batterieslifetime
Could anyone suggest, why lifetime of Pb batteries is so short in UPSes?
3 years till 0 mAh – I've seen that several times, and some UPSes were not low-end (~250$)…
Are there any ways to make it last longer when designing new device?
Same for LiIon – if we rarely charge/discharge them, how long they could last? (I've heard things like '2 years of shelf-life and they are garbage')….
My quick look into it:
The lifetime of lithium batteries decreases with the depth of discharge, looking like the following (this curve is for lead-acid batteries, but Lithium is stated as following a similar curve):
(source)
If the 100% DoD value is taken as a reference, one can plot what I call the "isoenergy" curve (I gave it a 2sec thought) which is basically how many cycles are required from the battery to deliver the same amount of energy as 100% discharges over its entire lifetime: $$isoenergy(DoD)=\frac{100\%DoDlifetimecycles}{DoD}$$ For example, 50% DoD require twice as many cycles as 100% DoD, 25% four times etc.
The results with this particular example:
Conclusion, it still holds that the depth of discharge should be minimised as much as possible.
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.
Best Answer
This is due primarily to internal corrosion of the lead frames and the electrodes, short circuits cause the sulfation of the lead. This causes sulfation, that makes the PbSO4 crystals merge into ever larger networks. This reduces the electrochemical active surface of the PbSO4. Due to the now smaller surface, the PbS04 cannot be "come off" that good , as it could before. So it takes very long until a sufficiently high concentration of Pb2+ is present. In addition, the electrical conductivity of the sulfate is lower than that of lead. The resulting increased Internal resistance of the load cell leads to increased voltage drop. Give that process enough time and your at the point where you wont have any cell voltage at all.
I hope this helped you.
P.S.: i didnt know a good word for "ablösen" in english, since unsolder is not the one i was looking for and peel of is not the right term. but i think youll still get it.