I’ve tried searching and can’t come up with a clear concise answer. With Li-Ion cells in series (2S2P in my specific case), how far out of balance (ie. voltage levels) can a pack be before it is unacceptable and requires balancing? Let’s assume cells are matched cells with identical capacities. Is 0.05v difference acceptable? Will continuing to use a pack with this much difference lead to bigger issues (ie. the lower voltage pair gradually losing capacity and therefore dropping further out of balance from the first pair)? Oh, and I’d like to see a reputable reference to support this information.
I’m reviewing a 2S2P bike light battery pack and found the pairs of cells at 4.20v and 4.15v and am wondering how significant this is. I understand it has reduced the packs capacity and voltage sags more than it should, but is it really “dangerous”? Will it lead to drastically reduced life expectancy?
As long as there is a protection circuit that checks each cell in a series (Bike light batteries often have that), there are no danger, only less capacity.
When capacity is getting too low, you just need to balance the pack with a hobby charger.
Yes, there is a protection circuit in place with a “balance” connection between the cells (which I guess only monitors balance, doesn’t actually rebalance). So the protection circuit would trip when they get “too far apart”? How far is too far?
All the ones i have seen have some kind of pair-splitting circuit, i assumed that balanced the 2 halves.
someone here said it just checks balance, i don;t really know
i don;t consider it dangerous, i have had at least 4 of these 4 cell 2s2p packs and usually they just fail before i expect them to.
on a bike at least you are not confined, it isn;t in your pocket.
even if it catches on fire you are not in that much danger, but i have never seen that…
(i don;t really know if the right term is 2s2p or 2p2s, what it is, is 2 pairs of cells. 2 cells are in parallel, then that is in series with the other pair. though the pairs are connected with a fet or something that can separate them for charging - i thought)
Yes,I’m familiar with 2S2P packs (I’ve even built my own 2S3P pack with pcm circuit & balance lead - see My Bike Lights thread in my signature.). Yes the cheap packs with mismatched cells often fail early just due to being out of balance. And yes I’m certain there is no balancing going on during charging, just monitoring (unless you’ve added a balance lead).
I’m just looking for some sort of spec or reputable guide on how close the cells should be.
I ran my first discharge with balance leads and even though they were balanced to start (balanced by me after finding them 0.05v different), one pair dropped about 0.20v below the other. I’m now cycling them 5 times without balancing and running the discharge test again to see how far they are out after 5 cycles.
When is the best time to check cell balance?
Is it under load, or during charging?
I might guess one good time is right after a full charge with no load, after a rest period…
Maybe that never happens
That is the ideal time to check lead-acid batteries for cell balance, anyway…
Sorry to post this on your thread but a similar device may be useful to you. I would be interested in a 3S 2P 18650 system that i could give to a non enthusiast, one that can be connected to a charger that correctly balances when charging and won’t over discharge. I don’t know of any such device that i can buy as a replacement for a 12V lead acid battery (not for automotive use but one that currently uses a 12V 12Ah lead acid battery).
why not one of those jump starter things?
they basically have 3 li ion cells inside
output 12V nominally
though it may not be precisely regulated since its main goal is starting a car
they come with chargers, and also output 5V via the USB ports
Checked my cells after re-charge and rest and they were within 0.01v of each other. Perhaps mine spread apart during discharge but come back close to where they started (i.e. similar voltage levels) when recharged.
Bort, if it were me I’d be building a 3S2P pack with 18650’s (probably using n.o.s. laptop pulls keeping the welded strips in place so I wouldn’t have to do too much soldering on bare cells) with a balance lead and find a simple stupid charger where the user only had to plug in the main lead, the balance lead, and then turn on the charger until a light turned green (I’ve seen what look like some really simple basic LiPo RC Battery chargers like I described). The pack could be plasti-dipped or at the very least wrapped up like a mummy with electrical tape (that’s all I did on my 2S3P pack).
What about a 12v Li-Ion drill battery pack and charger? You’d just need to fabricate some way of connecting your “equipment” to it. Of course that might be an expensive route to go. I don’t know how you’re going to keep it “budget” AND safe though.
Cool concept, but wow! The price, compared to a SLA deep-cycle battery, is insane! You’d really have to need a light-weight 12v battery to consider a LiFePO4 solution over lead-acid.
I will be posting my review of this pack and findings both here on BLF and over at MTBR. I’m just not sure I’ll have a proper conclusion on the cell imbalance issue. The pack has a “sealed” hard outer case, so it’s not so easy to open up to re-balance the cells either.