Mixing Battery Banks of different capacity, but with separate BMS'es

[Archerite]

For nearly a year now I have my DIY-ish self built 88Ah LiFePO4 batterybank. It's made up of 11 smaller 8Ah batteries, joining on a central busbar, individual cell fuses, disconnect switch, 80A main fuse and to monitor it all a Victron BMV-712 Smart. Each battery "cell" has an internal BMS that cuts off power when charged, low voltage and a few other things. These are not super high grade cells or BMS's I am sure...but they at least have something monitoring the internals instead of raw cells.
It's hooked into a Raspberry pi running VenusOS and I have it linked with Home Assistant through a custom NodeRed flow. When the battery is reaching 20% SoC the wall charger is turned on in Home Assistant until it reaches 50%. I also have a "SuperCharger" switch to charge to 80% on cloudy days. All these values are sliders in HA but these are the current settings I use. This is all working great but I thought I should explain the basic setup I have first.

Besides the grid/wall charger I also have 4 MPPT's with only 3 of them connected to panels at the moment. The 4th does not get much sun in winter because of shadow's and other reasons...so I disconnected the solar input. The solar charger are the only way the battery bank charges to 100% SoC most of the time.

My current "problem/situation" is that I would like to add more capacity. I have 2 100Ah batteries ready to install....but it will require a complete redesign of the battery cabinet, wiring, fuses and everything. I was just thinking why it would be a bad idea to have these 100Ah batteries join the system...but with a separate BMV/SmartShunt maybe. Of course before connecting them I would make sure all voltages are equalized and stable. The 100Ah batteries will also get their own fuses matching their limits and cable size of course.

I know it's "bad practice" in general to join cells of different capacity into a battery bank. But what if they are in different banks! it's kind of the same as what Andy from the OffGridGarage shows in his big batteryshelf. Each "bank/shelf" has its own BMS and balancer circuits, disconnects and then one central SmartShunt for the whole cabinet. I think I remember his capacities of the banks are also not matched exactly...280Ah vs 320Ah I think it was at 48v. While mine would then be 88Ah + 100Ah + 100Ah (or 88 + 200) both at 12V.

it would still require some rethinking to make things installed nice and safe...but it would bring my capacity to 288Ah with the batteies I already have! And every "Cell" has it's own internal BMS anyway...including those 100Ah! So when full or at a to low voltage they will disconnect either way. In theory.

Any way...just a thought and I would like to get some feedback if this is "ok", "good", "smart" or "terrible". Thanks in advance

 

[sunshine_eggo]

"OK"

Basically it's good once you've determined that under your extreme load and charge scenarios, all batteries are operating within specification.

 

[Archerite]

"OK"

Basically it's good once you've determined that under your extreme load and charge scenarios, all batteries are operating within specification.

Ok, great! It's not so much that I have an extreme high load or anything, I would just like to have a bit more runtime and collect more solar energy. Some days I have just nothing to charge anymore...and still 2 hours of full direct sunlight left. (this was in summer by the way).

But good to know it should be "ok" to put them in parallel. All batteries are 12V and before connecting them together I will balance them as close as possible on the voltages. Then put the grid charger on full power....would take it days to fill that much battery power! Poor thing Hahahah.

To give an idea of the current loads:
- 20-30watt constant draw
- goes to 70-80 when I run my TV from the inverter. (inverter is off when TV is not in use, otherwise 10watts extra draw with TV in standby)
- Ocasional powerbank recharge at 65W max
- I have an EcoFlow River MAX that can charge at 160W....through my Victron Orion at 24V
- I am planning on adding my modem and fiberbox as a constant battery load to. Already got the 12V DC-DC meanwell powersupply for them. Could be another 10-20watts
- Charging various devices with car chargers ranging from 5-30W. Usually only 2-3 at the same time.

Even with all this together I won't reach a 400W peak I think. Something the solar has done once in July last year! Got 23A at 12V going into this 88Ah bank that day! The individual 8Ah cells can deliver 10A max...but since they are in parallel 110A should be the upper limit. This is why I fused it at 80A as I knew I would never get close to even 40A in normal use.
And the 100Ah batteries can be charged and discharged at 100A

I also noticed a mistake in my calculation above. I can't discharge 288Ah if all are in parallel ofcourse...would be more like 264Ah I think. And by only going down to 20% that should give a usable range of a little above 200Ah I think. ?

And charging will be either from a slow-ish grid charger at 15A (mostly 100-110W) or from solar at 307W peak (19.2A) today. It was a short peak but it was there, hahaha.

Sorry for the long reply but I was also thinking out load a bit. Did I miss anything in my calculations?

And for the record: This is a for fun experimental and educational solar system. I do not rely on it for daily usage as we have quite a stable grid power system here. But prices are high so the more I use from solar the more we can safe on grid power.

 

[Robbert]

Should be possible

 

[Archerite]

Should be possible

Thanks. I am actually still getting the parts together to connect the batteries safely. ? I had done some "cleaning" and put a lot of the not so often used solar components into my storage unit. But it's not all in a single box! It's a bit of a search to find the "spares" for fuses, wires, connectors and the crimper together! I've got most of it now just need to take the time to actually DO the install...hahaha.