BMS/battery trouble

utsi

10 W
Joined
Oct 11, 2013
Messages
69
Location
Norway
My 72V China battery says it's fully charged but stops working after a few minutes usage.

I opened the pack and started measuring the small wires from the BMS connector to the battery. Here are the results:

connector 1-2: 0V
connector 2-3: 0V
connector 1-3: 7.2V


connector 3-4: 4.2V
connector 4-5: 4.2V
connector 5-6: 4.2V
.. and so on, all 4.2V all the way to the 20th connector.

Shouldn't the voltage between connector 1 and 2 and 2 and 3 also have been 4.2V? Connector 2 seems dead, there is no voltage reading from it even when measuring between the main negative output but I could be wrong, maybe it's supposed to be like this.

I have no idea, please help!

BTW: The 2. connector which is 0V goes into the BMS in case that matters.

IMG_0006.jpg
 
Try to follow the number 2 wire back to where it is connected to the battery and measure the voltage from that point to connectors 1 and 3. If there is a bad connection on either end of that wire the BMS will read a low voltage on both 1 and 2 then shut the battery down.
The 7.2 reading indicates that one or both of those cell groups are low so if you find and fix the bad wire or connection you should manually charge the low group. The BMS and charger could eventually bring them back into balance but it would take a long time. It is quicker to bring up the voltage with a single cell charger then reassemble the pack and connect the charger.
 
Thank you very much for your help, sir! You were absolutely right, a wire had fallen off so I soldered it back on. I find poking around in the pack a little scary so I probably wouldn't have done it if it wasn't for your advice :)

Now the 1-2 connector voltage is 3.8V and all the rest is 4.6V. Is the size of the diff so great that I have to manually charge the group or could I just take the bike for a long ride and see what happens the next time I need to charge?

In order to manually charge a single group, is it enough to connect a 4.2V charger to the 1-2 connectors and leave it there until it goes green?
 
My voltmeter battery was out, so the readings were way off.
I have started to charge the low voltage cell group with a 18650 charger for dual cells, 4.2V. Feels like I’m charging a Tesla with an iPhone charger, it will take days😅

Where can I find a good charger for scenarios like this?
 
aleks68 said:
https://aliexpress.ru/popular/imax-b6-b6.html (in such cases, a very convenient thing)

Thanks! As a matter of fact I already ordered and received this exact charger. What confused me was how inaccurate it seems to be - when measuring with a multimeter it's 0.1V off. And after searching the web I found no way to calibrate it, unlike the SkyRC version. Is this as expected? Maybe I just got a bad unit :?
 
if you have cells for 3.7v you need to set the charge to LI ion then it will charge up to almost 4.2v if you need to put LIfe po4 on 3.6
 
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