Iffy bms? 24v rack mount battery

[Rapido]

Hello friends,
Amateur ebike tinkerer here. Battery pack turns off intermittantly when off the bike. When on the bike it turns off within a second of moving. So I checked and cleaned contacts on the rocker power switch which looked fine so shorted out the switch poles and the intermittancy still present so my step would be to swap the bms for what looks like the correct type, 24V 7S I have seen on ebay.
Is there anything else I could check before investing a whole £7.60 on the replacement bms?
Thanks in advance for any experience in this area.

 

[lnanek]

Looks like you could unplug that white connector to what is likely the balance wires. Then you could check it has a stable voltage reading for each of the p-group's and that they are similar voltage. Many BMS will auto-shutdown if voltage difference between p-group's is too large and can be "fixed" just by manually balancing the battery with something like an old 1S cell phone charger on any low p-group's in turn.

 

[docw009]

An important part of evaluating the health of a battery is determining the cell voltage of each series group, The voltages for working battery will be between 3,0 and 4.2V, and they should be close to each other, If any voltages are below 3V, the BMS is supposed to shut off the battery, If too far below 3V, the BMS prevents charging,

So measure those cells. You can do it by pulling off the ribbon connector and checking there. No one really has made a decent youtube video explaining it. If someone did, they might get a million views and make a few bucks off clicks,

No video here. Just a pic. This guy hasn't got skinny test probes, so pins are used. It's a 10S battery, Every pair of pins, starting with the black pin is a cell. If you do that safely and come back with seven voltages readings, folks here can critique your battery. It's kind of easy to short circuit those pins, so be careful,

 

[d8veh]

When you check the cell voltages, you'll find that they're out of balance. I normally stick my probes in the slots on the side of the multi-pin connector. There's enough room to get most probes in there, which saves a lot of messing about. Often those cheap BMSs take their power from only one cell-group, so if you don't use the battery for a long time or never charge it to 100%, that cell group goes down to give problems like you describe. the solution is to charge up just that one group through the multi-pin connector until they're nicely balanced again. You can use any 5v USB charger to charge them up as long as you monitor progress carefully and frequently.

 

[Rapido]

An important part of evaluating the health of a battery is determining the cell voltage of each series group, The voltages for working battery will be between 3,0 and 4.2V, and they should be close to each other, If any voltages are below 3V, the BMS is supposed to shut off the battery, If too far below 3V, the BMS prevents charging,

So measure those cells. You can do it by pulling off the ribbon connector and checking there. No one really has made a decent youtube video explaining it. If someone did, they might get a million views and make a few bucks off clicks,

No video here. Just a pic. This guy hasn't got skinny test probes, so pins are used. It's a 10S battery, Every pair of pins, starting with the black pin is a cell. If you do that safely and come back with seven voltages readings, folks here can critique your battery. It's kind of easy to short circuit those pins, so be careful,

View attachment 368177

Update on this and seems encouraging. Here are the readings from the 7 cell groups as measured at the connector, the cells voltages range is within 4.04V and 4.06V. From my limited experience I'd say that's pretty great. Is the bms the prime suspect now? Thanks for support here.
4.06
8.10
12.15
16.19
20.25
24.31
28.36

 

[docw009]

Cells look good. Battery shuts off intermittently off the bike. Shuts off immediately on the bike. BMS is looking guilty.

What I would do is see if the cells can still run the bike with the BMS bypassed. As usual, take care with your wires.

 

[d8veh]

Cells look good. Battery shuts off intermittently off the bike. Shuts off immediately on the bike. BMS is looking guilty.

What I would do is see if the cells can still run the bike with the BMS bypassed. As usual, take care with your wires.

Yes, that's a good idea. Run a thickish wire across the BMS from where the negative cell-pack joins to where the negative output black joins. Try the bike out, if everything works properly, the fault is in the BMS. If the fault persists, it isn't the BMS. Don't run permanently like that because you've by-passed all the safety controls. It's OK for testing as long as you don't let the battery run too low, and don't charge it like that.

I've known some batteries, where the cells always measured around 4.0v, but as soon as you draw any current, the voltage collapses, so the BMS might be working as it should. The above test will confirm.