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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?

Davide Andrea
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Patrick Cook
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6 Answers6

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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.

Davide Andrea
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jms
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  • Yep -- for Li-Ion batteries there are three important protections: OCP (over-current protection), UVP (under-voltage protection) and OVP (over-voltage protection). OCP applies in both directions, charge and discharge, and the value at which it trips (especially charge) varies with temperature -- it's a bad idea to charge a Li-Ion battery at a high charge rate when it's cold (generally <10C or so, but varies on brand). – Krunal Desai Feb 25 '16 at 04:50
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    Thanks for the answer, but I just realized I was measuring the wrong voltage... The board I'm using to step up the voltage to 5v has an enable pinout that reads 2.5v... My battery was actually somewhere around 3.3v. I charged it again and discharged it to 3.3v and found that the mAh was still ~3990mAh, so everything seems to be ok. – Patrick Cook Feb 25 '16 at 05:39
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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.

Davide Andrea
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Chetan Bhargava
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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.

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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.

Passerby
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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.

Davide Andrea
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Michał Leon
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    10 to 20A? What capacity are you assuming here for the power tool cell? Or do you mean 10C to 20C? – nvd Mar 10 '22 at 00:23
  • @nvd fast-discharge batteries have lower capacity, usually 1.3..1.8 Ah, while modern "standard" cells reach almost 4Ah. – Michał Leon Mar 11 '22 at 01:03
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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.

Davide Andrea
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user309900
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    You are confused about the role of an over-discharge protection circuit. "Do not use over discharge protection as signal for when to charge batteries, it might wear the batteries down faster." ([source](https://lygte-info.dk/info/BatteryLowVoltage%20UK.html)). For redundancy and to reach max. battery life, your device has to track voltage and shut down before the over-discharge protection will. That would be at the 2.7 V under load that you mention. – tanius Jun 30 '22 at 18:00
  • the overdischarge circuit IS the device's function to prevent overdischarge. How is it a "protection board" with "overdischarge protection" if you need an overdischarge protection board after you install it? That makes no sense. – user309900 Jul 01 '22 at 19:06
  • Most of the sellers told me after refunding me for the mis advertised products that their suppliers had switched out the products for the defective 2.5v cutoff without their knowledge – user309900 Jul 01 '22 at 19:16
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    Device and battery can often be separated, that's why overdischarge protection circuit on the battery is added as a last resort protection against battery damage, but not to maximize battery lifetime. Just as lithium chargers have to stop at 4.2 V before the battery's overcharge protection will kick in. Please have a look [here](https://lygte-info.dk/info/BatteryLowVoltage%20UK.html) at "But my over-discharge protection do first trip at 2.3 volt, is it faulty?". – tanius Jul 02 '22 at 08:23
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    I'm not talking about batteries with internal protection circuits, but about bms boards that are intended to act as the battery protection in devices. – user309900 Jul 03 '22 at 11:00
  • and someone wanting to shorten battery life as much as possible without causing a severe fire hazard isn't in the interest of the consumer, that appears to be in someone else's interest. – user309900 Jul 03 '22 at 11:05
  • Full BMS is indeed another matter, though not all devices have one. Can you edit your answer please, as it stays a bit unclear there (mentioning "protection boards and circuits"). – tanius Jul 04 '22 at 13:20
  • honestly I don't follow your logic; If I make or get a battery with a built in protection board and use it in a flashlight or something that doesn't have battery specific bms built in for all types of batteries I don't want the battery running down to 2.4v. – user309900 Jul 05 '22 at 20:00
  • and if the device does have built in bms for a certain battery type, the use of protected cells isn't necessary or desired. And don't want the cell drained to 2.4v. – user309900 Jul 05 '22 at 20:01
  • under what circumstances is it acceptable to drain a lithium ion or lipo down to 2.4v? – user309900 Jul 05 '22 at 20:03
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    Final comment: You quote battery manufacturers but miss out on quoting them in full: they have **two** different cutoff values, one "voltage for terminate discharging" (maximizing service life) and one lower "voltage of over-discharging protection" (ensuring battery survival and safety; 2.0-2.5 V depending on battery and load). ([source](https://lygte-info.dk/info/BatteryLowVoltage%20UK.html), quoting Samsung specs). If I understand it right, BMS uses the first, while protection circuits are for removable batteries and use the second. – tanius Jul 06 '22 at 07:40
  • "Dear **** ********, The minimum voltage of 4.2v4000[mah] is 2.7V. Hence, 2.45V cutoff voltage is a bit too low for this [battery]. Overall, our products have at least 20,000 [cycles] long life under standard operation. Kind regards Larry" – user309900 Jul 07 '22 at 08:08
  • ....one of the manufacturer quotes you asked about – user309900 Jul 07 '22 at 08:09
  • so draining to 2.5v or lower may not cause a significant safety hazard, but it will shorten the life of the battery significantly. – user309900 Jul 07 '22 at 08:14
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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.

LloydofDSS
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