2 Battery Banks on 1 solar system - Looking for some kind of Switch to go from main bank to a reserve bank

[Echo]

Just a thought, but I have (4) 300AH LIFEPO4 batteries (ea. with 200A BMS) and an extra (4) 100AH LIFEPO4 batteries (ea. with 100A BMS).

I would like to use the 4 100AH bank as a reserve if my (4) 300AH bank ever gets too low/depleted in an extended grid down scenario.
Each 24V (Series/Parallel) bank would be kept separate from the other but using the same Solar Array, SCC and Inverter.

The minute details (bus bars etc.) can prob be worked out but my idea is to shut down SCC and then flip a switch and come back online with the reserve bank, if needed.

Does anyone know of a switch with high amp ratings that would work?

Thanks!

 

[sunshine_eggo]

I'll be the first to say, "why?"

Why not just lump them all as one big battery all the time? It's been well demonstrated that paralleling mismatched capacity batteries with best practices yields acceptable performance.

 

[timselectric]

 

[sunshine_eggo]

Had to get my glasses... I thought that was a middle finger at first...

 

[dopeassjackson]

A few more pannels and a charge controller may be the easiest method.

 

[Echo]

I'll be the first to say, "why?"

Why not just lump them all as one big battery all the time? It's been well demonstrated that paralleling mismatched capacity batteries with best practices yields acceptable performance.

Hi,
I am primarily concerned with keeping the 24v-300 batteries (2S2P) in balance and felt it might be too complicated in adding a mismatched 24v bank.

Also, since they are 2 different sets of BMS (200A vs 100A), I thought it's probably not a good idea, as I am not sure of any effects on the lower 100A BMS.

So, the next idea was keeping them separate, with one bank on standby as a way to hopefully keep everything running smooth.

 

[sunshine_eggo]

Hi,
I am primarily concerned with keeping the 24v-300 batteries (2S2P) in balance and felt it might be too complicated in adding a mismatched 24v bank.

There will be potential disruption, but a few cycles and charges to true 100% (28.4V) should resolve this. Connect the two battery banks when they are within 0.2V of each other to avoid excessive surge current between them.

Also, since they are 2 different sets of BMS (200A vs 100A), I thought it's probably not a good idea, as I am not sure of any effects on the lower 100A BMS.

A non-concern. Absolutely no effect whatsoever, especially if you ensure they are connected when within 0.2V of each other.

So, the next idea was keeping them separate, with one bank on standby as a way to hopefully keep everything running smooth.

This would actually create more imbalance issues. A sitting LFP battery is continuously going out of balance.

LOTS of folks have paralleled mismatched size batteries with zero issues after initial balance is achieved.

 

[sunshine_eggo]

Hi,
I am primarily concerned with keeping the 24v-300 batteries (2S2P)

Missed the 2S bit.

The 2S portion represents more issue than different sizes in parallel.

I do want to clarify that the SERIES elements of a battery must be the same size, i.e., you don't want to put a 12.8V 100Ah in series with a 12.8V300Ah.

A 2S2P bank of 12.8V 300Ah batteries in parallel with a 2S2P bank for 12.8V 100Ah batteries is no issue.

HOWEVER,

It can be challenging to keep the 12.8V elements in balance when in series for higher voltage. Your best best is something like this:

https://www.amazon.com/dp/B0CDBQFG2J/ref=emc_bcc_2_i

one for each 2S string to help keep the two 12.8V batteries at the same voltage.

 

[timselectric]

 

[Echo]

A few more pannels and a charge controller may be the easiest method.

Thanks. If I'm following you, I would have to pull more wire for a new solar array and that was what I was trying to avoid.

 

[Echo]

Missed the 2S bit.

The 2S portion represents more issue than different sizes in parallel.

I do want to clarify that the SERIES elements of a battery must be the same size, i.e., you don't want to put a 12.8V 100Ah in series with a 12.8V300Ah.

A 2S2P bank of 12.8V 300Ah batteries in parallel with a 2S2P bank for 12.8V 100Ah batteries is no issue.

HOWEVER,

It can be challenging to keep the 12.8V elements in balance when in series for higher voltage. Your best best is something like this:

https://www.amazon.com/dp/B0CDBQFG2J/ref=emc_bcc_2_i

one for each 2S string to help keep the two 12.8V batteries at the same voltage.

Thanks for the link. I just got a battery balancer in the mail today since I had planned to add one, and that's also why I was trying to go the separation route.

I had hoped with the 2 banks (one having the same type of 300's and the other 100's as another brand of the same type of batteries) could be separated with only a switch to change from one bank to the other as needed.

I feared a set of balancers for each 2S2P bank, with different BMS, battery manufacturers, amperage, etc... has a greater potential of failure.

 

[SoggyShoes]

What? no mike drop?

 

[740GLE]

What? no mike drop?

Its always implied