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diy solar

Mix a few 304ah in series with mostly 280ah

shanemgrey

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May 17, 2020
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I bought 32 grade b 280 ah lifepo4 cells to make a 2P16S bank. But 2 of the cells had damaged posts and I don't want to use them. 2 others looked like they were used, so I don't really want to use them either.

I also have 8 304 cells from another distributer. They are the same physical height as the 280s.

I am using 2 of the JKBMS active balancing BMSs at 200amp rating each.

I think I can add 4 of the 304 cells to the 12 280 cells to make the second 16s battery.

My understanding is that the larger capacity cells would just end up top balancing with the rest of them, and when drawn down, if any of the 280 cells goes under voltage before the 304s, the BMS would cut off the battery discharge. No harm to the 304 cells, nor any of the 12 280 cells.

I think that this setup would act exactly the same as a full 280ah battery. Essentially the 304ah extra capacity is just being wasted, but not harming anything.

Am I wrong with any of this? If so, what are the downsides to mixing in this way?
 
I've never tried it myself (so cannot speak from any actual experience), but my logic has always thought same like you, that the pack will just act like a 280Ah pack, and the 304Ah cells just won't exhale as much on the bottom end, but probably it would work fine, they would all be balanced on the top end. If balance on discharge was enabled on the BMS though, if you ever hit bottom knee, it could try to balance out, but in real world you may never notice any ill side effect.

I think that if no option is available to get same cell, I wouldn't lose any sleep setting up a pack this way. I might think it was better to not ever enable 'balance on discharge and charge', just leave it on the default 'balance on charge only', so if I ran the pack down to the lower end I wouldn't get a bunch of balancing trying to happen, but that may not even bother it too much either, in the real world, the SoC of the larger cells would always be straddling the SoC of the smaller cells at any given time I would think.
 
Yes as long as the combo battery is used as a 280 there should not be an issue. May as well stack the 304s together so it is easy to know who should be high or low. I have not built or tested such a battery. However I believe in going with what you have on hand whenever possible.
 
I bought 32 grade b 280 ah lifepo4 cells to make a 2P16S bank. But 2 of the cells had damaged posts and I don't want to use them. 2 others looked like they were used, so I don't really want to use them either.

I also have 8 304 cells from another distributer. They are the same physical height as the 280s.

I am using 2 of the JKBMS active balancing BMSs at 200amp rating each.

I think I can add 4 of the 304 cells to the 12 280 cells to make the second 16s battery.

My understanding is that the larger capacity cells would just end up top balancing with the rest of them, and when drawn down, if any of the 280 cells goes under voltage before the 304s, the BMS would cut off the battery discharge. No harm to the 304 cells, nor any of the 12 280 cells.

I think that this setup would act exactly the same as a full 280ah battery. Essentially the 304ah extra capacity is just being wasted, but not harming anything.

Am I wrong with any of this? If so, what are the downsides to mixing in this way?

I have two grade B Lifepo4 8s 24 volt battery banks in parallel with 271 ah cells except one of the battery banks has a single larger battery, but is the same height as mentioned so the bus bars lay flat on the terminals. The cell is wider and the terminals are wider apart, but that is not an issue since the bus bars just connect at a slight angle out to the larger battery terminals. It is placed at one end of the battery pack and it is the negative terminal connected to the BMS.

The larger battery charges slower at the top of the knee charge curve and discharges slower at the bottom knee. On the flat part of the curve there is little to no difference. I have no problems using the larger battery.

There are Heltec 5A active balancers activated on both battery banks when the voltage is above 27.3 volts. The balancer pulls the larger battery up to be equal to the others in the bank when the SCC is in boost (absorb) at 28 VDC for 10 to 20 minutes.

It essentially just ends up being the best (strongest) battery in the one battery pack.
 
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