Is my BMS defective?

I have a hailong style battery with a hp16sa BMS.
Few weeks ago it shut down and would not power my TSDZ2. I finally sat down and opened it up.
Voltage on battery output is 45v which seems okay but i assume its below the TSDZ2 cut-off.

But the problem is battery wont charge.
I have measured the cell groups and most a 3,6 except two which are at 3,5. I assume .1v difference is okay? Or could this be the problem causing the BMS to lock?

Another weird thing is; when i measure from the battery pack directly, the voltage is 48v. But if i measure from the output on the BMS its only 45v.

Is there a way i can check if the BMS charges okay? Or is the above symptons signs its just dead?

The battery output is less than the total internal cell voltage, so I would say the BMS has turned the battery off. Sometimes you get leakage on the output when a battery is turned off, and that could explain the 45V.

In my opinion, a quality BMS doesn't leak. One might see residual voltage on the output when it's off, but it should go away quickly with any load, Output quickly drops to zero. Hailongs have an on/off switch. What do you measure if you turn the battery off and put it on the bike?

If you have checked the individual cell voltages, and none are below the LVC of the BMS, then there is no obvious reason for the BMS being off, and you could assume it's defective. LVC is usually around 3V for BMS shutoff.

Sounds like a bunk BMS to me. If that 45V slowly drops as you measure it with a voltmeter, you can be sure the BMS has cut off.

I would replace it and see if that doesn't bring your pack back to life. If you have to wait a while to receive a new BMS or for any other reason, unplug the multi-pin connector in the meantime.

I have a Hailong style battery, which wouldnt charge.
After initial testing i replaced the BMS, which appeared to fix the problem.
But at first test ride, battery would cut whenever i used PAS. It could power the display only just fine.

I then measured and found all groups to be at 3,5 and one group at 3,2. So i assume thats part of the problem.
But shouldnt the BMS still be able to charge the rest of the groups, up to 4,2V? Or can it simply be that it detects an error on one group, and therefore wont charge anything?

Usually if one cell group is off or out of range. Let's say one groups is at 3,3v and the rest at 4,2v. It will not permit charge or discharge depending on the bms layout or functions. (Putting it in the charger won't equalize this. Maybe it will charge for sometime and then you'd get a 1km out of it before it cuts the power because charge cuts out if one cell group reaches 4,2v)

If the bms permits charge nor discharge:
The battery will very soon cut the power because the cell group at 3.3v goes below 3v, lowest possible voltage for a group and so in a sense the bms does its work. Because you shouldn't drain your battery more that 100%, should you?

If the bms doesn't permit charge nor discharge
It won't charge
It won't discharge

Solution
Fix(replace) the cell group or groups or remove all and put new ones in,
check the connections (as one lose connection to the balancing wires will act as zero voltage and deny charge)
or broken bms

leffex said:

Usually if one cell group is off or out of range.......

Click to expand...

Great. That was how i assumed it too. Just wanted to double check before i start dismounting the whole thing.
Time to buy a spot welder now. :wink:

Thanks for a good answer :thumb:

sjuft said:

I then measured and found all groups to be at 3,5 and one group at 3,2. So i assume thats part of the problem.
But shouldnt the BMS still be able to charge the rest of the groups, up to 4,2V? Or can it simply be that it detects an error on one group, and therefore wont charge anything?

Click to expand...

Yes, the BMS will allow that battery to charge. It only blocks charging when a cell is too low. The exact value will be in the spec of the BMS you just bought, but it is typically 2.50 - 2.60 volt.

The reason your battery shut off during PAS is that voltage sag cause that 3.2V group to drop below the low voltage cutoff (LVC). The LVC is also in your BMS spec, but is usually around 3.0V. If you do a partial charge and get that group up around 3,6V, you'll be high enough that voltage sag won't shut down the pack. Meanwhile, the other cells will likely be at 3.8V, and you'll have different group voltages. BMS don't care about that during discharge. It only looks for any group going under LVC.