Hey there -
You DO NOT want to wire dual batteries in series. These need to be wired in parallel; ie. you want a 12 volt source but double the amperage. When you wire the batteries in series you double the voltage to 24 volts and keep the amperage the same. I believe you are reading the positve 12 v. from one battery through the other if wired in series... because the positive on one battery is connected the negative (which goes to ground) on the other battery when series'd. Rewire the batteries in parallel; negative to negative and positive to positive.. then solidly ground the negative off one battery to the frame / cage (multiple spots is always best). 1/0 copper welding cable works great! Be sure to ground the engine to the frame as well with a grounding strap (you can get at any NAPA)... I bet this solves your problem.
Good luck - PM me if have any other questions or are using an alternator.. that's another issue, lol.. Take care,
Jeff
