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electric scooter battery powerpack - mixing cells
#1
hi!

im in the working of making a new power pack, upgrading the old pack from 13s to 16s to be able to keep a target speed that is the limit of my "ev-vespa" the limit is 45km/hr but have a hard time even driving 30 uphill. that aside.

the pack it self is a 16s and between 12-14 pr cell.
got around 500 cells mostly from electric bicyle.
the current draw from the factory is 33amp at nominal voltage 48V. at 12 cell row. giving a draw of 2.75A each cell on average.
the target is about 30-33amp at 60V nominal. so we are talking about 2.75amp draw pr cell at worst. and at best. 2.14amp on average.

most cells is about 2200mah each. so we are little over 1c of discharge. would there be any problems mixing cells from mostly samsung, sanyo and some unnamed brand that don't have any cell type writen on them.

sanyo: UR18650ZY T25A
samsung: N18650k P030
samsung: ICR18650-22F
to name a few
a guy that works with powerpacks for bicycle  saids i should never mix brands and last 3-4 letters/numbers like T25A and P030 should never be mixed in the same cell in parallel. 

what are you guys thoughts?

i test all cells for heat, capacity and internal resistance.

what i figured. if i mix the cells so every row has as close as possible of capacity using the tools this forum got. and don't go above 2.5A each cells. i should be pretty safe. and also check the data on the cells to make sure they are testet at this draw. or should i get another tester to see how the cell would react to this load?
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#2
Moved because posted in wrong section.
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#3
im thinking about getting a load tester. to see if the cell types can hold a load of atleast 5 amp. or can i asume as long im under 5 amp CDR it will be safe to mix cells?
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#4
Mixing cells in high drain packs is not ideal. You will need to match capacity and internal resistance as closely as possible. This is more for the sake of the cells, than the sake of your safety.
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#5
Most people here will be using cells in a configuration with as low a current per cell as they can manage.
My understanding is also that for higher current then internal resistance comes more into play.
If you are doing something out of the ordinary, then I would recommend lots of testing of what you are doing and lots of monitoring of the cells in the packs.
Maybe a "bench" setup to simulate the loads that the pack will be seeing and then monitoring of everything.
Voltage of.pack, voltage of cells, temperatures of all components.
Also check cell spec sheets and ensure that you will not be exceeding the currents that any of the cells are rated for.
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#6
i have calculated my way out of it. most cells has a CDR rating. im looking at about 10-12 amps test. figured ill get a load test that can handle lipo aswell sins i got a hungry drone.

the math involves difference in mAh. calculating it into % sins the difference in size would drain harder on the biggest capacity cell. and added on a safety margin of 1.5 so when cells get worse i still got some room in spare when the cells get worse.

best solution as most is to use fuse between each series. but this would be a big task because it's a power pack for a scooter, not a building. so i got limited space to work with.

FYI i do test for capacity and internal resistance and write it on the cells so i got that in mind.

man what a task. next time i will get new cells for a high drain powerpack like this. fun little experiment for sure.

and the monitor system for the powerpack when in use is set. the smart bms i got has it all.
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