precharge resistor question

[dcastillo]

Question, On my bigger gokart I used a mechanical Estop instead of a contactor to connect the Battery to the controller. Per the wiring diagram I installed a 1K precharge resistor across the EStop. If I understand correctly, the purpose is to prevent the large inrush current from the caps in the controller right?

My question is on my smaller 350Watt controller Im installing now I see no indication that I shoudl add one, but when I connect the battery to the controller I get a nice spark/arc which I assume is the Caps charging.... Should I add one to the smaller controller as well?

If it makes a difference, the controller has a two wire harness to connect the batteries to, I was going to break the 24V side with a 30A mechanical switch, should I just put the resistor across the terminals of the switch so when the switch is open, theres actually a 1K load to allow the caps to precharge?

thanx
Danny

 

[G1050]

Yes, you should add one if you regularly connect and disconnect the batteries from the controller. The spark will damage the connector/plug over time (or the contacts of a switch) and can hurt the electronics in the controller over time, as well. Once in a while should be fine, but otherwise I'd install a precharge resistor.

Since you're just using 24 volts, you could use a connector that already has a precharge resistor built in like this:
http://www.hobbyking.com/hobbyking/store/__61690__XT90_S_Anti_Spark_Connector_2pairs_bag_.html

Or you could wire up a switch with a precharge resistor, like you mention. One option someone else mentioned is you could use a three-position switch - ON-OFF-ON. One of the ON positions would go straight through, the other would go through a resistor. When you go to turn on the machine, turn it to CHARGE for a few seconds then ON. I don't use a switch between the battery and controller, so I don't have direct experience with that.

Or wire a resistor in with the connector so the resistor makes contact with the other connector before the connector does. Here's an example of both:
https://endless-sphere.com/forums/viewtopic.php?f=2&t=75048

 

[dcastillo]

Thanx for the reply... now I need to rethink this setup as I have finished the cart...

currently I have a 30A on/off switch that directly connects the battery to the controller. when closed it also connects a 2 wire volt meter. When i tried adding the precharge resistor, it allowed the volt meter to always stay on...
I guess one easy option is to add a very small switch to the voltmeter.

Question, if I do that and add a 1K resistor to the terminals of the switch. then when the gokart is just sitting in the garage, it will be discharging at about 24mA. Is that bad to have it always doing that? Maybe since the precharge resistor is always connected I can use a much larger value, like 10K?

thoughts?

thanx for the help
Danny

 

[G1050]

I think it would only be bad to do that if you left it alone for long enough to completely discharge the batteries. If it's plugged into a smart charger though, then I don't see a problem with it.

I believe that problem would be avoided by using an OFF-ON-OFF switch with the resistor on one leg and full power the other and no power whatsoever when off. If you just add a second small switch for the voltmeter, the controller will still be getting a small amount of power, which may be a problem if you leave it for a long time without charging.

I don't think upping the resistor to a 10K will help prevent the long term battery drain, but I'm no expert and can't say for sure.

 

[dcastillo]

Just had a thought, For an easy solution, I can just add a momentary button that connects the batteries to the controller thru a precharge resistor.... So I just press that button for a few seconds to charge the caps, then turn the switch on...

does that seem reasonable?

 

[G1050]

Yes, I think that would be a good solution. At least, I can't think of why not.