Cell balancing, someone explain

So i have a bike light
it has 4 18650s
i think it is 2S2P

there are 2 cells in parallel, then another 2 cells
those connect independently to the charge chip
i assume that for balancing the pairs

my question though is, does it matter that the pairs are not balanced independently?
wouldn;t it be better to split all 4 up when charging and charge them all the same?
they don;t balance the parallel pair, what happens if one gets weaker than the other one?

what is the best way to do it, if you only split the cells into 2 halves?

i ask because i have had at least 2 of these packs fail due to reduced run time

if i take them apart invariably i find one bad cell


is imbalance a cause of thermal runaway or is that just with lead acids?

What brand of batteries are you making these packs out of?

Imbalance in series Li-ions can result in a fire/explosion. It’s a fairly small risk in a 2S configuration, but grows with the length of the series.

If it’s easy to remove the cells, I’d charge them individually.
I’ve seen several cases of packs supposedly “protected” by a PCB, get so out of balance that the pack becomes quite compromised. Pull the cells apart, charge and test individually, and the cells are fine, if they haven’t been ruined. Most PCB’s do not balance, they ‘limit’. Each cycle of unequal cells, especially if pushed hard or to their limits can get them more out of balance. You end up working to the weakest cell.

I’m not making the packs, I just buy bike lights.
They are chinese ??? brand (cheap i am sure).
I took a failed one apart to see what was in it - how the 4 cells connect and to what.
The light itself runs on 8.4V.
It’s 2 cells in parallel, in series with another 2 cells in parallel.
The parallel pairs can [i guess] be un-seriesed so the charger chip can balance them.


it isn’t easy =- they are all connected and spot welded with steel connectors…
the things are so cheap [$25 for the battery pack, charger and light, with a free head mount] it doesn’t really pay to do that

like this


So is it possible for cells in parallel to get out of balance?

What is the worst out of balance problem, series or parallel connections?


Parallel cells are always balanced
Just series need to be balanced

Parallel is never a problem.

These bicycle light packs usual has protection in them, i.e. they will never get dangerous, you will just loose capacity while they go out of balance.

When they have lost to much capacity (Maybe once a year, but it depends on the quality of the batteries and how much you use them), take them out and use a hobby charger to balance them. It can balance cells in series.

That’s the problem. These cheap cells are so inconsistent of each other one cell has to help out the other until they get tired of helping.
I have built packs with cheap cells and the inconsistency between cells kills the pack within a few weeks of use. Its better to buy quality cells if their going to be used in a pack. Same reason laptops don’t use cheap china cells. It would eat up their profits with all the returns and refunds.
The better matched the cells the better the pack will perform and have the greatest usable life. The RC guys have known this for years.
Either keep buying what you have knowing they will be short lived or build your own with quality cells. Its not that hard to do but the quality cells in quantity gets costly. The protection circuits can be purchased fairly cheap.
Just my 2 cents. :wink: