# Precharge Circuits?



## Qer (May 7, 2008)

eco_sack said:


> In general, are precharge circuits built into the controllers?


No. In some it is but don't count on it without verifying.



eco_sack said:


> If they are not built into the controller origionally, is there one design that is better than another? Or does it depend on the brand?


The only thing the precharge does is limit the start current. You connect a resistor over the contactor either permanent or through a relay, then you don't activate the contactor until the voltage on both side of it is the same. If you activate the contactor with a voltage difference over it it will create arcing in the contactor and also risk to damage the capacitors in the controller due to excessive current rush.



eco_sack said:


> What are the main causes of controllers going bad? do people often hook up the precharge incorrectly the first time and cause the controller to go bad? Other mis-guided wiring?
> 
> Or is it the controller itself that has the issues?


Soooo not gonna touch that topic with a 10' pole...


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## eco_sack (May 29, 2009)

Qer said:


> Soooo not gonna touch that topic with a 10' pole...


Ha! Fair enough, I suppose I should have known better than to ask that.

But I will admit, I have let the smoke out of a couple of things here and there, some my fault, some not.

But that 'precharge circuit' is just like what is on the prius battery pack contactors. There is a relay that is controlled by the HV battery computer with a resistor going from one side of the contactor to the other, I just thought that it helped save the contactor.

Or similar to a light bulb charging a car stereo capacitor before you connect it directly to the battery.

Thanks!


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## blackpanther-st (Apr 4, 2009)

eco_sack said:


> But that 'precharge circuit' is just like what is on the prius battery pack contactors. There is a relay that is controlled by the HV battery computer with a resistor going from one side of the contactor to the other, I just thought that it helped save the contactor.
> 
> Or similar to a light bulb charging a car stereo capacitor before you connect it directly to the battery.
> 
> Thanks!


True, but more importantly is that is saves your capacitors in the controller from melting their own conductors due to excessive inrush current. that inrush can also weld the contactor closed  without the precharge, the inrush current is like short ciurciting your entire battery pack for a brief momment.


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