On 2/15/2013 12:12 AM, Bruce EVangel Parmenter wrote:
-What voltage would be a good disconnect voltage set point for that 12V Li-ion drop in battery? What voltage would prevent draining the Li-ion battery too low causing damage?
With a 12v lead-acid, the traditional cutoff voltage under load is 10.5v. I suspect that the commercial cutoff devices aren't that precise, and probably cut off at anything from 9v to 11v.
However, lead-acids have a useful feature. Their internal resistance goes up as their state of charge goes down. Before the cells go dead, their resistance gets high enough to cause an excessive voltage drop under load. This rather sudden voltage drop it what triggers the cutoff device to disconnect the load.
Suppose all the cells are matched (same amphour capacity, same initial state of charge). Then when you hit 10.5v, the 6 cells in a 12v battery are each 1.75v under load. They aren't actually dead at this point. If you remove the load, the resistive voltage drop goes away, and they immediately bounce up to 1.95-2.0v. You haven't actually hurt the battery. Charge it back up, and it will be fine (subject to whatever wear and tear a deep cycle takes).
Now, suppose the lead-acid cells are *not* all matched and balanced. When it reaches 10.5v, you may have 5 cells at 2.1v, and one cell at 0v. The one at 0v has just been *murdered*. This battery is not going to come back. Even if it seems to recharge, it will have lost most of its amphour capacity.
Now let's try this same cutoff device with lithium. If you have a BMS, at 10.5v each of the 4 lithium cells has 2.625v. That will be fine. They're not dead yet, and they can be safely recharged and will survive.
But suppose there is no BMS. Lithiums do not have the large resistance increase as they approach dead. The weakest cell keeps right on trying, right up to a few moments before it drops dead. At the 10.5v cutoff, you may have 3 cells at 3.1v, and one at 1.2v. The 1.2v cell's voltage won't bounce up to a "safe" voltage range. It has been seriously damaged, and won't recover. And without a BMS, you won't *know* you've damaged it, and may blithely keep using it, potentially leading to a fire!
So that's the situation as I see it. When everything is fine, it works. When something goes wrong (like the cells wandering out of balance, or leaving the lights on, etc.), there is a risk of failure. If things go very wrong, the failure can be dramatic!
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