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Need advice paralleling 18650s
#1
Hi to all! I'm pretty newbie with Ev's and batteries but i have some knowlege about electronics. So, it's better way to ask than do something wrong. And sorry about my rusty english language ? Found this forum month ago and still learnig from all of you, and thank you all! This is my first build so dont be harsh if i do mistakes ?

I bough old escooter and want to make 18650 pack for it. Right now i'm in process of harvesting cells from old laptop batteries and going well, testing it with liitokala 500, let them to stay about month so testing volatage again, separate them...
Motor on escoot is 350w 36v 13a, i order speed controller 500w 48v 30a, i know motor can draw much amps form start till reach speeds, and i tested on 48v sla and it goes well, just a little bit warm after 3-4 miles of ride.
Now back on batteries, my plan is to make larger parallel blocks because i know batteries from laptop is low discharge
1. if i put 14-15p in block is that will be enough for safe 30a draw?
2.can i make one parallel row with let say 15x2000mah and another 16x1900 just to compensate lost of mah? And how much difference in mah i can put in one row, like from 2000 to 2200 mah will be ok?
3. Any advice for good bms for 14s15p, it will be around 30ah pack and i want to charge it for around 8 hours, how much amps for safe charging 18650s? If my calculations are good i think i can safe charge it with 7-8a for row of 15cells? But what happen when i put them in 14 series, do i need bms with 8a or i need bms 14x8a?

Thanks in advice! Greetings from Montenegro
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#2
Welcome to the forum.

(08-28-2019, 02:47 PM)Funkadelicious Wrote: 1. if i put 14-15p in block is that will be enough for safe 30a draw?
2.can i make one parallel row with let say 15x2000mah and another 16x1900 just to compensate lost of mah? And how much difference in mah i can put in one row, like from 2000 to 2200 mah will be ok?
3. Any advice for good bms for 14s15p, it will be around 30ah pack and i want to charge it for around 8 hours?

1) Using laptop cell, in reality, no. With 15p with 30a is 2A per cell. WAY to high for laptop cells. These cell should only be used around 1A max on a continual basis.
However, even though your speed controller is rated for 30A, you won't see that because the motor is rated at 350W. Translate from 36V to 48V that makes it 7.29A (350W / 48V)
So if you figure 8A with 15A surge (mostly at start up), 15p could work.

2) Each pack "MUST" be as close to mAh capacity as possible. So the number in each pack doesn't matter in this instance. However, changing the number of cells from pack to pack will stress the cells in the smaller packs under heavier loads as the amps are shared across all cells. So basically the smaller packs would get worked harder than the larger packs and will die sooner.
Proceed with caution. Knowledge is Power! Literally! Cool 
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#3
Thank you, that is all i need! I go with same number of cells in parallel and look to get it close as i can
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#4
2A is really not a big issue with laptop cells as long as you tested it at that rate. I have multiple packs on ebikes doing 5+A per cell and still working fine... But I tested them at 5A to make sure their capacity equals and that their IR is still good.
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#5
I plan to test IR and discharge rates, just must get the equipment.. any advice for discarge board with 2a rates, or even better with discharge control, max 5a? Anyway my plan is to get bms with discarge limit control so i can start with low amps just to be sure and gradualy increase with testing. Its my first pack, still collecting 18650s, right now i have around 60psc 2000-2200mah branded, it's not huge difference in mAh right?
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