My 1970's plow truck has two 12 volt batteries (same battery, same installation date) and a battery isolator. It has worked fine. For some reason, everything goes dim and gets progressively weaker ever since I replaced the batteries. The batteries check out fine. Last year a mechanic friend scratched his head and ended up attaching a jumper cable between the positive terminals. This made everything work as normal. However, it seems like an inelegant solution. Anyone understand what is going on here? Solutions?