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I am running one of my projects from two 2000mAh Lithium Ion batteries 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 them, they had gone down to 2.5V.

I've read in many places that Li-Ions should be 3.7V when full and 3.2V when empty, but I've never seen anything about 2.5V or anything lower than 3V for that matter. I have heard and seen people talk about "overdraining" a Li-Ion/Lipo, and that when it goes below 3V 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 effect the longevity/performance of the battery?

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    \$\begingroup\$ Your project really should include a low battery cutoff if you are powering it from Li-Ion... \$\endgroup\$ – Ecnerwal Feb 25 '16 at 3:14
  • \$\begingroup\$ @Ecnerwal It does! The computer powered off but I can't power off the LED on the step up board. That was the only thing that kept running. \$\endgroup\$ – Patrick Cook Feb 25 '16 at 5:28
  • \$\begingroup\$ @PatrickCook You could cut it off. \$\endgroup\$ – Passerby Feb 25 '16 at 5:30
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Yes, lithium batteries undergo unwanted chemical reactions when discharged below 3V, 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 overdischarged 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 2.5V at no load, your batteries 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, 2Ah 18650s are cheap and not worth the risk of them blowing up.

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  • \$\begingroup\$ 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). \$\endgroup\$ – Krunal Desai Feb 25 '16 at 4:50
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    \$\begingroup\$ 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. \$\endgroup\$ – Patrick Cook Feb 25 '16 at 5:39
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Yes, depleting a rechargeable battery under certain voltage level is harmful to the battery. The discharge voltage level depends on the battery chemistry.

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

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

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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.

enter image description here

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.

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