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How to connect 8 x 272Ah + 4 x 280Ah together?

baipin

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Dec 25, 2020
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Hello everyone,

I had planned on building a single (1) 2p4s battery with a single BMS, or (2) 1p4s batteries each with their own BMS, to double the amount of current I could draw for brief moments, through two BMS. These cells were all Lishen 272Ah, matched, grade A, new. So that's eight total 272Ah cells.

As luck would have it, I got the deal of a lifetime on four additional grade A, brand new cells... except they're EVE 280Ah. That means, I have a total of 12 cells; (4) 280Ah, (8) 272Ah. A 10kWh system would be nice, so I'd like to use all 12 cells in a single battery or bank of batteries; use these new cells alongside the 272Ah somehow. My question is, what would be the safest/most reliable/most cost-effective way to do this? At the moment, I have (2) 120A JBD 4s BMS's to work with (or maybe sell). The way I see it, these are my options:

  • (1) Battery: (1) 3p4s battery made of (12) cells ((8) 272Ah cells + (4) 280Ah cells) with a BMS.
    The battery bank comprises of 12 cells in 1 battery.

  • (2) Batteries: (1) 2p4s battery made of (8) 272Ah cells, with a BMS + (1) 1p4s battery made of (4) 280Ah cells, with a BMS.
    The battery bank comprises of 12 cells in 2 batteries. The 2 batteries are connected in parallel after each battery's BMS. One battery is 280Ah capacity, the other is 544Ah capacity.

  • (3) Batteries: (1) 1p4s battery made of (4) 272Ah cells, with a BMS + (1) 1p4s battery made of (4) 272Ah cells, with a BMS +(1) 1p4s battery made of (4) 280Ah cells, with a BMS.
    The battery bank comprises of 12 cells in 3 batteries. The 3 batteries are connected in parallel after each battery's BMS. One battery is 272Ah capacity, the second battery is of 272Ah capacity, the third battery is of 280Ah capacity.
Perhaps I am missing another option, but this is all I can think of now. I'd rather not buy any new cells as they're expensive to buy and ship here in Canada. I would be open to trading the 280's for 272's if that's somehow better. I would also be open to options which have me sell/trade the (2) JBD BMS I currently have for a single, larger capacity BMS like a Chargery. Some insight would be much appreciated!
 
I think you have the options covered.

IMHO, (3) is the best option. It offers redundancy and protection from single cell failures. A single cell failing will tend to take out its parallel "cell mates." If you observe a cell having a problem, you have no idea which parallel cell it might be, so you've got to break them down anyway.
 
I think you have the options covered.

IMHO, (3) is the best option. It offers redundancy and protection from single cell failures. A single cell failing will tend to take out its parallel "cell mates." If you observe a cell having a problem, you have no idea which parallel cell it might be, so you've got to break them down anyway.
Sounds good, thank you.

My main worry with 3 batteries in a bank is load sharing and recharging. Or, is that less of an issue since each battery has the same JBD BMS, cable lengths will be the exact same until their common bus after the BMS, and each battery is of a similar capacity?

I assume mixing 8 272Ah and 4 280Ah cells in a single 3p4s pack is the worst option? No matter what I do, I should have multiple batteries unless I were to trade the 4 280Ah for someone else's 272Ah?
 
Configuring via best practices per "wiring" in link #2 in my signature minimizes the issue.

Once configured, it's a good idea to confirm the current passing through each battery is within the battery/BMS ratings via use of a DC clamp ammeter for both high charge and discharge conditions.

Battery manufacturers (and the "wiring" reference mentioned" indicate 3-4 parallel strings are acceptable.
 
Configuring via best practices per "wiring" in link #2 in my signature minimizes the issue.

Once configured, it's a good idea to confirm the current passing through each battery is within the battery/BMS ratings via use of a DC clamp ammeter for both high charge and discharge conditions.

Battery manufacturers (and the "wiring" reference mentioned" indicate 3-4 parallel strings are acceptable.

I would like to run a mini-split AC for a few hours, probably. So 600-1200 watts max. That should fit under the 1440 watt max load of a single BMS, and that'll be fine, I assume?

Once a week, for let's say 20 minutes, I might want to run 2000 watts on an induction cooktop; i.e. 1000 watts over two hobs. That's over the 1440 watt limit of a single BMS. Is that completely inadvisable, or okay to do if I have sufficient remaining capacity and not at risk of one let alone two batteries dropping out from LVD?
 
I would like to run a mini-split AC for a few hours, probably. So 600-1200 watts max. That should fit under the 1440 watt max load of a single BMS, and that'll be fine, I assume?

Once a week, for let's say 20 minutes, I might want to run 2000 watts on an induction cooktop; i.e. 1000 watts over two hobs. That's over the 1440 watt limit of a single BMS. Is that completely inadvisable, or okay to do if I have sufficient remaining capacity and not at risk of one let alone two batteries dropping out from LVD?

Divide the BTU/hr by the SEER, and that's your average Watts consumption.

You can't meaningfully exceed the BMS current limit or they will shut down; however, with 3 separate batteries, each with their own BMS, their current capacity adds, e.g., 3X 100A BMS would allow 300A (theoretically). Knock 10-20% off as a safety factor, and that has you running at least 2880W without issue.

2000W * 20 min/60min/hr = 667Wh = about 6.3% of your total capacity.
 
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