Me as my BMS

[MurphyGuy]

My 14s (480ah) lithium ion battery bank sits in storage most of the time doing nothing.. I store it isolated from everything.

I have one cell out of the 14 that has a higher self discharge rate then the others. Most of the cells have almost no self-discharge at only 0.001 to 0.002 volts per month, but one cell seems to lose about 0.008 to 0.010 volts per month of inactivity.

Since my battery bank is for grid outages, we can go several months without using the system.. All the other cells are within 5mV.. but my #12 cell was 60mV below the rest of the pack.

I already used a hobby charger to bring it up in line with the rest as the battery bank was isolated.. and monitoring it shows that it is losing about 1mV (0.001) every 3 days. In the past month, it has lost 10mV.

Anyone ever use a little hobby charger to charge a single cell while a system is running? I don't want to force the BMS to bleed off the other 13 cells just to equalize things.. much easier to just add some energy to that one cell..

Wondering if the BMS would even notice if the system was operating.. anyone?

 

[A.Justice]

I did it once, before I found this forum and learned to properly top balance my cells. I kept getting cell over voltage warnings during charging (because it wasn't balanced) and I used a 3.65v charger to top up each cell while it was in series. It worked just fine, and I don't see any reason it would be a problem.

 

[MurphyGuy]

I did it once, before I found this forum and learned to properly top balance my cells. I kept getting cell over voltage warnings during charging (because it wasn't balanced) and I used a 3.65v charger to top up each cell while it was in series. It worked just fine, and I don't see any reason it would be a problem.

My problem is not that the cells weren't top balanced, its that I have one cell that has a higher rate of self discharge than all the rest. I could top balance them to within 1/10,000 of a volt and that one cell would eventually bleed down over the course of a few months.

We don't use our system very often, most times I have it sitting at around 3.6 volts per cell.. only when a storm knocks us off the grid do I actually use it. In that case, during use, the BMS will keep it balanced.

 

[A.Justice]

My problem is not that the cells weren't top balanced, its that I have one cell that has a higher rate of self discharge than all the rest. I could top balance them to within 1/10,000 of a volt and that one cell would eventually bleed down over the course of a few months.

We don't use our system very often, most times I have it sitting at around 3.6 volts per cell.. only when a storm knocks us off the grid do I actually use it. In that case, during use, the BMS will keep it balanced.

I wasn't implying that your cells weren't top balanced, I was just giving you the reason why I did it. I figured if I could do it for that reason, you could definitely top it off as well, with no problems.

If you really wanted to get technical, you could disassemble your pack and let the cells rest for a few weeks, and then capacity test them individually, or really just the faster discharging one, against a regular one. Sometimes a slightly unequal voltage won't actually mean much of a difference in capacity, the cell could just settle a little bit lower, and may actually contain the same amount of power.

Also, maybe in your case an active balancer would be beneficial. Don't use it all the time, but plug it in once a week or so. I learned to use a resistor with mine, because the inrush current is crazy. I cut the black negative wire, and added a spade terminal that could be disconnected and have a resistor touched to each end to charge the capacitor bank in the active balancer and keep it from blowing up the capacitors.

 

[MurphyGuy]

I wasn't implying that your cells weren't top balanced, I was just giving you the reason why I did it. I figured if I could do it for that reason, you could definitely top it off as well, with no problems.

If you really wanted to get technical, you could disassemble your pack and let the cells rest for a few weeks, and then capacity test them individually, or really just the faster discharging one, against a regular one. Sometimes a slightly unequal voltage won't actually mean much of a difference in capacity, the cell could just settle a little bit lower, and may actually contain the same amount of power.

Also, maybe in your case an active balancer would be beneficial. Don't use it all the time, but plug it in once a week or so. I learned to use a resistor with mine, because the inrush current is crazy. I cut the black negative wire, and added a spade terminal that could be disconnected and have a resistor touched to each end to charge the capacitor bank in the active balancer and keep it from blowing up the capacitors.

I have a couple extra cells I ordered just for this potential issue.. I just charged them both and then monitored them for 10 days and I think they're good.

Next I will pull apart that self-discharging cell-bank and split it apart to see which one is dropping and I'll replace it.

My cells stay well balanced, this was an oddball, and didn't show its ugly until my 4th cycling and a charge up into the 4v range.. It had previously been charged to 3.8xx something and it didn't misbehave.

Thanks for your input.