# Why is it important to precharge the capacitors of your electrical vehicle's motor controller?

Play Video about Precharge system for lithium batteries

Every electric vehicle has an electrical motor with a motor controller and converters which input involves capacitors.

These famous capacitors require to be charged when starting the vehicle, which may cause an important current draw with potential destructive peaks.

The lack of precharge circuit can provoke the destruction of current components in the system: the battery, the PDU and capacitors.

In order to limit this current draw occurring every time the vehicle starts up, we are going to use a precharge circuit made up of a resistance and a contactor. Thus, we will be able to progressively charge the capacitors.

I can show you an PDU example, you can see the precharge resistance right here.

The precharge of capacitors involves 3 steps:

First step: the closure of the precharge track for an identified time in order to charge the system’s capacitors up to 100%.

Second step: the closure of the charge and discharge track.

Finally: the opening of the precharge track.

I am going to give you a tip to calculate the precharge time of your system. The formula is quite simple: t = 3RC.

“R” is the precharge resistance in Ohm, “C” is the capacitors value in farads and “t” is the time in seconds. Here is an example of a calculation with a 50 Ohm resistance and a 3300µf capacitor: you get a precharge time of 0.5 seconds. You have now calculated and dimensioned your precharge resistance.

There is a last point I would like to underline: it will be necessary to pick the appropriate precharge resistance and to dimension it with the proper power in order to absorb the energy during the precharge phase.

Thank you for your attention, see you next week for a new video.