bad charger or bad bms?

Hi all, I've had this 72v battery for a couple of years now and it goes great
https://em3ev.com/shop/em3ev-72v-20s10p-rectangle-battery-pack-24-5ah-29-5ah/

However about a year ago it stopped balance charging and just continues past the 84v fully charged point. I've tested the charger with a multimeter and it says its doing 83.5v which seems normal? It's just a chinese charger.

Is it the bms that determines when to start balancing and tell the charger to stop? should I look into a better charger first?

The battery and bike still go great but I just don't full charge anymore since I have to babysit it and don't know if anything is balanced now.

Cheers

It depends , there are high-end BMS designed to control chargers

But for cheap stuff, the BMS should only be used as redundancy, a backup for when the primary charger control fails

thus its HVC setpoint a bit higher than the pack should be charged.

Never trust the accuracy of either, test / calibrate using a known good DMM measuring at the posts.

That sort of balancing happens while the last stage of charging continues, when the latter stops so does the former.

Ideally the Start Balance threshold is lots lower than the termination point of the charger to give the process enough time to finish.

Ideally both devices let these setpoints be custom adjusted by the user

who is able to watch the cell-level voltages.

Probally the temp swing in the seasons changing. THey are adjustable, most, both voltage, current, and even some termination current.

Never use the BMS to stop the charge unless it controls the charger 100% ( disconnects from wall automatically). Always have the charger set to teh correct final voltage and termination current.

My understanding of the BMS and charger operation.....I'll take any corrections where I'm wrong.

The BMS charge circuit sits between the ground on the charger port and B- of the cell array. It uses pass transistors, but just think of it as a solid state switch. On a simple nonbalance BMS, the switch opens as soon as any series group hits 4.2 volts. CHarging stops.

On a balance BMS, the switch only closes when all 13 series groups hits 4,2 volts, In the meantime, the balance circuits are bleeding down the series groups already at 4.2 volts so they do not overcharge while the other groups catch up, Charging stops when balanced.

The charger's job is to stay below the 13S max of 4.2V. In your case, that's a 20S which 84 volts. As it gets nears this voltage, it automatically puts out less current. Prior to that, it supplies a constant current at its rated level, using whatever voltage is needed to support it. Meanwhile, the cells have a natural tendency to accept less current as the voltage approaches 4,2V, so it's an easy control process. Even if the charger glitches, the BMS should stop charging when everything balances.

The above allows me, for example, to use my latest 13S charger which I checked out as 45.11 volts. The BMS shuts off the charge at 54.6V. Now if I did something stupid, like hook the charger up to the discharge port, there is nothing the BMS can do to stop overcharge.I've done that in the past thru carelessness. The cells got hot and popped their safety vents, being good Panasonic GA cells. Whew,