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used 18650 in series and parallel - opinions.

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Simen:
I have 12 pc. 18650 Lithium cells, 2 different brands, which i am going to use in a powerbank. I'm going for 2S, 6P configuration.

One set of 6 cells have a capacity of around 2100 mA, and the other set, around 2250 mA. The internal resistance of each cell are all around 100 mOhm +/- 10 mOhm.

Question is; should i use all from each brand in its own parallel-string, or should i mix 3 cells from each brand in each parallel string to even out the capacity?

I will of course use a balancing bms board to protect the cells...

frackers:
Lithium are more voltage than current or capacity sensitive. If they are the same chemistry then they'll be fine in parallel.

I'd recommend undercharging them at 4.1 volts maximum rather than 4.2, they will last longer even though the capacity will be down by about 5%.

A mate is building packs into old ice cream containers that hold 64 cells per box, he charges to 3.8v (half capacity, 10 times the cycles) and expects to get another 1000 cycles at least from these second hand cells (ex-laptop) that he has tested and checked the capacity of. He has a full 48v worth now which gives him over 3kw-hrs of storage for pretty much nothing (apart from the testing gear).

Simen:
Thanks for the answer. :)

I'll have to see how well, and at what voltages the bms i've ordered (from ebay/china), balances the cells; if the balancing happens continuously, i can set the charger at a lower end-voltage, but if balancing only happens when one of the cells reach 4.20 Volt, i'll stick with that. ;)

The cells i have are also from laptops; unused - but stored on the shelf too long (Thus, free for me. ;) ) When i got them, they had a resting voltage at 1.1-1.6 Volt, so i charged each cell to 3.2 Volt with only a few mA's, before full charge... They should be 2600 mA cells, but resting at undervoltage have reduced the capacity.

They usually gets a little better capacity after a few cycles.

OperaHouse:
6P is a lot of energy.  There is always a chance that one cell will short. I don't like to put more than 2 in parallel without a fuse for each pair.  So, three fuses for every 6 cells.  A 5-8A would be sufficient to prevent a catastrophic failure. 5 cells feeding into one shorted one would be a real hazard.  I'm always dealing with recycled cells wit a more likely failure rate.

Simen:
Not a bad idea. :)
But wouldn't 2 fuses for each 3 pair be enough? (Between pair 1-2 and 2-3.) The rest should the bms manage. (Over-charge, Over-discharge, max. 15A charge/discharge current, short circuit, etc.)

I will use some buck converters to make up the rest of the power bank; 2 pc. 5V, 3A dual usb ports (6,5-35V in), and a dc-dc 0.1-5A CC/CV charger (4.5-30V in, 0,8-30V out).
With that CC/CV charger, i can use a 12V battery as a charging source, or connect a solar-panel directly.

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