Is draining a Li-Ion to 2.5 V harmful to a Li-ion cell?

Question

I am running one of my projects from two 2000 mAh Lithium Ion cells wired in parallel

I decided to let the battery run until it died, just once, to see how long it would last. It lasted 25.9 hours, and when I checked the voltage on the cells, they had gone down to 2.5 V.

I've read in many places that Li-Ions should be 3.7 V when full and 3.2 V when empty, but I've never seen anything about 2.5 V or anything lower than 3 V for that matter. I have heard and seen people talk about "over-draining" a Li-Ion cell, and that when it goes below 3 V a microchip disconnects the battery to protect it from discharging too far.

In this case, my battery still works, and it is charging right now, I don't plan to run it down that low again, but if it were to happen again, is it a big problem? Could this affect the longevity/performance of the cells?

Answer 1

Yes, lithium-ion cells undergo unwanted chemical reactions when discharged below 3 V, causing their internal resistance to be permanently and significantly raised. Their capacity will suffer as well, meaning that they won't accept the same amount of charge anymore. When such an over-discharged cell is "brought back to life", it will likely become chemically unstable, creating a risk of a short circuit developing inside the cell.

Even worse, assuming that you measured 2.5 V at no load, your cells have dropped even lower when they were being discharged and have subsequently rebounded to 2.5 V after the load was removed.

Li-ion cells have a maximum voltage of 4.2 V or less, I am not sure where you got the 4.7 V figure from but it's a recipe for fireworks. OP has since edited the question, to a still incorrect 3.7 V. 3.7 V is the nominal voltage (average voltage during a complete constant current discharge), while 4.2 V is the maximum voltage. These figures will vary slightly from cell to cell.

I would completely discharge the cells and get rid of them, 2 Ah 18650s are cheap and not worth the risk of them blowing up.

Answer 2

And that when it goes below 3V a microchip disconnects the battery to protect it from discharging too far.

That's true, for batteries that have built in protection circuits. Not all batteries do. Most Li-Ion batteries are raw cells that do not. The ones that do will be slightly longer than the raw cells.

There is also circuits for multiple cells, and in various variations of externally visible or not. You can purchase the cells with the protection built in or purchase the circuits by themselves. Not just for 18650, all form factors of Lithium cells can have them.

You obviously have a non-protected cell, and because you didn't add a low voltage lockout, drained it beyond the safe limits.

Answer 3

Yes, depleting a rechargeable cell under certain voltage level is harmful to it. The discharge voltage level depends on the cell chemistry.

The minimum discharge voltage varies between various sites, datasheets, etc. but 3.0 V - 2.7 V is an empirical value. If discharged under this voltage, the cell may be permanently damaged.

To get the precise value of min discharge voltage, consult the datasheet of your cell.

Answer 4

This requires an update in 2020:

For most modern Li-ion cells, 2.5 V is the discharge limit. Older batteries were usually rated at 2.75 V or 3.0 V, but as I've said, that's not the case in 2020. However, to be completely sure, you do need to consult the cell's manual, as the parameters vary wildly.

For example, a typical Sanyo cell will have safe discharge current at around 1C to 2 C, while a Sony power tool cell will be allowed to give 10 to 20 A. That's a 4 to 8 times difference.

Answer 5

Battery manufacturers in 2022 still firmly say that the cutoff voltage should be no lower than 2.7 V to avoid degrading the cell. Their specifications for mAh capacity are based on the minimum cutoff voltage so discharging below that will not add any additional capacity, it will only damage the cells. Under specified "standard" loads, the cell voltage will rise to about 3.2 V after the load is removed at the specified cutoff voltage. Much smaller loads the cutoff voltage should be higher, at 3.0 to 3.2 volts.

Despite this, very many 'protection' boards and circuits now use a "DWO1" or similar 'protection' IC that has a cutoff voltage around 2.4 V. It's actually becoming difficult to find such 'protection' boards with a correct cutoff voltage of 2.7-3 V.

There's a lot of trolls around trying to convince people that 2.4 - 2.5 volts is OK, while draining the cells that low actually significantly reduces their service life... or destroy the cell fairly quickly if it's used with small loads.

Answer 6

I just wanted to add to this thread some info I found. I tried to find out just what happens to cells when the are discharged below 2.5V. I have since learned thru experience that their rate of self discharge goes way up. If you just charge them up and measure the capacity of the pack, they will seem to work fine with capacity remaining stable for 4-5 cycles. But if you let them sit on the shelf for a few days or a week they will self discharge. This makes them useless for any normal battery application. Hope this is helpful.