Balanced charger using own power…

Hi

This may be a stupid question but here goes…

I have a 4s LFP battery pack. 270ah of 18650 cells. I have a BMS that only balances at 35ma when the pack voltages reach 4.18 which doesn’t happen as I’m not charging then to 100%.

I top balanced the packs about a month ago and checking them this evening they are now

4.04
4.07
3.80
4.08

There aren’t any bad cells as far as I can tell - no excess heat anywhere. The pack is working fine.

obviously I want to get the packs balanced.

I installed balance leads for my charger and was wondering this…

if I connect my balance charger to the pack and plug it in to the inverter which is also connected to the pack will it be ok? Will if not blow a fuse or blow up!! ?

I was thinking of something like a feedback loop but in electrical terms.. not good!!

my thoughts were I could connect it over night to keep the packs balanced before the MPPT charge controller kicked in.

Any advice welcome!

Cheers
Mat

Be careful hooking up a balance charger to a pack that has a BMS -- I messed up the readout on my Q8max by doing it --unplug the leads out of the BMS---looks like you have one row in you pack with less capacity to be that much lower

an isdt Q8 max will balance your pack and as chops mentioned above you should disable your BMS balance function.
actually, you should always disable your bms balance function, they do not do much of anything. Disable the BMS balance and pop on an active balancer...for $30-$40 all cells will be within a few millivolts of each other within a day!!

Thanks for the replies… I’ll take the BMS out of the circuit when I balance charge…

is it safe to balance charge the battery when the power source for balance charging is the actual battery? Is it safe to do so via the inverter?

mathewhgreen said:

Thanks for the replies… I’ll take the BMS out of the circuit when I balance charge…

is it safe to balance charge the battery when the power source for balance charging is the actual battery? Is it safe to do so via the inverter?

Click to expand...

well, you could but,,,
If you just get an active balancer and plug it into your balance leads it will sort it all out in time (unlike the balance circutry in the BMS).
I actually make a "Y" cable so that I have two connector on each balance lead which lets me leave the BMS attach and plug(or unplug) my active balancer whenever I want.
FYI on active balancers, some are not good at all and only kick in when your battery is wayyyyy out of balance (100-200mv); and then they stop balancing as soon as the cell diff is less than soemthing aroudn 50mv...

I plug in my capacitive active balancers and within a day cell differential is less than 5mv.

Have you looked into changing the voltage at which the BMS starts balancing? That should kick in the balancing sooner.

Unfortunately I can’t change that and it’s set as 4.18 per cell!

Something is not matching. LFP is 3.65V max, and 4.18/4.2 is lithium ion chemistery. I would be very cautious when doing such a loop, it may happen the inverter is not isolated such as transformerless solar inverter.

Regarding the unbalance of your pack, there is definitely a problem with the cells at 3.80V. It is very low compared to the 3 others, and it is an unbalance that can be corrected with a basic BMS.
If you charge the pack with solar power, how this cell is behaving? Does voltage increase quickly?

I add the exact same problem with an ebike pack of a friend. Less power, but same principle 13S 5P: 1 cell was dead and was unbalancing the pack. No excessive heat neither...

Definitely, I would investigate this cells to find the culprit...

ericgrand said:

Something is not matching. LFP is 3.65V max, and 4.18/4.2 is lithium ion chemistery. I would be very cautious when doing such a loop, it may happen the inverter is not isolated such as transformerless solar inverter.

Regarding the unbalance of your pack, there is definitely a problem with the cells at 3.80V. It is very low compared to the 3 others, and it is an unbalance that can be corrected with a basic BMS.
If you charge the pack with solar power, how this cell is behaving? Does voltage increase quickly?

I add the exact same problem with an ebike pack of a friend. Less power, but same principle 13S 5P: 1 cell was dead and was unbalancing the pack. No excessive heat neither...

Definitely, I would investigate this cells to find the culprit...

Click to expand...

yes it’s Lithium Ion and the inverter is a Victron inverter so should be fine!?
One of the other packs went out of balance now so I’m beginning to think perhaps it’s a BMS issue?
Will continue to investigate!

Put this battery in safe zone, it's can fire fast if damaged cell is paralleled.
You can try to charge only the lowest cell with an AC 18650 charger (normaly there are isolated and no ground loop) and see