another BMS not BMSing :(

[jimmyhackers]

my dads 250w ebike was cutting out under load.... lots of voltage sag.

its got 18650 tesson cells (2200mah) in a 10s5p arrangement. cell one was at 4.1v and every other cells was from 3.4v-3.9v

it has a tiny bms but its pretty evident its not doing what it should be, or is it?

id imagine a bms to keep all cells level/balanced.... but ive heard some just cut off the main output if one cell discharges too far (no balance charging).

can anyone tell by the bad pics of it what this bms should actually do?

 

[docw009]

Can't tell without detailed pics of the BMS circuitry. Some folks can recognize what a set of 10 balance resistors will look like. I cannot. I always use balance BMS, and when they are balancing, there will be about 50 ma flowing in the balance leads. I use a clamp on ammeter to see that,. Most people don't own clamp on ammeters,. I find them useful. Otherwise, you have to cut the wire an insert an ammeter. Too invasive and messy.

The other way to tell is to let it charge in a safe place for a few days. See if those cell groups start to equalize in voltage. If the low ones don't change much. the BMS probably doesn't balance. Most inexpensive commercial batteries don't balance.

 

[jimmyhackers]

i have a clamp ameter but it says it only does ac havnt tried it on dc.

its the bms cutting off charging once cell one reaches roughly 4.25v. so i guess no balancing. :s

im seperately charging each cell with a my turnigy accucel.
5 cells in, so far so good....no heater cells, cells are holding charge and seems to be a resonable mah going into each cell.

how it got so unbalanced is another matter, it may be that the bms should be balancing but its "borked" and it's making it uneven.

i will have to wait for the other 5 cells to charge and then i can see what happens after a ride....if all the cells are a similar voltage i could assume the cells are ok but the charge portion of my bms is borked.... if they are wildly different voltages though....i can assume the cell/s with the lowest voltage have some issues.

 

[Chalo]

The BMSes we're familiar with are cheap electronic devices with a relatively high failure rate. So the BMS malfunctioning isn't out of the question.

But a lot of this stuff comes down to rider habits. If you run the battery all the way down until cutoff, that pushes cell groups out of balance. To restore balance, the battery must sit on the charger after it's indicated as full, for long enough that the bleed current allows the weakest cell groups to catch up. How long this takes depends on the degree of imbalance, the capacity of the cell groups, and the amount of current that the BMS can bleed from each cell group.

If your dad runs his battery flat often, and if he takes it off the charger as soon he sees the indicator light has turned green, then his habits are likely to progressively exaggerate any imbalances between the cell groups in his pack. I'm not saying this is definitely the problem with your dad's pack, but I think it's at least as likely as a failure of the BMS.

 

[jimmyhackers]

that could be a posibility, im not sure how low he has run it and how often. im not even sure how long he leaves it on charge. i will inquire
this is why i prefer to set up my bikes without a bms, and use a "smart" charger.
slightly more hassle to charge....but i can check/access each cell quite easily.
i can put the battery on a storage charge, which im sure not being able to hasnt helped my fathers battery.
ive seen fancier bms's that have these features and more, some phone app wifi bt thingy, but are a lot of money.

 

[jimmyhackers]

well... 3 cycles in....the thing charges up fine...discharges fine. nothing is out of balance...
my dad says hes never seen the battery light go below 2 bars while hes used it.... he leaves it on charge for a while after use.... usually for a day/24hrs.
chargers rated at 2A (manufacturer of it could be lying)

hes used it maybe 200-250 times (cycles)

i noticed in these three most recent cycles that it usually only takes 5-6 hours before the red light goes green on the charger and the cells are as said before...all balanced and good :S i have left it on for the day/24hrs each time and again...all still in balance.

seems weird it went out of balance to begin with :s

 

[Eastwood]

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seems weird it went out of balance to begin with :s

Yeah, seems weird it went out of balance, I would like to know the answer too.