Changing from 4S to 8S; how to prep batteries

I have two 12V LiFePO3 batteries, each 280 AHr, connected in series for a 24V output. When I set these batteries up, I installed a 4S BMS on each one, which I was advised to do but I’m having regrets, and want to use a single 8S BMS for both.

Irrespective of the above, should I equalize the battery cell voltages before installing the 8S BMS, and if so, what’s the best way to do this? Should I just connect all the positive terminals together, and likewise all the negative terminals? Or should I discharge one battery to meet the other battery’s voltage?

Awsmits said:

I have two 12V LiFePO3 batteries, each 280 AHr, connected in series for a 24V output. When I set these batteries up, I installed a 4S BMS on each one, which I was advised to do but I’m having regrets, and want to use a single 8S BMS for both.

Irrespective of the above, should I equalize the battery cell voltages before installing the 8S BMS, and if so, what’s the best way to do this? Should I just connect all the positive terminals together, and likewise all the negative terminals? Or should I discharge one battery to meet the other battery’s voltage?

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I would simply rewire as 8S with the 8S BMS and evaluate if top balancing is needed.

The BMS will balance everything out on its own once it's online.

Even if the two batteries are at quite different voltages? The cells within each battery are very similar, but the difference between the two batteries is that one battery the cells are near full (3.5 V) and the other battery the cells are about 3.1V. Seems like it would be better if I discharged the full one down closer to the lower one.

Or charge them both first so they're the same SOC before you disassemble.

Awsmits said:

Even if the two batteries are at quite different voltages? The cells within each battery are very similar, but the difference between the two batteries is that one battery the cells are near full (3.5 V) and the other battery the cells are about 3.1V. Seems like it would be better if I discharged the full one down closer to the lower one.

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That wasn't clear from your post.

Rednecktek said:

Or charge them both first so they're the same SOC before you disassemble.

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This.

Charge each individually as 12V and then build 8S.

Now... something that really bothers me.

These two 12V were in series, yet one is full and the other is at a VERY low state of charge. WTF?

Were you pulling 12V loads off one of the 12V? If so, NEVER EVER do this. When you have two 12V in series, you have a 24V battery. Period. You should only pull 24V loads from it. Period. No exceptions. Pulling from PART of any battery built from a string of elements is a recipe for a screwed up battery that will eventually have 0Ah capacity.

Yes, using them in series, connected to a Growatt 24v inverter, so they should be charging and discharging together. I don’t understand how the two batteries get so far apart in their SOC either. That’s why I want to connect them as an 8S.

And the 4S BMS of each battery is set to the identical charging and discharging parameters….

Given your use & setup this is what I suggest:
! Assuming these are all identical cells (Make & Model) !
Disassemble the packs and set 4 cells in Parallel and then charge that "set" to 3.650V and allow the amperage taken to drop down to 2.5A, then leave the cells in parallel so they balance between themselves while "settling". Once done, then do the next set of 4 and when done leave them for about 2 hours to settle as well. You will more than likely see a bit of difference between the sets but that is OK. Next set all 8 Cells in Parallel again and charge the "lot" to 3.600 again and allow the charge taken to drop down again to around 2.5A.

Now the cells will be Top Charged & Fully reactivated (in case some got lazy, they can if charged in weird ways) and ready to assemble in an 8S Battery Pack. Assemble them into your box and use a Decent BMS like a JK With Active Balancing (Inverter Model if you have compatible Inverter/AIO). Set balancing to start @ 3.400 to 3.420 leaving 0.010 Deviation between cells. From this point forward use a Moderate Charge Profile and after a couple of cycles all the cells should be quite happy & balanced out.

See here for Moderate Profile & BMS settings:

In a perfect world @Steve_S has got it down (other than the JK BMS, but that's my opinion ) but that requires building harnesses and controllable DC power supplies and a LOT of time. The quicker way is to charge both 4s packs in parallel with a car charger or the like, then tear apart and re-assemble. If the car charger has both banks at the same SOC then the voltages of each cell are going to be close enough that you don't have to worry about which cell goes in which order, and the balancing of the new 8s BMS will clean up any leftover imbalances. In the BMS app turn the "balance while charging" to OFF and throw it on your system, the cells will continue to balance all night even if they're not being charged. When the sun comes back up the next day your BMS should have everything all evened out inside.

Rednecktek said:

In a perfect world @Steve_S has got it down (other than the JK BMS, but that's my opinion ) but that requires building harnesses and controllable DC power supplies and a LOT of time. The quicker way is to charge both 4s packs in parallel with a car charger or the like, then tear apart and re-assemble. If the car charger has both banks at the same SOC then the voltages of each cell are going to be close enough that you don't have to worry about which cell goes in which order, and the balancing of the new 8s BMS will clean up any leftover imbalances. In the BMS app turn the "balance while charging" to OFF and throw it on your system, the cells will continue to balance all night even if they're not being charged. When the sun comes back up the next day your BMS should have everything all evened out inside.

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And then turn the “balance while charging” back on?

No, just leave it off.

Balance while charging means "Only balance cells while charging" so turning it off tells the BMS to balance all the time your cell volts are full enough. That way if it takes all day to charge up the batteries, it'll balance in the evening or night when there's no solar rather than ONLY when the sun is bright enough to charge the batteries.

Got, it, thanks...so at what cell voltage is "full enough"? Is this a parameter setting, and if so, what should it be set to? 3.3, 3.4V?