best pre-charge solution

[nonlineartom]

I'm moving up to 24s battery on my latest build and with the spark I'm getting at 18s I'm thinking a Pre-charge would a good idea. What resistor will I need for the best solution?

I was hoping to use a Rally cutoff switch like this..

Mounting it in a small junction box and run the pre-charge circuit parallel to the cut off switch with a momentary push switch, so the start up would go like this

Plug battery in (no current flowing yet)

hold down the push switch to complete the circuit with the resistor to charge the capacitors

Turn the key to complete the main circuit

release pre-charge circuit button.


Does this sound about right? and are those 12V cut off keys really ok with taking 100V 80A?

 

[fechter]

That should work. On many setups you can just leave the precharge resistor permanently connected as there is near zero drain when the controller is off. This assumes you have separate switch for the controller logic.

 

[nonlineartom]

what rating resistor to use?

Is there a risk putting a potential 8Kw through that switch?

 

[Gow864]

That should work. On many setups you can just leave the precharge resistor permanently connected as there is near zero drain when the controller is off. This assumes you have separate switch for the controller logic.

This is how I've done it in the past. works like a charm.

Gow.

 

[wesnewell]

what rating resistor to use?

Is there a risk putting a potential 8Kw through that switch?

I'm using 170 ohm 30W resistor circuit. Actual 3 510 ohm 10W resistors in parallel. Works great for 12-24s lipo. Still got a pretty good spark using only 2 resistors at ~ 255 ohm at 100V. If I were buying, I'd get 150 ohm 25W. Don't know about that switch. I just use bullets to connect mine.

 

[jonescg]

On my wife's e-bike, I put a precharge resistor across the terminals of a switch on the front of the battery box. In the OFF position, the current can only flow through the resistor, so plugging the controller in doesn't result in a massive spark. Then when it's plugged in she flicks it to on. If she tries to ride away without flicking the switch, it just limps along. But the resistor doesn't seem phased :?

On a bigger system I'd use a precharge circuit with a contactor, but this needs a key, contactor and auxiliary power supply; things you probably wouldn't bother with on an e-bike.

 

[fechter]

I use a 1K 5W resistor. If you forget to turn on the main switch and fire up the controller, the resistor will limit the current to less than 1mA per volt of battery. So, with a 72v battery, 72mA is the most the resistor can pass and nothing blows up.

 

[Electricdirtbiker]

I like using a light bulb of some higher voltage than the battery.
There is still a tiny spark, but the charge up is faster and more complete, because
a bulbs resistance is lower when it's cold and higher when its hot.
On my 50v system, I ended up with a 20 watt, 110vac halogen bulb.
Watch it with a volt meter and see the charge up.
Good luck

 

[Hardcarve1]

I have done the same as what you are going to do and it works great.
Just went with a bigger marine switch that can carry 300amp continuous