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robert-welling avatar image
robert-welling asked

Calculate DYI Lithium battery pack Ah capacity

Hi All,


I build 3 DYI jehugarcia (youtube) 18650 battery packs, each pack consist of 9 times 7x18650 cells with a 24v output. So, in total 63 cells. Each 18650 has an average 2400mAh.

So, in a grand total I have 3 packs of 63c = 189 cells.

Each pack has it’s own BMS and they are charged by a Victron 100/30 MPPT Solar charge controller. A BMV 702 keeps an eye on things and a Raspberry PI with VictronOS on it to log statistics to a VRM dashboard.

All well so far and after weeks of charging and discharging the cells are still very much balanced. A Victron Battery Protect as a measure to prevent to low a discharge and opens at float voltage to keep the MPPT as maximum power. Basically, this pack functions as an equalizer as the total capacity isn’t enough for off-grid purposes. (In The Netherlands we have clouds now and then with the sun disappearing).

The issue I have is to calculate the correct total Ah settings in the BMV shunt. I think each pack of 9 has a 21Ah capacity, so a total 63Ah. But the key question is, did I calculate it correctly?

Hope one of you can assist.

Kind regards,

Robert





battery capacity
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3 Answers
seb71 avatar image
seb71 answered ·

Make a drawing how you connected the cells.

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JohnC avatar image
JohnC answered ·

Hi @Robert Welling

Without dropping the decimal point early, I make it 64.8 Ah at 24V nominally.

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robert-welling avatar image
robert-welling answered ·

@JohnC and @Seb71 thanks for the reply, I made an error in my summary;

each pack consists of 10 stacks x 7 (70) 18650 cells. Each stack has 7 in series connected 18650 making @ max 28.4 volts. If I make the math myselves that makes 2500mAh per stack times 10 (total pack) is 25Ah.

So having 3 of them makes ~ 75Ah, agree?


/rw

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seb71 avatar image seb71 commented ·

So 10p7s in each of those 3 strings?

If you have groups of 10 cells in parallel and each cell is 2.5Ah, that's 2.5Ah x 10 = 25Ah per string.


Then three 25Ah strings in parallel =>

25Ah x 3 = 75Ah

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