grounding issue?

When I try to hook up a 24/12 volt 20 amp converter to the 24 vol positive post (going to the starter) inside the battery box, I get an arc at the positive post.
any ideas what is happening?

thanks,

Common on any switched mode power supply. There is a large capacitance on the input side (24 volt) which acts like an instant short until the voltage equals battery voltage. For safety, most have a high value bleed resistor or other load to drain this voltage when off. Which means the converter will eventually drain the battery. It would be beneficial if you installed a switch on the input if you have to store the vehicle.

Make sure you have the correct polarity, converters don't take reverse polarity well. Better ones are fuse protected.

thanks for the reply!
ok, so found out the 24/12v -20amp converter was bad (smoking wires when hooked up to 12v battery).
Now, what is the correct connection to the batteries?

I have heard.... take 24v source from the post inside the battery box and ground wire from the shunt to the 24/12v converter....
this will take from both batteries, not just one.
is this correct?
or is it best to take from the 24v battery post and rear ground post on the rear battery? (like I have now)

All power supplies will have a draw on the supply side (24 volts in this case) so it is best to take from a switched source so you won't drain batteries when the vehicle is off. Get your negative from the rear battery.

ok,couple more questions
I had the positive coming off the center (24 volt)post of the front battery with an in line fuse.
found that in line fuse holder is now bad too.
I used to remove the fuse each time I shut the truck off.

could I set up a rocker/toggle switch....between the inline fuse and the 24/12 step down converter?
this would have to be a 24 volt switch, correct?

I would use a large cutoff switch or a relay to switch it. A small toggle switch cant handle the current of the converter under load.