Help me troubleshoot this pack please. How are these voltages?

E

eikido

10 W
Joined
May 8, 2019
Messages
94
I have a battery pack (13s) that dies at around 45-46v (~40% capacity left) when loaded. I'm convinced (but not 100%) that I have one or more bad cells or unbalanced cells. Do you guys think I can solve this by charging it a very long time to balance any unbalanced cells?
 
To balance effectively you need to have access to the balance leads to see the cell voltages.
 
Ok, thanks. Here goes. See the list below with all cell voltages. Does anything look bad? I'm reacting to cell 11. Could that cause this issue? Because if they look good, then it's most likely a bad BMS? The battery pack has not been used for 24h so i guess the voltages has settled.

1: 3.488

2: 3.479

3: 3.478

4: 3.470

5: 3.481

6: 3.480

7: 3.484

8: 3.480

9: 3.487

10: 3.475

11: 3.449

12: 3.480

13: 3.485
 
You could get the top balance perfect

but if a cell (group) is weak it will still cut out early, range is limited by the weakest link.

The ideal is to capacity load test each cell.

Otherwise, do get them as well balanced as possible at the top, then discharge using a coulometer to do say 20% DoD, note the voltages, then another 20%, etc.

Then of course it will cut out, see if one cell/group is way out from the others.
 
What I would do:
1. Fully charge the pack
2. Check that all cell groups are fully charged/balanced. If you can't get them charged/balanced by the internal BMS, then I would individually charge the low cell group with an RC charger through the balance wires.
3. Go and give it a thrashing
4. Recheck cell group voltages
5. If group 11 is low again, then it probably has a problem

eikido said:
Do you guys think I can solve this by charging it a very long time to balance any unbalanced cells?
Click to expand...
BMSs are usually pretty slow at balancing, so it might take a long time. Like I said above, if it doesn't balance in a reasonable time (I would leave it on charge overnight - it'll also depend on your charger. If your charger cuts off at full charge, then the BMS might not even attempt to balance), use a hobby charger to bump up the low group(s). Once you have a fully charged and balanced pack, you can load test it and see what happens to the balance. In my experience, in a healthy pack the cell groups will stay in balance, and really shouldn't need balancing very often, if at all.
 
Thanks for your suggestions all. I will do my best trying them.
I don't have access to a Coulometer so i won't be able to do that. But i'm looking for something to discharge my battery pack but i'm discussing this in another thread.

I do know that the optimal way to test this is to load each cell but i don't think it will be possible for me to do. Because a cell might look good when measured unloaded but as soon as you load it, it might act strangely.

I will start by balancing all cells with my charger first and if it doesn't work, use a balancer charger.

The difference between the max and min cell charges here are 3,488-3,449=0,039v. Is that a lot?
 
What loads are we talking about, and what battery do you have?
Individual cell voltage is looking normal to me.
A smaller 13s pack at 45v could easily sag enough during high load that the controller shuts dow due to low voltage protection (often around 39v). Are you able to read the voltage right when it shut down?
 
Jan-Erik-86 said:
What loads are we talking about, and what battery do you have?
Individual cell voltage is looking normal to me.
A smaller 13s pack at 45v could easily sag enough during high load that the controller shuts dow due to low voltage protection (often around 39v). Are you able to read the voltage right when it shut down?
Click to expand...

I have 18650 rated 2.6Ah with 6 in parallel (13s6p).
I can adjust the torque on my controller and i set it to the lowest torque and it still doesn't work. It shouldn't happen for sure.
The controller LCD is way to slow to update me on voltage but i will connect a multimeter to the charging port and monitor it while riding.

How common is it that a single cell measures fine when unloaded and behaves bad when loaded?
Especially after some usage, i measure it and it measures the same voltage as the other cells but is still bad.
 
Voltage reveals next to nothing about SoH% condition of the battery.

It is **very** common for a batt to appear fine "hold a charge" but have near zero capacity.

Again, you need to CC load test the cell, can be crude just a $20 lamp kit and a stopwatch.

Or a coulometer, under $20 also
 
One cell group (or several groups) may have a diminished capacity. When the pack voltage gets down to the drop-off of the weakest cell group(s), then the whole pack will sag on hard acceleration.

This is a common occurance in the spring, when ebikes have been stored at 4.2V per cell over the winter. I recommend long-term storage at 3.3V-3.7V, and to proactively avoid storage at anything over 4.0V per cell. For 13S, that is 52.0V max.
 
john61ct said:
Voltage reveals next to nothing about SoH% condition of the battery.

It is **very** common for a batt to appear fine "hold a charge" but have near zero capacity.

Again, you need to CC load test the cell, can be crude just a $20 lamp kit and a stopwatch.

Or a coulometer, under $20 also
Click to expand...

I suspected this. Do i need to load each cell or is it enough if i did a whole s group?
 
You can put a load on the whole battery with a hair dryer an see which one drops all going to be at 1
that drops voltagee and you might have one string of the 13 drop meaning it has no capacity.. Yes check under load. if someone has a better way please chime in. Please chime in.
 
Just deplete the pack and check the balance. If it's a balance issue causing early cutoff you will see it. Your cells were balanced ok in the previous post. Did you happen to know the actual cell used, or at least the name of the pack?
 
2600mah cells of which make, for instance if they are say Sam's they will be a crappy variant of the 26F. A very low current rated cell.
 
eikido said:
Do i need to load each cell or is it enough if i did a whole s group?
Click to expand...

You could get the top balance perfect

but if a cell (group) is weak it will still cut out early, range is limited by the weakest link.

The ideal is to capacity load test each cell.

Otherwise, do get them as well balanced as possible at the top, then discharge using a coulometer to do say 20% DoD, note the voltages, then another 20%, etc.

Then of course it will cut out, see if one cell/group is way out from the others.
Click to expand...


 
The charger is fine for your setup. Especially at this time. Please charge the pack start from there they sit on the charger for a while check the voltage of each parallel group and then use it I put a load on it and check each group sales in the one that just dives dive down and run you having problems with it
Maybe or it may be something else. ?
 
I just found out that my BMS does not have a balancing function.

Back to squre 1.

Here is my plan.
Change bms to a balancing one.
Balance the cells

If it doesn't help, balance manually.
If it doesn't help, disassemble the whole pack and find the bad cell.
 
will get it remember a lot of different people on here I've never met each other and we're just guessing and trying to be the most righteous of right of right
Lol
 
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