Lithium Ion Lowest Discharge?

What is the lowest voltage that a Lithium Ion cell can be discharged to without damage? I bought a couple Protected cells from Lighthound that cut off at 3.2 volts.
Thanks,
Mike

I did a search before I posted this. Some posts said 2-2 1/2 volts?
Mike

It's supposed to be 2.5v but I've had protected RCR123As get killed from discharging to 2.5v (their cutoff point) . ~3v reduces available capacity but is safe and should maximize cycle life.

At the other end, I wouldn't go above 4.15v.

Vetkaw63;

I've used 100's of lithium ion cells over a few years.

With a quality cell such as Sanyo, Sony and Panasonic, I have discharged them to 2 Volts with absolutely no loss of capacity.

Lower quality cells may suffer when discharged to those levels. Only capacity or impedance testing will give the answer for those cells.

Larry Cobb

Thanks Guys,
Thats what I needed. I put a Cree XRE in a Dorcy 1AAA and am using a Lithium Ion 500 mah battery. It is down to 3.2 volts in about 20 minutes. I was hoping that I could extend that by a few minutes. It is amazing how bright it is with the lithium ion cell.
Mike

Hello Larry,

You either have been very lucky, are using older cells, or are referring to voltage under a reasonable load...

What are the capacities of your Sanyo, Sony, and Panasonic cells?

What was the open circuit resting voltage of them after they were discharged down to 2.0 volts?

What kind of discharge currents are you using?

If you are going all the way down to 2.0 volts open circuit resting, how long do you let them remain at that low voltage?

I may have to pick up some of those cells and do some testing. I know my LG 2400 and 2600 mAh cells will not endure that kind of abuse.

Tom

SilverFox;

Lots of them have been discharged to 2.0 under a light 0.5 amp load. I don't recall the resting voltage, but they did recover slightly.

The capacities have been 2000, 2200 & 2400 mah.

Some of the cells have been at 2.0 Volts for a few days before recharging.

Maybe, I need to send you some new & used "name brand" cells to test out.

Larry Cobb

All the manufacturers specify the minimum voltage under load. The unloaded "resting" voltage is only a VERY rough way to judge if they've been over discharged and will vary a LOT depending on cell capability and discharge current.

Sanyo LiOn discharge tests are only to 2.75 V under load. I think I've seen others specify 2.5 V under load for LiOn but can't find them now.

If 3.7 V LiOn don't recover to 3.3+ V resting after a discharge under moderate load, it's a pretty safe bet, they've been over discharged.

In the r/c world where 10 C and much higher rates are common on Lithium Polymer cells, 3.0 V under load is the normal spec with some moving to 3.2 V. Anything below a resting voltage of 3.7 V is generally considered questionable but I've found high current packs discharged under lighter loads (eg 12C pack discharged at 6C) to 3.0 V cells will recover a bit below 3.7 V and still be ok long term.

LiOn cells discharged to 2.0 V have been seriously discharged... Personally I'd be reluctant to try recharging them if they've been left there for long.

Mike

Not convinced by those figures - just checked my battery which was cut off at 6.1V under a 1A load (which is C/2), and it's only recovered to 6.56V - so less than your 3.3V a cell, and I also cut off at the same voltage when running at C/4, which will presumably recover even lower. Fairly sure I've not damaged it though, as I've been running LiIon batteries down to that sort of voltage for years. Having said that I'm not convinced it's worth running down any more than that (and why I cut off at 6.1V rather than lower), since you get very little extra energy out - even the drop from 6.6V down to 6.1V only gives me about 2 minutes at 1A with that battery!

I just recieved 3 new 14500's (protected) and charger from PTS. Running them in a Jetbeam MKII, all 3 batteries shut off at about 2.51v. After resting them
for them for 20 minutes they jump to about 2.57v.

I was just reading some information from Panasonic. They recommend a low voltage cut off at 3.0 volts. They go on to say that if you are trying to charge a cell that has a voltage of 2.9 volts or less, you should initiate the charge at a 0.1C rate.

From this I would suggest a low voltage cut off in your circuit of 3.0 volts under load, and if your resting voltage is under 3.0 volts, you should adjust your charge rate to 0.1C and expect a loss in capacity or cycle life.

I use 3.5 volts (open circuit resting) as my cut off. Under moderate loads of 1 - 2 amps, there is not much left when the voltage drops below this. This changes with light loads of 1 - 500 mA. There is some useful energy left, and the voltage will not rebound as much when you remove the load.

My view of the protection circuits in protected cells is that they are a last safety if everything else fails. Li-Ion cells maintain better performance with less than a 100% discharge, so why run the cell all the way down in normal use. If your light accidently comes on and you are not around, it is nice to have the battery shut it off, but I don't think this is the best thing to do every time.

Larry is the only one I know of that regularly claims to drain Li-Ion cells to 2.0 volts under light loads without problems. If I ever get my testing schedule cleared out, this may be something to look into.

Tom

Tom:

I've read somewhere to use around 50ma till the cell reaches the nominal 3.2V and then resume normal charging procedure.

An absolute current isn't that useful information given batteries have all sorts of different capacities.

Don't necessarily try this at home, but I've recovered LiIon cells from totally dead by charging with C/10 until they reach 3V. Lost ~25% of capacity, but otherwise seem to work fine - though I do have to make sure I keep them balanced also, as the amount lost by each cell is not consistent.

Hello Chris,

I might just point out that a batteries useful life is defined to the point where it reduces to 80% of its initial capacity.

If your cells lost 25%, they are dead for scientific purposes. Thus the statement that over discharging your cells kills them.

There may be some useful life left in them, but they will take longer to charge, heat up more during charging (and discharging), and can develop a higher rate of self discharge.

I have a few of these cells, but prefer not to use them.

Tom

SilverFox said:

If your cells lost 25%, they are dead for scientific purposes. Thus the statement that over discharging your cells kills them.

Click to expand...

Yep, they are still dead as far as I am concerned - I only use them for testing driver circuits and such, as I don't have to worry too much if the low voltage cutout is working correctly. Very frustrating though, as I killed a whole 21.6V pack before I'd even totally discharged it once by leaving it connected to a driver circuit for months, having not realised that the driver wasn't going into sleep mode properly, and so still drawing 1mA or so

Big difference between losing 25% of capacity and going

Hello Chris,

Extreme bummer... I feel your pain.

Tom