Age is a silent killer to the batteries, no matter how you treated them. That's why warranties cut out at 8 years tops, because they know that with current tech, it's going to dendrite and kill itself sooner or later. Some people get lucky and get 10 or so out of a battery, others not so much. it's not so much a matter of 'if' anymore, but more of a 'how fast'. The other components in the battery break down over time as well, the separators, etc. the cellophane wrap ages, gets brittle, weakens, may start leaking, etc. Age affects everything in it, not just the chemical to electric side of things.
Granted one could argue that each 'module' in an overall 'pack' has in theory you could look at it and say, it's own personal bms. which is why you can stack a bunch of bms'd ... say 48 volt batteries in an array and they'll all police and take care of themselves, BUT the overall decay, pushes the community 'conditions' to where some are constantly having to perform a sort of 'maintenance' on themselves, and it eventually causes the older ones to age and wear out even faster.
while it's a different chemistry and works a bit different, this scenario will kind of help explain.
look at lead acid's, lets go with agm's.
lets say you have some strings, in parallel, and each string 'module' has it's own bms on it. for the entire 'pack' of them.
if they are new they can take, ehh lets call it 14.2 volts, that's where they stop charging,
now lets add 3 years on that, now instead of 14.2 they may be happy at 13.9, they also dried out a bit, so instead of 100 ah you may now only be getting 90 ah out of the pack.
lets mix some new ones into that, so now we have some strings that are 3 years old, and some that are new. lets put a charge to them. the ones that are happy at 13.9, will finish first, but the overall pack want's 14.2 so the charge will kind of glide / glaze over them, the current will just pass thru them, their voltage will rise very quickly to 14.2 because there's nothing left to charge, it's already at it's peak, BUT, that gloss over and current flow, even though its really.. passive at this point, causes heat, even though the bms, sees this, and may limit the flow and it works it's way through the other parallel arms on a more sophisticated bms, you still put extra heat into them, which dries them out even faster, making their capacity drop even faster, now a month later they are only good for 70 ah, another month later 45 ah, ... you cook them...
if you just kept them with batteries all of the same age bracket, there wouldn't be this extra push voltage sliding thru them to dry them out.
now lithium does not really have glass mats of acid that dry out and all that but, the nuances of an older pack, being bullied by newer packs, is still there. now i left a lot out of this explanation, but the basic point is there.
can you do it, yes
will it work... yes
but it's not the best practices for keeping your batteries happy and lasting as long as possible.
these things get expensive, we don't want to be throwing 100's or even 1000's of dollars away every 2 years if we can make it last 5 by slightly different maintenance practices.
just some helpful advice, hopefully something that helps.
thank you
Aaron
aaron
Author
Topic: Power tank upgrade (Read 13370 times)