ping 36V20Ah problem

[tef]

hi ebiker,

I have bought two ping 36V 20Ah packs last year and use them for my bmc geared hub @36V.
After a few rides, I realized that one pack had a slightly higher voltage drop under load. I did not care about this because I have never had a bms low voltage disconnect, even a 40km ride (with pedalling) was no problem.

During winter time I stored both packs at 39V.
Yesterday I fully charged them for next week and measured the voltages.
One was charged to 44,5V, the other to 40.5. Because of this big difference I removed the bms cover and checked the cell voltages.

here my measurements:

3.766
3.329
3.342
3.328
3.339
3.329
3.335
3.337
3.344
3.344
3.791

When I look at those values, I guess that I have a serious problem. The pack is fully charged which means that all cells should be at about 3.7V, right?

What do you think could be the problem, maybe the bms?
How could I check this?

I have tried both 3A ping chargers, no change.

thanks,
tef

 

[dnmun]

no, 3.37V seems ok for the average, 3.7V avg seems high. i don't know why the 2 end cells are different, but since you have 2 packs, why not swap the BMS between them and see if the problem replicates, but you should not make an initial assumption until you discharge the cells and can measure the capacity they still have. all jmho

 

[slayer]

if this is just after the being stored for winter and never discharged yet i would not make a judgement cause it could be that they self discharged just a little bit over the winter and when you put the charger it wont reach to its full 3 stage because it is still in float mode because the voltage is high enough ...

 

[Aerowhatt]

It sounds like you were measuring the voltages right after disconnecting the charger, is that correct?

If so then the voltages are low. I've got a similar ping on the bench which hasn't been topped up in a few weeks and it measures 40.4 Volts.

Since you have the BMS uncovered one simple check that you can do is to reconnect the charger and monitor the temperature of the resistors. The two on the end cells (high ones) should heat up pretty quickly. The others should warm up later. When they are all near the same temp then the cells are balanced (if the BMS is working properly) Yours is pretty far out of balance by my standards. It would take a good long time for the charger and BMS to bring all those lower cells up.

If you don't get the resistors for the two high cells heating up pretty quickly then the BMS isn't working properly and you want to disconnect the charger to avoid overcharging those two cells.

Aerowhatt

 

[dnmun]

that is a good idea, just monitoring the resistor for heat, so the BMS is shutting down the charge cycle because the 2 cells are high individually? before the others can get to a higher voltage?

you had only measured one pack, did the other pack have 2 high on the end?

 

[dogman dan]

Mabye try discharging just the end cells a bit, and recharging with the ping charger? Have they been run at all? Maybe if you just go for a few miles riding, and then recharge it may sort out? You are definitely out of balance, but it does sound a bit similar to when the pack first arrives and balances after a few discharge cycles to me. I wouldn't panic till the symptoms persist. Another possiblility, a difference in two chargers?

 

[tef]

I have measured the voltages directly after charging.
After I plug the charger in again, all resistors have the same temperature (not even warm).
I discharged the pack about 10-15 times 70%, maybe I should try discharging till the bms shuts down and then recharge the pack.
I have tried both chargers so this is not the problem.

Is it possible to discharge the two high cells without opening the pack? If yes, how should I connect the sensor wires to the load?

thank you all for your help,
tef

 

[dnmun]

maybe it will stabilize after you go through several charge cycles, you know now to watch for some cells being too low, and it may just be a characteristic of the lifepo4 after it sits for months.

 

[Aerowhatt]

I have measured the voltages directly after charging.
After I plug the charger in again, all resistors have the same temperature (not even warm).
I discharged the pack about 10-15 times 70%, maybe I should try discharging till the bms shuts down and then recharge the pack.
I have tried both chargers so this is not the problem.

Is it possible to discharge the two high cells without opening the pack? If yes, how should I connect the sensor wires to the load?

thank you all for your help,
tef

It sounds to me like the BMS is kicking the charger out at too low of a voltage for the balancing circuits to be sending power to the resistors to be burned off on the high cells. Those two resistors on the high cells should be getting warm in just a few minutes after plugging the charger back in.

It would be possible to pull some off the two high cells with out taking the battery apart. I wouldn't recommend it though you can just cause more trouble with a slip.

If it were me I would do the following. First I would check the voltage at which the charger changes mode when charging the other battery. Then do the same for this problem battery. Check the open circuit voltage on the charger if it registers one. All of these should be above 45 volts. I had one charger from ping V1 that wasn't putting out enough voltage to initiate the BMS balancing. It's possible (however unlikely) that you have two of those. Then it would be that neither pack is getting balanced and one has just stayed balanced better than the other. I wouldn't trust a possibly defective BMS with a discharge to cut off. Plus it won't help the balance situation anyway. The lowest cell would trip the BMS to shut off power (if it's working properly) leaving the higher cells just as far ahead of the lowest cell as they are now. If the charger voltages check out at 45.6 or above then you need a new BMS board. Ping has them and they are reasonable if not free depending on how long you have had the battery. IMO it's best not to monkey around more than necessary to find the problem. If it is the BMS and you balance the cells manually you still need a new BMS so why take the risks involved in trying to balance it out manually?

Aerowhatt

 

[dnmun]

several people have torn sense wires off tabs and torn tabs off too when opening their ping, so don't go there for sure, swapping charger like aerowhatt recommended and swapping BMS too might tell you something, does the ping charger use CC-CV type charge cycle?

 

[tef]

Check the open circuit voltage on the charger if it registers one. All of these should be above 45 volts

Both charger have an open circuit voltage of 45.9V.

I have connected the charger again and one resistor (the first) got a bit warm, but it was hard to feel any difference in temperature.
The charger swiches from green to red every ~3 minutes for a few seconds.

I think that I'll give the other bms a try.

does the ping charger use CC-CV type charge cycle?

Sorry, I can only tell you the tech. info written down on the charger housing:

Li-Ion Charger
Model: FH - 42030
Output: 46.2V 3A
LPS

thanks,
tef

 

[Aerowhatt]

It sounds like the BMS and charger are both doing what they are supposed to. As you can imagine with the charger only connected to the cells by the BMS a few seconds every three minutes the effective charge rate on those low cells is in the neighborhood of .15 amps instead of 3 amps. It will take significant time to bring them up. In the meantime keep checking on those resistors they should warm up pretty good and there should be more and more warming up over time. You are probably looking at at least 48 hours of this cycling to get them balanced up closer.

Aerowhatt

 

[tef]

The charger was plugged in up to now. No change regarding the two ~3.8V cells but the others got ballanced perfectly.

Today I tried to discharge the two high cells to 3.34V, but it was nearly impossible because of this scary voltage sag.
Even with only 100mA discharging current the voltage falls from 3.79V to 3.4V within minutes.

thanks,
tef

 

[Aerowhatt]

The charger was plugged in up to now. No change regarding the two ~3.8V cells but the others got ballanced perfectly.

Today I tried to discharge the two high cells to 3.34V, but it was nearly impossible because of this scary voltage sag.
Even with only 100mA discharging current the voltage falls from 3.79V to 3.4V within minutes.

thanks,
tef

That's normal. The higher voltage is mostly surface charge so it has no depth to it. Probably 30 to 60 minutes at the 100ma discharge will bring them down pretty close to the others. You will know how close at the end of the following charge cycle of the whole battery. The other thing you can do is to put a small load on the whole battery and measure the individual cell voltages to see how close you are at any given time during the process.

Aerowhatt