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Old 24-06-2024, 10:36   #1
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Overkill BMS

Hi all,

I bought 3 overkill bms's when they 1st came out. Customer service is non-existent

I have 3, Eve 280 ah batteries (cells bought from reputable seller, brand new cells). Each battery is independent and has it's own BMS, fuse, on/off switch.

I recently connected my solar panels and installed a victron multiplus. Victron solar controllers.

The issue I'm having with the BMS's is that the FET' trip at the end of the charge. Cell overvolt. The Victron is set to charge to 13.8v. Screenshots of my BMS

Appreciate any help you can provide. Hope the attachments go through
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Old 24-06-2024, 10:56   #2
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Re: Overkill BMS

Cell overvoltage is set to 3.5V which is somewhat low relative to the target of 3.45V. What is happening is one cell is hitting 3.5V before the pack voltage hits 13.8V (3.45V per cell) that isn't that unusual cell voltages aren't going to be identical.

One option would be to bump up the cell overvoltage to 3.55V on the second to last screenshot. The other option would be to keep the 3.5V overvoltage but lower the absorb charging voltage slightly but that would mean giving up a small amount of capacity. You may also wish to slightly lower the balancing voltage to give the BMS more time to lower the highest cell voltage cell.

Keep in mind that while at rest LFP 100% SoC is 3.40V it will build up a surface charge as high as 3.6V while charging depending on charging voltage. This surface voltage will eventually dissipate even if there are no loads because it is phantom (100% SoC at rest after standing is 3.4V) but it can still trigger the cell overvoltage.
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Old 24-06-2024, 12:22   #3
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Re: Overkill BMS

The cells are pretty far out of balance, and so one cell is hitting the OV limit before the battery is fully charged. As Statistical pointed out, 3.5 volts is a pretty low setpoint. I would set the OV limit no lower than 3.65Vpc. Set the release at 3.55V. The battery is fully charged at 14.6V, and so anything lower than 3.65Vpc would not allow the battery to fully charge. Going up to 3.7 is even acceptable, because remember this is not a voltage you charge too, it is the "oh ****, something is wrong and I need to stop" voltage. And charging to 3.65V is the normal full charged voltage, not the oh **** voltage.

So, step one, set the cell OV to 3.65V.

Second, the switch to balance only when charging. Turn that off. That is contributing why your cells are out of balance. Balancing doesn't start until 3.45Vpc (because that is what it is set for in your screenshot, and is the correct setting) But then the battery promptly stops charging due to the OV cell, and thus stops balancing.

Also contributing to your cells being out of balance. Balancing doesn't start until 3.45Vpc. And your Victron charger shuts off at 13.8V. So, just when your battery starts to balance, the charger shuts off. I would set the Victron to 14V. Many people recommend charging to 13.8V, and that is a good recommendation. But you need to charge higher than that once in a while for balancing to happen.

I would also set the delta to balance to 10mV or 15mV. That would be good enough, and not work the balancer so hard. 5mV will be hard to achieve, and so the balancer will always work to get there and never turn off.

If you make those changes, your battery should work well right off the bat, but will still require a long time to get back in balance because the balancers are rather slow. You can either wait (might take a month or more of normal usage) or take it all apart and perform a top balance. I would probably just leave it be and wait.
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Old 24-06-2024, 12:27   #4
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Re: Overkill BMS

One thing I would add is to avoid chasing ghosts measure using a multimeter the cell voltage of all 4 cells and compare it to what BMS is reporting. Also measure the pack voltage and compare it to both what the bMS and charger are reporting. Very likely is is correct but always good to verify.
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Old 24-06-2024, 12:52   #5
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Re: Overkill BMS

Thank you both. When my batteries hit full charge at current settings my cell delta is normally between .004 and .006

Battery 3 (which was taken out, load tested and then top balanced and reinstalled) is showing a super high balance issue.



I'll try your suggestions and monitor.

Thank you
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Old 24-06-2024, 20:10   #6
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Re: Overkill BMS

What do you make of this?

It was reading delta of .086.

After inputting Wholly Bee's suggestions in 1 charge cycle today, it is down to .006!

I'm finding the numbers a bit difficult to believe.
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Old 24-06-2024, 20:54   #7
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Re: Overkill BMS

That is faster than I would have expected, but not surprising. The new settings let the balancer do it's job, and the results sound great! The cells probably just weren't as far out of balance as I believed they were.
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