Building a 48V battery with 24 Lifepo4 3V cells

d2po

New Member
Joined
Dec 28, 2021
Messages
1
Hello solar enthusiasts,

I'm building the blueprints for my next house, where I want to reach 80% electrical autonomy.
Doing this does require 22KWH of energy storage in batteries, so I need to arrange 24 batteries (@310ah each) to reach the required 48V.

The simplest way to achieve this would be by using 16 large capacity batteries, or to reduce my storage needs, but none of these help my return on investment, as larger batteries are more expensive per kwh.

So my solution is to arrange 24 batteries the first 16 being connected by bundles of 2 in parallel, the next 8 being connected in serial.

The BMS would sense battery voltage on the 2nd of every parallel pair (blue lines are BMW sense wires)

What are your thoughts on this? Did anyone build something like this?

Has anyone got a better recommendation on building this 24 cells 48V battery?

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meetyg

Solar Addict
Joined
Jun 4, 2021
Messages
356
I'm not an expert, but I don't think this will work well. When draining the battery pack, the single 8S cells will drain first, as they have the least capacity. They will bring down you pack voltage.
You could use the BMS's parameters (given its a programmable one) to shut down the load when the pack goes under this voltage, but then you wouldn't utilize the pack's full capacity.
Also, the BMS is there to handle edge case scenarios, not an uneven pack.

If you don't protect from this, you will simply drain the 8S cells to death (literally).

In my opinion you have three options:
1. Just build a 16S1P pack. That would give you a nice 15kwh pack. You could use the remaining 8 cells for something else.
2. Buy 8 more identical cells, and build a proper 16S2P pack. That would give you a whopping 30kwh pack! But of course more costly.
3. Maybe you could go with a 24v system?
Probably 24v is less convenient to work with (larger guage wire, etc...) and there are less high power 24v components out there (chargers, inverters, etc...).
But, then you can utilize all your 24 cells, and build a 8S3P pack which will give you a wonderful 23kwh pack.

To overcome the limitations of some 24v components, you could use some in parallel (like multiple solar chargers and/or inverters).

That's my 2 cents...
 

TomC4306

Solar Addict
Joined
Mar 2, 2021
Messages
286
I agree with @meetyg above. You cannot mix parallel and serial connections inside a single pack the way you describe.
 
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