LiFePO4 or regular lithium ion cell?

[fechter]

I found the reading of the multimeter first showed a reading of 42.5V. But after 0.5 second, the reading changed to 37.2V. This had been confirmed to be the same with another multimeter. Is the charger with problem or it is designed like this? Do I need another charger? (Other voltages had been confirmed to be correct with another multimeter.)

That actually sounds about right for some Li-ion chargers. It would charge until it reaches 42.5v, then when it senses the current drop off, it changes output to 37.2v to stop feeding current into the battery. Anytime the pack voltage is under 37.2v, the charger goes back into the 42.5v mode. When just attaching a meter to the output, there isn't enough current to make it stay at the higher setting, so it drops. I have a some little single cell chargers that work like that.

37.2v sounds a little low, but a partially discharged pack should be somewhere around that.

It would be best to try measuring with the pack connected and see what happens, though this requires a lot of baby sitting.
You can also test by attaching a load to the output. Something like a 120V incandescent light bulb could work, or the right value of resistor. With a big enough light bulb attached, the charger would think the pack still needs juice and stay at the higher setting.

 

[fivari]

Later, I checked the voltages of each cell group with results as follows:
3.95, 3.96, 4.17, 3.91, 3.90, 3.89, 3.90, 3.89, 3.89, 3.89

Then I will say the hvc should be about 4.2V at cell group no.3. But at this moment, the balancing function of the bms is not working properly. Am I correct?

You definitely have Li-ion and no LiFePO4. To me, the third series of cells is fully charged and therefore cuts out the charger (HVC). When the charger is cutt off, the voltages rises to 42V, then after some bleeding, the charger pick in again and the voltage drops to the voltage of the pack (=3.95 + 3.96 + ... + 3.89) until the third string hits HVC again. This cycle repeats until, due to the balancing, all cells are charged to 4.2V.

Try charging the other series manually.There are two possibilities:

• This gives you the full capacity of the battery pack and solves the problem

• charging the others series provides only limited amount of the original capacity and the third series of cells is causing LVC: the thirds string is causing the problems and should be (partially) replaced

 

[mushymelon]

I think fetchers on the right track.
You have some old tech there so you need to determine if your charger is functioning correctly.
If you can open the charger and test the output close to the board while charging under a small load. this may give you a more accurate reading.

I would still bring down that high cell group first. I use a 12v hair dryer but any dumb 12v accessory or incandescent bulb will work.

 

[tmho]

Thanks for everybody's suggestions and comments.
Today I found something that it may mean that the charger is normal. Before whenever I charged the pack with the charger, I plugged the charger to the AC source first and then plugged the charger to the pack. In such a way, if the pack is higher than 37.2V, then the pack could not be charged. Today, I plugged the charger to the pack first, then plugged the charger to the AC source. Then even if the pack was 39.0V, it was still charging by the charger until the pack reached 41.0V. If that means the charger is normal, then I will charge the pack in such a way and everything will be okay except the balancing.

 

[fechter]

I have one charger that's like that. The instructions say to plug the charger into the pack before turning on the AC.

The balancing function on most BMSs is pretty weak. Doing a bunch of shallow discharge/recharge cycles will usually get them back in balance eventually if everything is working.

You may still have a problem with cell 3, draining that one manually with a light bulb might get it back. If that group is bad, I would expect it to have a lower voltage than the rest during discharge, then go higher during charge.

 

[tmho]

May I ask one more question? Does the balancing (via the bms) only work when the pack is connected to the charger? How long will it take for the balancing in my case so that I can keep the charger connected?

 

[fechter]

May I ask one more question? Does the balancing (via the bms) only work when the pack is connected to the charger? How long will it take for the balancing in my case so that I can keep the charger connected?

Yes, it can only balance when it is charging. There is a lot of variation in the way these things work, but most of them can only balance a little at the end of each charge cycle. Once the charge terminates, no further balancing happens. If the cells are significantly out of balance, it will take many cycles. If you charge the pack, then drain a few% of the capacity, then charge again, you can speed up the cycles.

Manually balancing can get a pack balanced a lot faster. Once balanced, it should stay balanced if it is used frequently enough and there are no bad cells. Manual balancing can be done by draining high cells with a resistor or light bulb, or by charging low cells with a single cell charger or power supply.

It is common for a pack to get out of balance if it has been sitting around unused for a long time.

 

[tmho]

Thanks a lot fechter.