Spark when connecting the batteries to the controller?

[Flx]

Hi
I am using four large 12V lead gel batteries in a series configuration (which gives me 48V) for driving a 5kW BLDC motor. When connecting the battery to the VEC300 controller, there is a bright spark and a pretty load "bang". 
I assume that means that a lot of current is flowing at this pont of time even though the throttle is on 0-speed and the motor is not rotating.

My wiring seems to be correct though. I can control the motor speed, reverse, brake, etc..  So, my questions are:
- Is it normal that there is a spark when connecting the controller to the batteries?
- Any technical explanation why there is a spark? I am an engineer and I would like to understand... 

[Bikemad]

Hi Flx andto the forum.

The spark you have observed is normal and is caused by a very short but very high inrush current as voltage is quickly being transferred from the battery to charge the capacitors in the controller up from 0V to full battery voltage.

High current controllers (which tend to have bigger capacitors and a higher residual current draw), tend to use sealed contactors (heavy duty relays) to eliminate any external sparks.
The contactor is usually controlled by the controller after a built in precharge system allows the capacitors to be charged using a limited current until they are close to battery voltage, allowing the contactor to then be switched on.

A manual option uses simple resistor and a momentary push button switch to precharge the capacitors for a few seconds  before the main switch is closed. The following simple precharge switch circuit works great with lower current hub motors:



But you would need to use higher wattage resistors and a switch capable of carrying a much higher current. 

Alan
 

[Flx]

Ok, that makes sense. I like the idea of adding a resistor for charging the capacitors.
Thank you for the fast and useful answer.