DIY Ebike battery quite warm under load

[CUDAcores89]

I finished building my electric bike a couple of weeks ago, and I have been using it to commute to and from where I work for the past couple of weeks.



I actually built the battery myself out of "dead" laptop cells as many others have done. My battery is 10S5P 36v 10AH for a total of 360 watt-hour(and yes, I DID charge and discharge each individual 18650 cell to verify it's capacity was about at least 2000mah to go into the pack). I am using a cheap 250w hub motor from ebay and counter to what others have told me, it actually works really well. Also to add, the motor was sold as being 250 watts. Except I put a watt-meter on the battery to measure the draw under load and it was reporting 450 watts, almost double what the motor was specified. Now my ride to work is a poultry 2 miles, so the battery appeared to work fine. Today I decided to go for a much longer ride and really push my battery to the limits. I went on a bike ride and (with absolutely ZERO pedaling, I wanted to see how far I could go without assist) I managed to go 15 miles on the battery. I got home and my battery felt quite warm. I quickly pulled out my IR temp sensor and recorded 110 degrees F .

Now converting that to Celsius that is around 40C. Not enough for me to worry about the battery going nuclear, but high temperatures on almost all batteries is very bad for them and can shorten their lifespan quite drastically. Theoretically, the pack should be able to take this discharge since I have chosen cells whose datasheets had specified a maximum continuous discharge of 2C. Although the cells still seem to be getting quite warm. Now that I have discovered this, I have a few options:

1. I can find a way to actively cool the battery pack. I am thinking of using something like a fan, any suggestions?

2. I can double the size of the battery to 36v 20AH. I already have the cells laying around to do this (that have been charged, discharged and resistance tested) but this will double the battery weight to 10lbs.

3. I can buy new cells and assemble a pack out of proper high-drain 18650s. This is the most expensive.

4. Just not care at all. My commute is only 2 miles anyway so the batteries are not going to spend a tremendous amount of time under full load.

Any suggestions?

 

[skeetab5780]

In a perfect world you could use higher C rated cells, but if you have the extra old cells i would put more cells in parallel. 5p wih low C rated cells will be fine for short rides at 10a but if you get to 10p your range will likely more than double since the cells arent being discharged as fast. You could always make two 5p packs and split it when going on short rides.

 

[Voltron]

Re batteries... Yeah, what he just said.

And as an fyi, if the battery is getting hot like that, possibly the motor is too, and if it's a geared motor, it's harder to notice from the outside.. also the stock connectors to the power and phase wires sometimes aren't up to the long hard sessions either.
It's worth doing lots of quick pullovers mid ride to hand check stuff as you're getting the system broken in.

 

[CUDAcores89]

Thanks for the advice guys. I'm going to add another 10S5P battery pack to my current one for really long trips. Hopefully that should solve the heat problem.

 

[Mündawg]

I agree that you should just use the extra cells you have laying around and add some parallel groups to your battery. this will add range but also will decrease the amp draw on individual parallel groups making them last longer. Specially if you already have them laying around.

 

[dogman dan]

If the pack gets too hot, it's too small. Add more for every ride.

 

[KarlJ]

My 29E 16.5AH pack gets quite warm on my rides too.
RE 60F... pfft thats nothing dont worry.

Im pulling 3C from start easing quickly to 0.5C rates of discharge. battery getting to 60C or more!

Lappie cells simply wont do 3C to start with!

On the road well done!

 

[Rconway]

my 48v 20ah battery I just added to my bike is having an issue the power wires from the battery to the controller are getting really hot? why?? can some one help I am concerned Thanks Bob C

 

[Marin]

my 48v 20ah battery I just added to my bike is having an issue the power wires from the battery to the controller are getting really hot? why?? can some one help I am concerned Thanks Bob C

Bob we need more information,

Did this happen all of a sudden?

Are you riding at full throttle and the wires are too small?

Fill us in,