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Large DIY Battery Bank (14kWh) 24V 2P8S vs. 2P4S * 2

edclarke

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Apr 25, 2021
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I'm planning a battery bank for a van camper conversion, and hoping the community here can help me understand the pros and cons of these two scenarios (or offer more elegant solutions). Thank you in advance for your help!

Design goals
- Fully power a 3,000 Watt inverter
- Handle up to 500W of DC load

24 Volt 560Ah
Pros
- Some smaller cable sizes
- Only need one BMS

Cons
- Will need a 24-12V converter
- a little more complexity
- slightly more expensive 24V devices (when available)

Two 12 Volt 560Ah banks
Pros
- Less complexity
- Some redundancy

Cons
- Need 2 BMS

Concerns
- In both of the depicted scenarios two cells are paralleled before the series connection, and the BMS connects to the paralleled positive terminals. How does the BMS respond to a failing cell when it is connected to another?

batt_design.png
 
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2P4S X 2 is just a bad idea given the other options with DIY.

You omitted the best option:

8S X 2.

2X 8S batteries each with their own BMS. Should one battery fail, you have immediate redundancy.
 
8S X 2.

2X 8S batteries each with their own BMS. Should one battery fail, you have immediate redundancy.

I love that option.

Just started to look for a battery for a 3000W Growatt for 24v - that's 125A

A lot to handle for all one BMS. But when you do 8S X 2 with 2x 120A BMS - the battery bank would be able to supply 240A - lots of safety margin and I like the redundancy part.

Most of the time the batteries would run at very low C ratings. Usually I need around 1000-1500w top. But sometimes the refrigerator decides to start when I already got the coffeemaker on.
 
I love that option.

Just started to look for a battery for a 3000W Growatt for 24v - that's 125A

A lot to handle for all one BMS. But when you do 8S X 2 with 2x 120A BMS - the battery bank would be able to supply 240A - lots of safety margin and I like the redundancy part.

Most of the time the batteries would run at very low C ratings. Usually I need around 1000-1500w top. But sometimes the refrigerator decides to start when I already got the coffeemaker on.

In that setup, it's good to apply a bit of a safety margin (maybe only 200A instead of 240A) and confirm that the batteries are "splitting" the load via use of a clamp DC ammeter under a heavy load.
 
In that setup, it's good to apply a bit of a safety margin (maybe only 200A instead of 240A) and confirm that the batteries are "splitting" the load via use of a clamp DC ammeter under a heavy load.
I would only use maximum 125A from 240A possible. So almost 50% safety margin.

Very likely far less - my average high load is around 1000-1500w @ 24v somewhere in the 40-60A range.
Perfect for redundancy. Even if one BMS or string of batteries would crap out, I still would be able to power my normal loads.
 
2P4S X 2 is just a bad idea given the other options with DIY.

You omitted the best option:

8S X 2.

2X 8S batteries each with their own BMS. Should one battery fail, you have immediate redundancy.
I really like your suggestion. I was trying to omit the second BMS, but it's a reasonable price to pay for so much more redundancy.
 
I used to be a big XPYS proponent, but I've switched loyalties. Saving $100 on only getting 1 BMS sacrifices a lot.
 
I really like your suggestion. I was trying to omit the second BMS, but it's a reasonable price to pay for so much more redundancy.
Redundancy was actually the reasons I stayed with Lead for a long time, Since each battery just works by itself.
My old Van conversion has 3x 12V subsystem, which can work independent from each other. 4 Charging sources (2x Solar charge controller, 120V charger, Alternator)

In my current RV built,
I think I'm going to have 2x 24V batteries and 2x 12V (house and chassis batteries)
With a Growatt 3000W 24V inverter/charger/solar and a small 1000-1500w 12V MSW Inverter

I mean you are sinking $2000-3000 in a battery here. Adding $100 for a BMS to have redundancy is really cheap insurance.
You see plenty of failing BMS when you just search here in the discussion board.

I'm even planning using similar rated BMS from different manufacturers, just to be on the safe side. Belt and Suspenders :p
 
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