Dual battery installation problems

[wetfootwilly]

I've revamped my charging system to include a 2nd battery/battery switch and a 30 amp breaker but I'm getting spark when I connect the two neg terminals of the batteries. I have both batteries connected to a 3 terminal battery switch and a lead from the battery switch to a 30 amp circuit breaker and from there my power line goes to the fuse box. I've checked my connections and everything is attached properly...I've read where it best to ground both batteries separately and directly to the engine as the single ground sometimes isn't strong enough - i'm thinkin maybe this is my problem? Any advice would be appreciated...thx

 

[bruceb58]

Are both batteries fully charged and do you have the batteries on Both while making the ground connection?

If you have the batteries connected together with the switch and one is discharged more than the other, of course you will get a big spark. The same as you would hooking up jumper cables.