# Estimating Precharge time



## PTCruisin (Nov 19, 2009)

I'm in the middle of installing the HV wiring in my EV and will be using a 820 ohm, 10W precharge resistor across the contactor connected to the B+ terminal of the controller (Logisystems 144V/750A). 

I want to delay the closure of this contactor relative to the key-switch closure of the first contactor (on the neg. terminal of the traction pack).

I've built a simple circuit that uses the charge time of a capacitor across a 6.2V zener diode to turn on a darlington pair of transistors that close the contactor. I can vary the delay by changing the RC values in the 12V charge path.

So, what would be a reasonable length of time to allow for precharging?

I've tested the circuit at 4 seconds and at 8 seconds. I'm hoping that 4 to 5 seconds is sufficient.

If I knew the total capacitance of the controller then I could calculate the time constant, yes?


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## major (Apr 4, 2008)

PTCruisin said:


> If I knew the total capacitance of the controller then I could calculate the time constant, yes?


Hi PT,

Just put a voltmeter across the resistor and time it down to a volt or two after you close the negative contactor. Maybe 5 or 10 volts would be close enough.

major


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## PTCruisin (Nov 19, 2009)

major said:


> Hi PT,
> 
> Just put a voltmeter across the resistor and time it down to a volt or two after you close the negative contactor. Maybe 5 or 10 volts would be close enough.
> 
> major


Thanks major. I would just measure it if I had the LiFePO4 batteries I ordered back in January. Still waiting on EV Components to deliver.


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