LiCo cells is a bit harder at voltages of ~3.80 Volts and below. This is due to minor differences in the actual chemical composition and physical construction of different manufacturer's cells.

Also, I've found that particular chart to be on the low side. I think this one is a bit more accurate, most of the time.

4.2V – 100%
4.1V – 90%
4.0V – 80%
3.9V – 60%
3.8V – 40%
3.7V – 15–20%
3.6V – basically discharged

Even this one seems conservitive sometimes, however I think it's pretty close with older/used, conventional LiCo cells.

When you get into other chemistries, it gets even more complicated. Even LiMn/IMR cells are quite different. They start out about the same, but as the OC voltage gets lower, they're pretty far off. I estimate my AW 18650 cells, for example, to be at about a 60% SOC at 3.75 Volts OC, but are still able to maintain higher voltage under load than LiCo cells with a higher OC voltage. And when you get into LiFe/IFR it's even worse. The discharge curve for these cells is so flat, that it's really difficult to estimate remaining capacity, at all.

Dave

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I know for a fact that the NCR18650/NCR18650A's have about 30% capacity left @ 3.6v. In fact, Panasonic ships these cells from the factory at ~3.6 - 3.65v. The other thing thats difficult is what you consider resting voltage of a cell. Is it 5 minutes after you pull it out from a light? 1 hour? 1 day? Can make a pretty big difference, as to the voltage reading of the cell.

I do agree with you that it may vary from chemistry to chemistry, and can depend characteristics like: the cells internal resistance, nominal voltage, and cutoff voltage.

Maybe someday if I have some spare time I'll take a cell with a known capacity (mAh), discharge it by 10% of that rated capacity, let it sit for an hour, record the voltage, and repeat till its completely discharged.

Most of the Li-ion cells I've tested using a CBA II have shown very little capacity left below 3.2V.

Mike moderator007 was saying that flat Li-ions should be charged ASAP, 40% is far from flat and is as you say the ideal level for storage. I store my spare cells at about 3.7V to 3.8v in a sealed plastic food storage box in the fridge.

Norm

Norm said:

Most of the Li-ion cells I've tested using a CBA II have shown very little capacity left below 3.2V.

Mike moderator007 was saying that flat Li-ions should be charged ASAP, 40% is far from flat and is as you say the ideal level for storage. I store my spare cells at about 3.7V to 3.8v in a sealed plastic food storage box in the fridge.

Norm

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I'm confused by his use of the qualifier "or pcb cutoff" which would indicate he meant functionally flat. That is, his use of "0%" meant when a battery's protection cut-off tells you there's no more juice left and not really when there is literally no juice left.

Are you saying it's unsafe to store Li-ion batteries at their cut-off or below cut-off?

Here is some interesting info about the effect on capacity of 18650 li-ion cells in long term storage at different SOC levels and at either 23 degrees C or 45 degrees C.
These curves are based on 18650 cells with graphitic anodes, such as the Panasonic NCR18650 has:

-Source: http://www.sony.com.cn/products/ed/battery/download.pdf

This graph from this PDF http://gltrs.grc.nasa.gov/reports/2010/TM-2010-216926.pdf found in this thread seems to confirm my 3.2V as mentioned in post #10

m

bl4zd said:

I'm confused by his use of the qualifier "or pcb cutoff" which would indicate he meant functionally flat. That is, his use of "0%" meant when a battery's protection cut-off tells you there's no more juice left and not really when there is literally no juice left.

Are you saying it's unsafe to store Li-ion batteries at their cut-off or below cut-off?

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Not unsafe as far as I know (I'm sure someone will correct me if I'm wrong), but isn't good practice if you want the maximum number of cycles from your cells. Any and all abuse is cumulative and will shorten the cells life.

Don't rely on a cells protection circuit to trip at the specified Voltage, if you discharge a cell at low current it is possible to run the cell well below the cut off voltage. Think of the protection circuit as an air bag something you don't want to be using everyday.

There are people who know their Li-ions a lot better than I, the above is a combination of experience and reading it shouldn't be taken as gospel.

Norm

samgab said:

Here is some interesting info about the effect on capacity of 18650 li-ion cells in long term storage at different SOC levels and at either 23 degrees C or 45 degrees C.
These curves are based on 18650 cells with graphitic anodes, such as the Panasonic NCR18650 has:

Click to expand...

Those charts are interesting, and they highlight the fact that with lithium ion cells there is a 0% state of charge (say about 3.3 V open circuit), and there is a less than 0% state of charge (below 3.3 V in the Sony example).

