# Precharging?



## bblocher (Jul 30, 2008)

I just ran across this term twice in the last day doing some research. I also keep finding mixed opinions on whether it's even necessary. I read it has to do with precharging the internal capacitors to the controller which avoids an inrush of current and will prolong the life of the controller.

How exactly do you do this? Contect x voltage to A and B terminals on the controller until y happens?

Anybody who can explain this in detail please help


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## Qer (May 7, 2008)

bblocher said:


> How exactly do you do this? Contect x voltage to A and B terminals on the controller until y happens?


Precharge is that you slowly charge the capacitors in the controller to the pack voltage in an orderly fashion. If you just turn on the controller with the contactor, the capacitors in the controller will act like a brief short circuit that will result in a current rush so to be nice to the capacitors you can choose to precharge them. Like soft start for light bulbs extends their life span, the precharge will extend the life span of the capacitors and the controller.

The simplest approach is to connect a resistor of some hundred or thousands of Ohm across the contactor so that the controller always have the pack voltage over itself but without the ability to draw power. It's simple and only needs an extra resistor, but it also means for example that touching the controllers terminals will be dangerous even when the car is turned off.

The somewhat more expensive solution is to put a relay in series with the resistor and connect it to the ignition key so that the relay connects at the first position of the key, giving power to the controller through the resistor so the resistor keeps the current rush low. Then, when you turn the key to the second position the main contactor goes on and bypass the resistor.

The deluxe version is that you have a timer circuit that always guarantee that the relay (with the resistor) is activated first and the timer delays the activation of the contactor.


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## Twilly (Jan 22, 2008)

I had never heard that not having a precharge resistor was bad for the controller, but it is bad for your main contactor. The constant arcing every time you turn on the ignition will damage the contactors contacts.


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## frodus (Apr 12, 2008)

Caps in the controller don't like to be slammed with current. When you slam a full pack across the contactor and into the caps at full current, their lives can be decreased. Caps are usually across the B+ and B- of the controller. When you slam them full on, they don't really like it.

Simply put, they have a max "rate of charging" and anything past that, can stress them.

Its also bad for the contactor to arc, it pits the contacts and decreases the life. If you switch while there is little or no current, you increase the life to almost infinite (not switching under load).


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## bblocher (Jul 30, 2008)

I was planning on using two contactors, one ignition triggered, one accelerator triggered. Also some safety features that would disengage one. 

So if you do run resistors across the terminals on the contactor you're basically powering up your controller. So now lets say I left my fuel door open which is one of my safety features and now when I give it the throttle the second main contactor does not close. However the controller is powered up and ready to go trying to draw current for the motor. Does the little resistor burn out in this scenario or does the car just not move because the current is so low?

So if I did use say a 750 ohm resistor and I'm running at 144 volts this would allow .192 amps through and I would need more than a 27 watt resistor to make sure I didn't fry it right? The motor would receive power it would seem, not enough to make anything happen I guess?

I'm I on the right track? Anybody have a setup like this where it could happen?


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## frodus (Apr 12, 2008)

> I was planning on using two contactors, one ignition triggered, one accelerator triggered. Also some safety features that would disengage one.


why? just trigger one contactor through a series of "conditions" that must be met, and if they're not, drop the contactor.



> So if you do run resistors across the terminals on the contactor you're basically powering up your controller.


you're only precharging the caps to pack voltage. The controller won't turn on without a key input to it, and/or a start input to tell it to go.



> So now lets say I left my fuel door open which is one of my safety features and now when I give it the throttle the second main contactor does not close. However the controller is powered up and ready to go trying to draw current for the motor. Does the little resistor burn out in this scenario or does the car just not move because the current is so low?


 You shouldn't enable the controller in that case. All of your "safety disconnects" like fuel door, accelerator NOT in the 0 position, keyswitch off, hood up... they should all drop out the relay that POWERS the contactor. If any one of those is not made, it won't turn on the contactor. This will also drop out any enable signal to the controller, which will stop the controller from going.



> So if I did use say a 750 ohm resistor and I'm running at 144 volts this would allow .192 amps through and I would need more than a 27 watt resistor to make sure I didn't fry it right? The motor would receive power it would seem, not enough to make anything happen I guess?


if there is no supply of current, the controller will discharge the caps as soon as you try to go, and go into low voltage cutoff and fault.


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## bblocher (Jul 30, 2008)

I have two contactors because it was "recommended" for extra safety. Basically you could turn on the key, go under the hood to check something out and there isn't a complete circuit yet until all the safety checks pass and the accelerator is pressed. Not having the experience I just spent the extra money. Sounds like a hood sensor would be a cheaper solution 

I have the Curtis 1231C with the PB6 potbox. So when you start to press the accelerator there is a switch that triggers a lead to the controller. So if I understand correctly the controller isn't really on until that signal lead from the potbox is closed, or maybe open (I didn't check to see which way it works).

