help! Problems with kit.

[cherrytree]

I live in Belfast and am new to electric bikes. I bought a kit of ebay for my brother. Its a 36v 250w front wheel. Conhismotor I think.We used 3.6v li-ion batteries, 11 in series to power it.This worked for about 2 mile and then stopped. Now its developed a fault. Until about 3/4 throttle it turns slowly. on full throttle it goes for about 2 seconds then cuts out for about 2 seconds and then goes again for about 2 seconds and so on. Has anyone any idea what would cause this. As I am new to pedelecs any advice will be appreciated.

 

[flecc]

Li-ion batteries are usually supplied as a one piece unit complete with an electronic battery management system (BMS). We need to know more about those batteries you are using, were they supplied as part of the kit?
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[Deleted member 4366]

A rough guess: It sounds like you don't have enough amp-hours in the batteries. As Flecc says, we need to know more about them. What are they exactly?

 

[cherrytree]

Batteries to blame

Thanks Flecc & d8veh. It is the batteries. Connected 3 car batteries in series and the wheel flew around. My brother decided to test it under resistance by pressing a piece of wood on the tyre and to quote his words,"shes burning rubber" So the little kit works great.The batteries were li-ion 16340, 1Ah 3.7v from china. I put 11 into a tube(a piece of an old glass fibre cabling rod). They fitted snugly and were connected under compression,not soldered. The meter reading showed 41v. We used 6 rods connected in parallel which should give 6Ah. I know I could Buy a branded 36v battery but that would spoil the fun as i love experimenting. I read somewhere that Li-ion batteries need to be fully discharged and recharged a few times to reach there maximum performance. Is this correct? I was also thinking of using 12 batteries instead of 11. This would pump the voltage up to 45v. Would the kit cope. I will also solder them maybe in banks of 4, giving 15v per bank, What do you think. If all else fails can you recommend a good cheap 36v battery. Thanks.

 

[vhfman]

You should never fully discharge Li-ion batteries. For the Li-ion 16340 cell chemistry, discharge to a minimum 2.75V; probably safer not to go down below 3V per cell. Charge to max of 4.2V per cell. Charge has to be constant current and constant voltage. You need to monitor the cell voltages, so some form of BMS (Battery Management System) is needed. To be honest I think you are wasting your time with these cells. They are probable not capable of supplying even 1C rate. So with only 6 cells in parallel, the battery will not have enough capacity to run the hub motor. These 16340 cells are meant to be used in torches not power a BLDC motor.

If you really want to build your own battery; it would be better to go with A123 26650 cells, LiFePO4 chemistry. A123 cells seem extremely robust and stable. They have a high C rate; although the cell capacity is only 2.3Ah, they can support at least 30C constant discharge. LiFePO4 cells are nominally 3.2V; they can be run between 2.5V discharged and 3.65V fully charged. You need 12 in series for a 38V battery, 43.8V fresh off the charger; and as many in parallel to increase the Ah capacity to your requirements. They are more expensive than the cells you have been experimenting with but you can get them on ebay. Like others on this forum, I have used this supplier OMG who seems reliable and price wise about the best. I have put together a battery with 72 A123 cells, 12s6p; similar construction to jerrysimon. He has a full description of his small A123 battery build, well worth reading if you want to go for the home made battery.
Of course your other option is to get a readymade battery form BMS Battery or Ping etc.

But you sound like an experimenter to me, so.....

Cheers

Chris