Hello, I'm trying to make a 72v battery pack for an ebike. I have two 48v battery packs about to arrive which I originally planned to use before wanting to switch to 72v. problem is, if I re-configure the battery cells for 20s 5p, then my max continuous output would be about 37a. I'm pushing about 45A max so I need a higher current battery.
BMS Link
I already bought this bms for the battery.
I plan on adding more battery cells to this battery by connecting more in parallel, but does amp hours or differences in discharge current matter between the batteries? These are some of the cheapest cells I could find and they output 20A continuous which is much higher than the other cells. The battery packs I bought are from aliexpress so finding the exact same cells would be pretty difficult.
If you buy 50 batteries, you will get 50 different battery capacities. No two are exactly alike. If you isolate each parallel pack with a sufficient diode, you should be OK. Series need no isolation.
SteveMann:
If you buy 50 batteries, you will get 50 different battery capacities. No two are exactly alike. If you isolate each parallel pack with a sufficient diode, you should be OK. Series need no isolation.
Thanks for the reply. Do you know of a sufficient diode? Do you think I would be able to get by without diodes?
Seems like finding a diode that can handle 72v(86v fully charged) and 10-35A would be prety difficult. I talked with my brother in law today who is also an electric engineer and he also suggested the diodes. After I told him that might be unrealistic, he believes I shouldn't have any trouble without them to his knowledge. However, he also pointed out that he is no battery expert. He said the higher discharge batteries would drain first until the voltage dropped below the other ones in which the others would kick in.
Lithium batteries in a series and parallel arrangement must be very well matched.
A weak cell can be driven too low (or even in reverse) on discharge or be overcharged on charge.
Lithium batteries can explode or catch fire when abused in this way.
srnet:
Lithium batteries in a series and parallel arrangement must be very well matched.A weak cell can be driven too low (or even in reverse) on discharge or be overcharged on charge.
Lithium batteries can explode or catch fire when abused in this way.
Edit: I'm using a BMS which I believe is intended to prevent overcharging.
Well my issue is I bought two identical 48v battery packs each with 52 cells. So 104 total. They are arranged 13s 4p.
Bought them from a Chinese website so not sure how reliable this information is but they claim 30A max continuous and 90A instantaneous per battery pack. 30/4 =7.5 So the battery cells are rated at 7.5A continuous 22.5A max. Website also says their capacity is 3400mah . So, if I were to take 100 of the cells and combine them into 20s 5p, I would end up with 37.5A continuous and 112.5A instantaneous.
My controller is 30A continuous 60A max. I have the max set at 45A at the moment though. From my experience the controller will push over 40a without issues from quite some time.
So, now the question is, is it safer to run the battery near it's limits or add some different cells to help out with the discharge. I can always add a second battery on a relay in the future instead for more range.
Any thoughts?
05silgto:
I'm using a BMS which I believe is intended to prevent overcharging.
Does it monitor each of the 100 individual cells seperatly ?