If that's the case it sounds like my safety features need to keep the secondary contactor from closing AND not send the potbox switch signal until all safety checks pass.


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## frodus (Apr 12, 2008)

thats how I'd do it.

Basically on the low voltage side you have:

+12V goes through the key switch, then to the hood switch, then to the gas door switch then to an aux interrupt (if used) then to one side of the coil on the contactor. Lets call the "node" at the 12V in to the contactor N1.

The other side of the contactor, lets call that N2. That goes to COM of the 12V system. Remember that you need to put a diode across the contactor coils (across N1 and N2. unless its an EV200, in which case it has an economizer with diode in it).

I don't know how your switch goes (some are pack voltage, some are 12V). If its 12V, then tap off the N1 node, go through the throttle contacts (the ones that are closed when the foot is off throttle) and then to the keyswitch input on your controller. That way when you're sitting there with the key on, hood shut and the gas door shut, the contactor closes and the controller is energized. If the throttle is in the fully off position, the controller is ENABLED, and as soon as you push down, it should accelerate. If you turn the key on, hood shut with gas door shut with your foot on the throttle, the car won't go until the throttle goes to 0, and THEN its enabled.

Its called safe start. Controllers won't go unless the throttle position is at 0.


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## bblocher (Jul 30, 2008)

Sounds good.

I do have 12v coils on my contactors but was looking at them now that you asked. They sold me two Albright Contactor SW-200 (12V coil). I was looking at EVParts and found specs (if they are correct) and they say 120v, 250amp cont and 360max. I plan on running at 144v (which is slightly over but maybe ok?) but it seems I want to have the 500 amp max support that my controller does. Should I replace both of these or just run them in parrallel?


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## jlsawell (Apr 4, 2008)

frodus said:


> That way when you're sitting there with the key on, hood shut and the gas door shut, the contactor closes and the controller is energized. If the throttle is in the fully off position, the controller is ENABLED, and as soon as you push down, it should accelerate. If you turn the key on, hood shut with gas door shut with your foot on the throttle, the car won't go until the throttle goes to 0, and THEN its enabled.
> 
> Its called safe start. Controllers won't go unless the throttle position is at 0.


Fordus, I just wanna add one thing to your setup to explain mine. Since I'm using an auto gearbox, I have a missile switch next to the ignition which will supply 12v power to a small "idle" pot first. That way I'm sitting there with the doors closed and controller charged, but no signal. I plan to shout "Warp 9, engage!" in my best Kirk, flick the missile switch and the controller will start to turn the motor about 200RPM. As I press the pedal, the PB6 will take over from the small pot and we drive away like everyone else.

It also means for extended delays I can easily turn off the juice and save pack power while stopped at lights. With throttle at 0, I can just close the switch and the motor will stop. As the lights go green I can flick the switch and about 1 second later - away we go.

I just need to find out the specs for that small pot. There's a post in here somewhere with a circuit diagram...


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## bblocher (Jul 30, 2008)

bblocher said:


> Sounds good.
> 
> I do have 12v coils on my contactors but was looking at them now that you asked. They sold me two Albright Contactor SW-200 (12V coil). I was looking at EVParts and found specs (if they are correct) and they say 120v, 250amp cont and 360max. I plan on running at 144v (which is slightly over but maybe ok?) but it seems I want to have the 500 amp max support that my controller does. Should I replace both of these or just run them in parrallel?


I found a graph that shows these amp ratings aren't quite accurate. The 250amp is continuous but it shows amps all the way up to 1500 and how many seconds it can handle those amps. They can handle 500 amps up to 5 minutes. I believe controller time at max amps is that or less. 300 amps was close to 15 minutes straight so looks like these are fine for my application.


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## bblocher (Jul 30, 2008)

Ok so now I have another question in regards to the resistor to use. So I found a resistor that is rated for a working voltage of 200V, it has 250 ohm resistance and is rated for 3 watts. These are just some numbers from a resistor, not one that is necessarily picked out for my application.

Anyways, what I don't understand is the math on this. Using ohms law the amps that should be going through a 200 volt circuit with 250 ohms of resistance would be 200/250 = .8 amps. Am I wrong to then also say that 200v * .8 amps = 160 watts, not 3. Why is this rated for only 3 watts when the volts and ohms suggest it would need to be higher. Is this because you would need something else in the circuit to make up the rest of the resistance?

Just trying to really know why a certain resistor is rated for this application as oppose to just buying somebody is saying will work


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