DC power is reduced by the length of the cables. A voltage drop of lower than 10 volts will negate the dual battery due to distance. A many stranded cable ( much more expensive) will conduct much easier than a course stranded one. Find the voltage drop, you find the problem area. A clamp over the cable meter will help immediately to find the drop in current flow. It can simply be the contacts, ie, worn burned won't conduct power completely or not at all at times.
If you have a bad cell in one of the batteries it can rob the system of capability.
Last edited by papa tiger; 09-07-2012 at 12:15 PM.