Question about multiple smaller battery packs.

wtfemery

New Member
Hello, New here,

I was doing some research on cheap lithium battery packs for a 48v 1000w bike I'm planning on building this winter on a very cheap college student budget. Even on aliexpress, 48V 24+AH batteries are well over $500, but 24V 12AH packs are as cheap as $65 shipped (given a very long shipping time, but thats not very important to me). My question is, what are the pros and cons of running four of these packs in series parallel to get 48V and 24AH? Am I missing something? From what I am seeing, this method is cheaper than a larger battery pack by FAR. Ideas?

Heres the link for a 24v pack:

https://www.aliexpress.com/item/24-...lgo_pvid=eba230cc-bc36-4972-bf97-75a56662452d

Thanks
 

indianajo

Well-Known Member
If you get a bad battery pack you can't ship it back, you are not a certified haz mat material shipper.
I recieved one from btrbattery of amazon, went full charge to dead in 7 miles, got my money back. I received one from sun.lc vie ebay that was okay at 1 amp but collapsed to 16 v at 3 amps, did that test too late to get my money back.
Parallel LIPolimer batteries does not work without a battery management system. They are prone to developing lower voltage and only half the battery is used until a full balance charge is managed by the BMS. Full balance charges shorten the life of the battery, only do that every 3 months or so.
 

wtfemery

New Member
If you get a bad battery pack you can't ship it back, you are not a certified haz mat material shipper.
I recieved one from btrbattery of amazon, went full charge to dead in 7 miles, got my money back. I received one from sun.lc vie ebay that was okay at 1 amp but collapsed to 16 v at 3 amps, did that test too late to get my money back.
Parallel LIPolimer batteries does not work without a battery management system. They are prone to developing lower voltage and only half the battery is used until a full balance charge is managed by the BMS. Full balance charges shorten the life of the battery, only do that every 3 months or so.

Thank you, that helps a lot. But it still doesn't explain why these companies charge so much more for larger packs if they contain the same amount of cells as multiple smaller ones. ugh. guess the demand is a lot higher...
 

indianajo

Well-Known Member
Another scam is using laptop or flashlight cells for bike batteries. there are several types of 18650's. My ebike battery has a max discharge rate of 50 amps. That is fine for my controller of 26.5 max amps @ 1000W. Search engines can't find spec sheets for the import cells, only for unicorn like US made cells
I had a "new" defective battery just fail to deliver 3 amps - 48 v bat collapsed to 16 v with 3 amp load. The BMS makes sure the current is shared between the parallal stacks. If one or more stack goes low voltage, that stack no longer delivers current to the shared load. That just stresses the working cell stacks more.
So if the 24 v battery vendor has specifications (not usual in the alibaba world) check the max discharge rate. You may have found a flashlight battery.
 
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harryS

Well-Known Member
There's nothing intrinsically wrong with the concept of connecting stand alone battery packs in parallel and in series. It's not a fail safe deal though, and there are some risks with doing this. I leave it to you to research them. I often connect two 36V4ah packs to get 8Ah, and I sometimes put a booster 12V battery on my 36V packs to get 48V.

The main problem I see here is low price batteries of unknown quality. I have to figure the seller is pricing these for about 100% margin. That means the 40 cells are around 60-80 cents each. How that could be anything better than cheap junk is pretty unlikely, in my opinion. They won't last long and hopefully will not catch on fire, but they could.
 
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