36V 500Watt motor

[slimmy101]

Hello people of Endless-Sphere, this is my first post, I have scoured the internet far and wide and cannot find an answer to my battery question, hopefully someone has some insight here.
I bought one of those cheap Chinese 500 watt 36 volt hub motor kits from ebay and a $70 mountain bike to put it on, I have since collected about 120 18650 cells from old laptops, tested them and they average about 1500-1800 mah each. My question is, the battery back is going to be 10s but how many in parallel?? I am afraid that i may be pulling too much power from these relatively low drain batteries and cause one of them to short and flame, causing the rest of the pack to go up like a firecracker between my legs.
P.S. the pack will be balance charged with IMax b6 so no worries about an out of balance situation.

 

[slimmy101]

Hello people of Endless-Sphere, this is my first post, I have scoured the internet far and wide and cannot find an answer to my battery question, hopefully someone has some insight here.
I bought one of those cheap Chinese 500 watt 36 volt hub motor kits from ebay and a $70 mountain bike to put it on, I have since collected about 120 18650 cells from old laptops, tested them and they average about 1500-1800 mah each. My question is, the battery back is going to be 10s but how many in parallel?? I am afraid that i may be pulling too much power from these relatively low drain batteries and cause one of them to short and flame, causing the rest of the pack to go up like a firecracker between my legs.
P.S. the pack will be balance charged with IMax b6 so no worries about an out of balance situation.
Any help with what yall are running will be greatly appreciated.

 

[amberwolf]

I merged your two threads on the same thing into this first one for you.

There are actually a lot of threads here describing builds for such packs and often why they are building the way they are, if you choose to read around. There is even a very specific thread here in this subforum about how to determine what you need, test used/recycled cells, and build packs from them, by DrkAngel. It and other threads are linked in the sticky index at the top of this subforum.

But it will depend on what actual cells you have, and how good they still are, as to how many you will need to parallel. So we can't answer that for you until you know that; you'll probably have to test the cells first for their full capabilities (not just capacity) to find out.

 

[Voltron]

It's not the wattage label on the motor that matters....it's the draw from the controller. And the label on that is a who knows kind of thing until you measure it...My generic ebay controller turned out to pull about 48 amps at peak, way more than the 1500 watt label would imply. Since you're starting with low c rate cells, probably paralleling as many as you can manage to carry is the direction to go....I had to double the paralleling on mine from the initial size to keep them from heating up during hard accelerating stop and go traffic type riding

 

[slimmy101]

i'm assuming that at most, durring a hill climb the controller will pull 30A, a safe number for these cells will be 1700 mah on the absolute lowest end, and about 2000 mah on average. that gives me a range of about 25500 mah - 30000 mah if i did my math right that should be enough not to overheat the cells??

coments would be greatly appreciated.

 

[wesnewell]

Laptop cells are usually 1C cells, so if you have 120 of them, build a 10s12p pack. That will probably be about a 20-25ah pack and should work ok, but not great, with a 30A controller

 

[dogman dan]

I agree, you will need a very minimum of 20-30 ah, to make low c rate laptop cells work. I've heard of .5c being good for continuous discharge, so assuming your 20 mph cruise takes about 15 amps, 30 ah would be good.

 

[999zip999]

The battery is the heart of the system.

 

[slimmy101]

has anybody heard of what would happen if one of the cells shorts out or reverses polarity? would it be like a cascade failure and vent, causing the rest of the pack to start flaming up one by one in a glorious fireball???

 

[999zip999]

I have had 350 used sony cells from returned Makita tool packs. I had seen 4 of the cells shoot a small flame out the top. Just moving and taking the cover off. Yes no tinkering just flame out the top. These cells could have dropped or water and rust damage. This lead to a very expensive garage fire. I think the vendor tried to charge the 18v packs to 12v for sale. Many came to me at 8-10v by the time they hit my doorstep. Plus the factory plug had been removed, so the person had looked inside for the newer constructed cells. There profit my garage fire. I lost ski's, fishing poles, jewelry equipment ect. Lots of money.
I would never mess with used or low rated cells. If you want do as learning or hobby. Do be careful. Best to store in safe place. Not you garage that attached to your house. An old barbeque is best. As fire is bad.