When looking at the lithium ion cell chemistry, the 0% state of charge is most stable and lasts longest. Any voltage over or under the 0% point causes a decrease in stability and reduces storage life. This is unlike NiMH or NiCd cells where the most stable 0% point occurs effectively at 0 V and you can't really go below that (unless you reverse the voltage, which can be damaging).

The worst possible scenarios with lithium ion cells are to go dramatically below the 0% point or dramatically above the 100% point. Doing either can cause serious damage to the cell and may render it unsafe. When you use lithium ion cells you must be very careful not to take the voltage too low on discharge, and this is one of the primary functions of the protection circuit--it will cut off the circuit at if the voltage does go too low.

Mr Happy said:

Those charts are interesting, and they highlight the fact that with lithium ion cells there is a 0% state of charge (say about 3.3 V open circuit), and there is a less than 0% state of charge (below 3.3 V in the Sony example).

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Below 3.3V I'm guessing the cell is considered flat because it can no longer supply the required current. Your car battery can be flat, but the open Voltage can still read close to 12V.

Norm

Thanks for all the replies.

Its interesting to note that the answer is not clear cut and differs for different chemistries and different current drainage demands.

The particular application that led me to pose this question to myself was for the SPY007-XML where I was configuring it to stretch it to its limit of drawing 3A at the highest level with 2 AW 16340 IMR 550mah cells.

They were both 4.16V when I put them in and then I had to program in the 6 knob positions for the 2nd Configuration slot. I chose 3A for knob position 6, 1.5A for P5, 750ma for P4 etc.

The UI calibrates each position and during the calibration one can see the light output changing and then reaching the desired level.

After this was done, I played around with the light for 5 or 10 minutes trying to compare the output with a ceiling bounce test to other lights in my possession.

I then took out the cells and one of them measured 3.8V and the other measured 3.88V. The SPY007-XML seemed to suck the juice out of them pretty quickly, but the calibration itself probably used up quite a lot of current.

Anyway, at that point I charged them up again, but I was wondering if it would have been safe to leave them to go lower than that, say 3.6V?

With no automatic protection or cut-off for IMRs, its not easy to know what to do for the best sometimes.

EDIT: By the way the SPY007-XML does not allow a current level of 3A for more than about 45 seconds before winding down to 1A and I did not keep it at P6 for a sustained period during my testing either.

easilyled said:

With no automatic protection or cut-off for IMRs, its not easy to know what to do for the best sometimes.

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I say if in doubt; recharge.
There is absolutely no harm to a li-ion in recharging sooner than necessary, eg; with only a very low DOD.
You can happily recharge at 2% DOD, 5% DOD, 10% DOD, whatever you like.
So if you have the cells out, and are near the charger, give them a charge, it's better than over discharging.
It doesn't count as a full cycle until all of the partial charge/discharge cycles add up to one complete cycle.
Eg, 5 part cycles of only using 20% capacity add up to 1 cycle.

Something that hasn't been mentioned, while Li-Ion cells do not exhibit much self discharge, if a cell is stored in a zero SOC, at this level of charge, the voltage will drop exponentially fast, whatever the reason. This is where the 40% SOC idea comes from. It allows a "cushion" to allow for self discharge, parasitic effect form protection circuits, temperature changes and so on.

I'd say if a cell were stored at the cutoff voltage of the PCB (2.50-2.75V), particularly if it had been discharged at a lower rate, it probably wouldn't take very long before the cell voltage dropped to a damaging level. As I recall, this is around 1.5-2.0 Volts, for conventional LiCo cells.

Dave

45/70 said:

Something that hasn't been mentioned, while Li-Ion cells do not exhibit much self discharge, if a cell is stored in a zero SOC, at this level of charge, the voltage will drop exponentially fast, whatever the reason. . . . . .

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The Sony graphs above show that there is no problem storing cells at 0% state of charge.

If you discharge a cell to cut off at 2.7 volts, the voltage will recover once the load is disconnected.

MikeAusC said:

The Sony graphs above show that there is no problem storing cells at 0% state of charge.

If you discharge a cell to cut off at 2.7 volts, the voltage will recover once the load is disconnected.

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Unfortunately my Milwaukee 28v Li-ion battery pack hasn't seen that graph because it's refusing to charge beyond 23.8 after ~5 years of storage in a shed. It was never charged...just stored as a spare while I used the first battery pack.

I don't know if that voltage means a cell is damaged, can be reconditioned, can be charged at all with the correct type of charger, or anything really other than it was an expensive mistake/lesson