36v controller cutoff


New member
Local time
7:36 AM
Jul 8, 2018
I have a 36v hub motor and controller (don't the current rating). I used to have a 36v, 7AH SLA battery pack installed. It works fine until I just upgrade my battery to a 36v, 15AH LiFePO4 battery pack. Everytime when I push the throttle to the maximum, the motor runs about 5 seconds then stops. In order to make it run again, I have to turn the power key off then on again.

I wonder is this because my controller is designed for 7AH battery only, not the 15AH? Does this make sense?
Have you tried with the headlight on? Or some other way of measuring voltage? It could be the voltage drops to a critical level and caused the controller to fail and then need to be reset.
I have this happen with a pingping battery pack on my bicycle. The pack in this case has a battery management system that's pretty eager about limiting the current to the controller. Hitting the throttle makes the controller draw more than the BMS wishes to send and to preserve the battery it cuts the connection.

Maybe your bicycle is teaching you about moderation? e.g. what happens when you do not floor the accelerator but do a more gentle start? ;-)
Which would be the opposite my theory. Instead of a voltage sag, the failure would be voltage not dropping enough causing current to go too high.

A voltage measurement would tell us exactly what is going on. Home depot carries a cheap ananlog voltimeter for $3.99
Maybe your bicycle is teaching you about moderation? e.g. what happens when you do not floor the accelerator but do a more gentle start? ;-)

The new battery has quite a bit more reserve, so it stands to reason any initial surge will be greater. That increased surge will drop voltage and perhaps increase amp draw beyond the parameters of the controller so OFF it goes. I remember reading about this before. The operator discovered he had to start out s-l-o-w-l-y until he had about 25% of maximum speed and THEN he could "floor it" and the controller would stay on.
To start gently and gradually increase the throttle does work. However, when I start climbing, I can never turn throttle to the maximum.

The thing really bugs me is I don't know if this is because of the new battery's BMS system or my hub motor's controller does this cutoff. If hub motor's controller cannot handle the big current drain, I might be able to update my controller. If it is battery's issue, I cannot do anything about it.
In my case -- I also have a rather large rheostat that has high wattage capacity. I have attached it to the battery pack along with an amp meter and played with different discharge rates. The battery pack itself cuts out at "high" discharge rates which appear to be related to the wattage of the discharge rather than the amps of the discharge.
Using your rheostat to apply different-sized loads to your BMS to see when it cuts out is fine but you're also indirectly measuring amps. If your voltage is constant, amps will increase as the wattage increases, right? If the voltage drops significantly (under load -- like full acceleration from a stop OR climbing a hill), then the amperage increases to compensate . . . attempting to meet the demand wattage. I suspect voltage is dropping below the BMS parameter OR the amperage under load is exceeding the controller parameter (OR both but more likely one of 'em is cutting out first). Any difference when the battery is freshly charged vs. partially discharged? Assuming all your contacts are fresh and clean?
Your LiFePO4 battery pack has a built in protection circuit that limits the maximum current that the pack will source. Many of these monitor circuits (mine included) will shut the flow of current off rather than just limit it at it's peak level.
I measured the voltage when it's been cutoff. The output of battery is still 40V. However, the input on the controller PCB has dropped to 0. Does this mean the controller shuts it off? Another thing I don't understand is why I turn the key off then on, the controller will go back to normal.

If it is controller to shut off the power supply, what can I do to fix this issue? Buy a controller with larger current rating? Thanks!
There should be matching between the power of motor, the controller and the battery pack otherwise the things go wrong.
Your controllers only source of power is from the battery pack so that when the current flow gets interrupted the MOSFET driver circuit in the controller shuts off the MOSFETS and the load that the battery sees goes from very low to almost open circuit. This is the state that the controller is in when you probe it with the meter. Turning the key to off forces the MOSFET driver circuit to reset and return back to it's normal Pulse Width Modulation operation.
If it is controller to shut off the power supply, what can I do to fix this issue? Buy a controller with larger current rating?
If the input on your controller has dropped to 0, it sounds to me like the BMS on the battery is what it is cutting out -- which would be consistent with you not having controller problems before the battery was replaced. Again, do you have any recourse against the dealer you bought the battery from? If your bike was working fine before . . . and NOW it's not, it would seem the dealer should have SOME motivation to help you find the problem?

If nothing else, he should be able to provide the design parameters for the BMS. In other words, at what voltage does the BMS cut out on the low side? I suspect full acceleration (from a stop or while going up a hill) causes the voltage to drop below the BMS parameter and it's just cutting out. When you turn the key OFF and then back ON, it resets and has no memory of what just happened. Again, do you know how many watts your hub drive will pull? What would tell the REAL story is a low-voltage amp probe which would actually measure the amps of draw under such maximum load situations. Maybe someone at an electronics store might help you out? A friend of mine does adaptive equipment for developmentally disabled people in a state-run institution where I used to work. I borrowed his to do some 12-volt measuring . . . I don't remember if it went up to 40 volts are not . . . but they are available if you can't borrow one.