Problem with new Battery


New member
Local time
9:27 PM
Jun 12, 2024
New Zealand
I have a 6 year old 36v ebike conversion, with 250w front hub motor and 12AH Controller and 12AH battery.
I recently bought a new 36v 30AH battery, but kept the old controller.
The new battery charges ok, up to 40v . But after a few miles of riding , about 20% discharged , the battery cuts out. Voltage then shows about 37v.
Do I need to get a new controller, or is it likely that the fault is in the new 30AH battery?
What brand battery? And how much? For many batteries, the price and brand name is telling of it’s quality.
A voltage of 37V when the charge is at 80% makes some sense. The open-circuit voltage of a 36VDC lithium-ion battery remains well above the nominal value until the charge is above 25-50%. After that it drops rapidly. I add a typical “voltage vs. remaining charge” chart of a lithium-ion battery.


Certainly there is a possibility that the battery is sized incorrectly, or some cells are not working, and thus the voltage goes below the set minimum.

Have you noticed any overheating anywhere? Is your path flat?
Thanks for the reply. There's no overheating and the terrain is flat. I've had 2 rides and the battery has cut out at the same place on the ride. I'll try a new controller, the battery seems to be behaving normally, apart from cutting out! Cheers.
You welcome....

How many miles are "a few miles"?

Your battery has a nominal 432 Wh capacity. For an average ebike on a flat and sealed road, I would expect a 28 miles range from such a battery. That means you should theoretically reach 80% at 5-6 miles. If your "few miles" is more or less that, it may actually be the controller.
It was about 5 miles, so I think you're right about the controller. I would have got a new controller earlier, but I think the new controllers may have different connecting plugs, but hopefully I can figure that out. Thanks again.