18650 2s3p proyect

E

emiliozv

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Hello!
I'm planning to build a 7,4v - 8,4v portable battery from 18650's, so I know It has to be 2SNP or NP2S.
In terms of size and capacity I think 6 cells will be perfect, so it would be 2S3P or 3P2S. 2 x 3,7v = 7.4v ; 3 x 3000mAh = 9000mAh.
My initial doubts are:
1. Is there any advantage / disadvantage for 2S3P or 3P2S?
2. Is there a bms that can handle the balance for each of the 6 cells, so I can charge it with 8.4v input, and use the same cable to charge and discharge? And so not having to use balance wires such as in RC lipo batteries.

Thanks in advance.
 
** If you connect the cells in parallel then connect them in series, that's XsYp

* very easy to keep all cells balanced
* with Batrium, Xs longmons are needed
* with inexpensive dumb bms boards, 1 in needed

** If you connect the cells in series then connect them in parallel, that's YpXs

* more difficult to keep all cells balanced
* a bit more difficult because you have twice as many connections to monitor
* with Batrium 2 * X2 longmons are needed
* with inexpensive dumb bms boards, 2 are needed


There are plenty of bms boards on the market that can do what you are looking for. From a few dollars to as much as $50 or more. There are several reviews on the forum that have tested various different bms units. Daromer has a few videos of tested bms's.

You will always need to use balance wires if you plan on balancing. You cannot keep cells properly balanced if you don't have the extra wires; unless you take the cells out and charge them independently (even then, they may not be charged to the exact amount)
 
Thanks for your extense and fast response!

Korishan said:
You will always need to use balance wires if you plan on balancing. You cannot keep cells properly balanced if you don't have the extra wires; unless you take the cells out and charge them independently (even then, they may not be charged to the exact amount)
Click to expand...

Yeah, I meant I'd like to keep them inside the battery (the balance wires), and not having to plug them to the "smart" charger.

Talking about smart chargers, would it have any problem with a bms?
Thanks again.
 
chargers are usually independent from the bms. the charger sees the full voltage and dumps in according to the SoC. Then the bms watches each bank of cell(s) and if one goes higher than the rest, it starts working to keep it in check while the rest of the cells are charging. If the cell rises too fast, the bms disconnects the input charge. If the charger doesn't disconnect at the top voltage of the pack that the bms thinks it should stop at, the bms will disconnect the charge current.
Note, it doesn't normally turn off the charger, just disconnects it. However, most bms units can be worked to control the charger with some circuitry reworks and extra wiring.
 
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