Manual cell balancing ?

[Hightechpete]

I have a 4 year old 36v 11Ah rack battery which has been neglected for the past year or so. It has only had around 20 charge cycles in it's life. Fully charged (left it on charge overnight to balance cells) shows 36.8 on dischage connections, so I took it apart to take a look.
It has fifty unbranded cells in 5 groups, I'm assuming 10s 5p. Measuring each group shows results between 3.4 and 3.6 volts but the group nearest the BMS is at 4.1v. Do you think a manual balance charge would help, if so how is this achieved?. The BMS looks like a cheap bit of kit and I'm wondering whether it's up to the job.

 

[Deleted member 16246]

I would say that if you can get those cells balanced that battery will come back good and strong. I have no idea how the battery is put together, but if you can get access to each individual cell (top and bottom) and have access to a single cell charger, you could try to charge each low cell individually to get them to 4.1 or 4.2 volts. Connect with croc clips or something of the sort BEING CAREFUL not to short out any cells while connecting and charging. Use a single cell charger (4.2v) and gently charge each cell. It will be inconvenient and time consuming, but it will be a very safe way of bringing the battery up to spec. Once all the cells are at the full charge, you could part discharge, and then connect up your bike charger and check them again.

I just recently recovered a battery that had not been charged in over a year and it was dead to all intents and purposes. The bike, a Pemberton Somerby bought by my partner for £350 has just done 25 miles of tracks and roads set on PAS level 2 of 3 and unloaded, the battery is showing 37 volts. I just took it out and ran it up a steep, mile long hill at PAS level 3, really working the battery and motor, and the battery lights dropped to two of four under load and offload, immediately returned to three. I reckon she will easily get thirty miles out of this 8.5 AHr battery from a full charge. It is well worth trying to balance that battery as long as you take care.

 

[vfr400]

You should drain down the 4.1v one to the average of the others rather than charge all the others up to it. A 12v bulb would probably do it. A 15w indicator bulb would drain at about 0.4A, so it should take about 10 hours, but check the voltage regularly to make sure.

 

[Hightechpete]

Thanks Tony and vfr, I'd thought of charging each pack with a 4.2v charger at 450mA just didn't know if this was acceptable. I'm assuming the 42v charger sees the voltage of the best group and stops charging, so bringing that voltage down will allow all the cells to charge simultaneously?. Either way it's going to take some time, thanks for the advice guys.

 

[Deleted member 16246]

VFR400 has the best solution I'd say. Go with that.

 

[Hightechpete]

I'm trying that now, seemed like the safest and quickest option.

 

[Sturmey]

I'm trying that now, seemed like the safest and quickest option.

I think the safest and quickest option would be to bin the battery. Here why. I would be worried that the 4.1v group has serious problems. When you attempted to charge the pack, the BMS appeared to have done its job by shutting down the charging and not allowing that group to go any higher. Had the BMS failed in this function or you tried charging through the output socket, you would likely have raised that groups voltage beyond 4.35 volts and thats a potential fire hazard.
Just be careful with that 4.1 volt group. If you do balance, you probably need to monitor the individual cell groups as you do a full charge and discharge test afterwards.

 

[Nealh]

As vfr has said the best option is to discharge the high cell group down to that of the others and then carry out a balance charge, leave the battery open so that all 10 groups can be checked for balance when done. Don't leave the battery to charge without being there to keep an eye on it.

 

[vfr400]

See how you get on. You have nothing to lose, but for one cell to be higher than the rest, something is probably wrong, like a broken weld.

 

[Hightechpete]

As you say, nothing to lose. All welds look good and battery will be left open until discharge/charging is complete, I'll check the voltages of the individual groups before deciding how to proceed. If there's any doubt about safety it will be recycled.
Thanks for all the advice, I've learned a lot and will let you know the final outcome.

 

[Hightechpete]

I've discovered an EV recycling centre 20 mins away, their website offers battery diagnosis and repair/remanufacture. No mention of ebike batteries but I'll be happier knowing that they're set up for proper disposal.