I have an existing 4S48P Lifepo4 battery bank put together using 26650 batteries. Each one of these 192 batteries are between 2400 - 2800 mah. Is it okay to add four 25 ah lifepo4 batteries to each parallel string of my existing bank? So technically the battery bank would become a 4S52P bank, but four of the batteries in each parallel string would be significantly larger (like ten times larger) than the rest of the parallel connections.
Parallel Battery Connection With Different Capacities
- Thread starter Schtevie
- Start date
It's actually the other way around. I have four sets of 48 batteries in parallel. I then put those four parallel strings of 48 batteries each into series under a single BMS for the entire 192 battery pack. Does that make sense or am I not explaining it clearly?
battery = collection of cells.
cell = individual unit of a battery's construction
You have a 48P4S battery = 4 groups of 48 parallel cells in series for 12.8V nominal. Using an average of 2.6Ah, your 48P4S capacity is 124.8Ah
Do you then intend to parallel 4 more 4S 25Ah batteries (not made from the 26650 cells) each with their own BMS?
cell = individual unit of a battery's construction
You have a 48P4S battery = 4 groups of 48 parallel cells in series for 12.8V nominal. Using an average of 2.6Ah, your 48P4S capacity is 124.8Ah
Do you then intend to parallel 4 more 4S 25Ah batteries (not made from the 26650 cells) each with their own BMS?
You've indicated you're adding 16 new batteries when you previously indicated 4 new batteries. Do you mean cells?
Again, a cell is the individual 3.2V element (assuming we're talking LFP. If not, please clarify). A battery is when you arrange more than one of those together in parallel or series or both. it's important to use proper terminology, or we're not going to be able to communicate.
Do you have 16 25Ah cells you which to add do your bank? If so, I would recommend you create a 4P4S battery constructed from those different cells and use a new BMS. Then take those two batteries and join them in parallel at the main (+) and (-) using best practices in "Wiring" in link #2 of my signature.
That way you're joining a 125Ah battery with a 100Ah battery if I'm understanding your intent.
Again, a cell is the individual 3.2V element (assuming we're talking LFP. If not, please clarify). A battery is when you arrange more than one of those together in parallel or series or both. it's important to use proper terminology, or we're not going to be able to communicate.
Do you have 16 25Ah cells you which to add do your bank? If so, I would recommend you create a 4P4S battery constructed from those different cells and use a new BMS. Then take those two batteries and join them in parallel at the main (+) and (-) using best practices in "Wiring" in link #2 of my signature.
That way you're joining a 125Ah battery with a 100Ah battery if I'm understanding your intent.
I'm sorry for not using the correct terminology. Thanks for being patient with me and bridging the communication gap!
I'm guessing I need the second BMS to monitor the 4P4S battery because it's cell capacities are so different from the existing 48P4S cells? Are there any rules of thumb for how closely matched in capacity the cells in parallel need to be? I saw elsewhere that when cells are wired in parallel, it just becomes one big cell. I guess that is true if the cells in parallel are identical or closely matched, but what are the limits?
I'm guessing I need the second BMS to monitor the 4P4S battery because it's cell capacities are so different from the existing 48P4S cells? Are there any rules of thumb for how closely matched in capacity the cells in parallel need to be? I saw elsewhere that when cells are wired in parallel, it just becomes one big cell. I guess that is true if the cells in parallel are identical or closely matched, but what are the limits?
Now the 10X makes sense. 2.5Ah cells vs. 25Ah cells.
When you're building a battery, the components should be equal capacity. Each cell group should have the same capacity and all cells within each cell group should have the same capacity. Natural variation is normal, but 10X is out there. The biggest issue is getting equal current distribution within each cell group. The 25Ah cells in parallel with the 2.5Ah cells will not behave optimally.
It's much easier to load balance a 125Ah battery in parallel with a 100Ah battery. If your current needs are low, then an additional $20-30 for a 4S BMS seems like a tiny price to pay.
When you're building a battery, the components should be equal capacity. Each cell group should have the same capacity and all cells within each cell group should have the same capacity. Natural variation is normal, but 10X is out there. The biggest issue is getting equal current distribution within each cell group. The 25Ah cells in parallel with the 2.5Ah cells will not behave optimally.
It's much easier to load balance a 125Ah battery in parallel with a 100Ah battery. If your current needs are low, then an additional $20-30 for a 4S BMS seems like a tiny price to pay.
Makes sense, but how does the load get balanced once the two cell groups are hooked up in parallel? I just want to make sure I'm not missing something in the wiring. I'll have one BMS on the 125 ah 48P4S cell group, and a second BMS on the 100 ah 4P4S cell group. The fact that the two cell groups are off by 25 ah is less than optimal, but not a problem from a safety standpoint?
I see how to wire the cells in your 'wiring' link, but it doesn't mention a BMS. Do the two BMS units fight with each other once the two cell packs are connected in parallel? So here's what I see from your 'wiring' link (four cell groups instead of my two cell groups, but same idea):
This is from Will's site with multiple BMS units:
So I guess my question is can I follow the diagram above for multiple BMS units, or is there a section in your 'wiring' link I'm missing?
This is from Will's site with multiple BMS units:
So I guess my question is can I follow the diagram above for multiple BMS units, or is there a section in your 'wiring' link I'm missing?
Polyneutron2
New Member
I have half that number so 24P4S and I was wondering if it is advisable to put the 4 in series in the middle of the parallel or across 12P but the BMS only looks at one ground and because it not precisely in the middle put the series at the end. What is the best practice for large numbers of batteries in series?
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