BMS cutoff while cells are fully charged

roadrider

100 µW
Joined
Sep 29, 2014
Messages
8
Hi.

A 3P10S 18650 (panasonic) battery pack.
Charged using a Turnigy charger, cell by cell.
All cells took the same amount amperes.
Total voltage of the pack = 40,3 volts
Just replaced the BMS because I thought it was the faulty part.

When plugin in the BMS, voltmeter show between 3,8 and 3,9 volts for each cells while BMS is plugge in. BUT the output of BMS = 0,01 volts. Any idea why ?

I've applied a load to each cell one by one (a 12v automotive bulb), voltage of the pack stayed at 40,3 volts. So I doubt that a cell is weak in go quickly under the 2,7 over-discharge protection voltage when I plug the BMS, which could make the BMS cutoff. Plus the BMS should cutoff the cutoff when each cell comes back over the 3 volts from over-discharge recovery voltage.

I've really no idea...
 
As additional informations, I can say that I've unplugged/replugged the balance plug several times.
That I use a Chinese multimeter, so 10x3.9 volts may equal to 40.3 volts.

Otherwise I'm still hoping someone will answer me because I'm lost, I can imagine why I've no power at the BMS output...
 
Same thing, full battery and 0 volts on BMS output.

I charged every cell using a Turnigy accucell 6 charger, which lets me set the max charging amperage, I set it to 1 ampere. When charging, the charger decreases the amperage gradually to 0,1 ampere. Some cells took 10 hours to charge 3 amperes, those celles stood at 0.1 ampere charging current for several hours.

I used balance plug to charge cells individually, so I know balancing plug and wires were correct.
 
The battery pack did work, for several months. But one day, while riding, it suddenly stopped working.
So I checked voltage of each cell, to be sure they were all balanced I charged them one by one with my charger. BMS still refused to work, so I replaced it.
Note that if I plug in the battery charger, I mean the big one which charges the whole battery pack at once, that means connecting it to the pack via the BMS, it doesn't even begin to charge either, whether with the old or with the new BMS.
 
roadrider said:
The battery pack did work, for several months. But one day, while riding, it suddenly stopped working.
So I checked voltage of each cell, to be sure they were all balanced I charged them one by one with my charger. BMS still refused to work, so I replaced it.
Note that if I plug in the battery charger, I mean the big one which charges the whole battery pack at once, that means connecting it to the pack via the BMS, it doesn't even begin to charge either, whether with the old or with the new BMS.

How are you measuring pack voltage? From the balancing connector or main wires from the pack (do you have a pic)?
 
Either the BMS is bad or one of the wires going to it is broken or mis-connected. Best to try measuring right on the BMS balance wire plug to check the wires.

Also, I've seen BMS boards blow if the wires are connected in the wrong sequence. I suggest always:
Connect the main B- wire fist.
Attach balance wires and try to mate the B- end of the connector first.
Make the B+ connection last. Usually the B+ is also connected to one of the balance wires.
 
B+ is directly connected to the load, it is not connected on the BMS.

I've soldered all the balance wires on the connector. Then I've soldered B- on both the battery pack and the BMS. Only then I've connected the balance connector to the BMS.

I've checked voltage on the balance connector, with COM of the voltmeter connected to the first pin (which is actually B-) and the measured all the pins one by one. I get a constant increase voltage from about 4V each time. So the balance wires are connected in the right sequence.

Did I do the things right ?
 
roadrider said:
B+ is directly connected to the load, it is not connected on the BMS.

I've soldered all the balance wires on the connector. Then I've soldered B- on both the battery pack and the BMS. Only then I've connected the balance connector to the BMS.

I've checked voltage on the balance connector, with COM of the voltmeter connected to the first pin (which is actually B-) and the measured all the pins one by one. I get a constant increase voltage from about 4V each time. So the balance wires are connected in the right sequence.

Did I do the things right ?

Did you measure the pack voltage from the balance wires or main battery conductors?
If the main conductors don't show pack voltage, but the balance connector does, maybe there's a fuse that's blown.
 
The BMS is this one
https://www.aliexpress.com/item/1005003203344199.html

The BMS has a B- (battery negative) and a P- (power negative) terminal.

I measured voltage from balance connector, the side that plugs in the BMS. I also measured main + and B-. All give me correct voltages.

The problem is between + and P-, 0 volts.

BUT only when balance connector is connected, when I unplug it, I get a voltage reading the increases slowly from 0 volt to 36 volts (that takes a bit more than 10 seconds), but as soon as I put a load between + and P-, it falls back to 0. When I remove the load, it increases again slowly to 36~38 volts.
 
Your link comes up with a $5 BMS with $5 shipping from China. It's 5 bucks and they claim it can also balance? Mayve if they stood it on its end.

I buy balance BMS for $20 shipped and still get 10-20% junk, boards that are dead or are soon dead. On the other hand, I've had good result with these guys.

https://www.aliexpress.com/item/2251832734589511.html?spm=a2g0o.order_list.0.0.21ef1802w5nMiQ
 
The voltage behavior is pretty typical when the BMS is tripped. You can get slowly drifting readings but they are not "real" readings. Under load is what counts.

I've seen some boards that need to be manually reset when tripped. Usually do this by charging briefly through the discharge port. You might try that and see if it comes on and has voltage under load. Those are a suckky design and should be avoided.
 
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