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Time for a top balance?

Aphers

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Nov 17, 2020
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Been using my 4S 271Ah system for about two months now, it's installed on the boat that I live on. I have a JBD BMS.
Recently I've noticed that when drawing a next load the balance is no longer good- up to 0.2v. I cleaned up some contacts which helped a bit but still not happy.
I was watching the system when it reached full charge today, cell 3 was a bit below the others so I think that's where the problem lies. The JBD doesn't seem to be able to do any balancing, possibly I've got the HVD set too low to allow that?
I guess I should take the pack out of use and do a top balance. Only problem is I don't currently have a PSU, I managed to fry it when I was charging another battery and mixed up the polarity ?‍♂️

Other than generating heat, will I be damaging the peck continuing to use it like this? It seems happy enough at low discharge rates, and returns to 0.01v balance fairly quickly even after going out of balance under load.
 
I would not take it apart yet. If you want to do something to reduce the delta, fully charge till the charger cuts out then put a resistor across cell 3 till the voltage is in line with the others.

Use a high wattage resistor. (25W or more) I like to use aluminum case power resistors.

A 3 ohm resister will drain at ~3.5/3=1.17A or ~4Watts. This will get a 25W resister warm maybe even hot over time.
A 1 ohm resister will drain at 3.5/1 = 3.5A or ~12.3W. This will get a 25W resister very hot.

Other people use an incandescent lightbulb as the resistor.... but I have never tried that.
 
cell 3 was a bit below the others
Cell 3 needs a bit more charge not less.

However you could pull the others down with a load.
If you have not got power resistors to hand, filament bulbs are great, I use a 50 watt headlamp bulbs, the fact that it lights up is rewarding! Other things I have used, 12v soldering iron, 12v bilge pump motor, to load the cell.
Apply the load to the high cell for a few seconds whilst watching the BMS display of cell volts. Repeat as necessary to the 'high' cell of the group in turn, ideally carry out the process whilst under charge at the point where cells start to diverge.
to do any balancing, possibly I've got the HVD set too low
Most BMS default to 3.4v per cell for balance to start ( 13.6 for a 4 cell pack). If this is the case and charging voltage is higher, then cell balancing should take place. (assuming its enabled in the BMS parameters and suitable values are set).

Mike
 
Oops. I misread.

You could also put a power supply set to the target voltage across the low cell.
Once I've replaced my PSU I could try that.
Newbie question but is it ok to do that with the pack still connected up?
 
Once I've replaced my PSU I could try that.
Newbie question but is it ok to do that with the pack still connected up?
You can with care use the existing 12 charger and the magic 12v bulb. Connect in series with the 12v charger and apply across the low cell. Apply the power to the cell making sure the polarity is correct, apply for several seconds at a time whilst watching cell volts.
All the 'pulling down' or 'pushing up' can be done with the cells connected as a pack.

Mike
 
Update: I used a 12v bulb to drag down cells 1, 2, and 4 whilst the pack was at full charge, and got everything within about 0.01v. So far, so good.
But putting on a 60A load, I've still got cell 3 about 0.1v below the others.
Does this point to a bad connection... or possibly a not so good cell?

Also on a side note, I noticed that the BMS cut off charging, although I didn't catch whether it was cell or pack voltage which tripped it. Should I be changing my MPPT settings to ensure that the BMS doesn't have to be doing this?
 
Does this point to a bad connection... or possibly a not so good cell?
It could be either...or it could be that they are still a bit out of balance. Since the BMS but off charging, there was probably a cell that had the voltage run away. Try finding the run-away and drain it down a bit.
 
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