Battery Terminal Connections

My plan is to replace the terminals with something heavy duty and buy a wire brush terminal cleaner so I can clean the heck out of the terminals and crank down on the connection. The only question I now have is the gauge of the wire so can buy the proper size. Any idea?

The battery might be 5 years old, but it shows a full charge and the problem existed even when the battery was new. I expext the cables are as old as the boat.

Every year, I fully charge my battery, clean the posts and terminals, and attach the terminals to the posts with enough pressure to make them secure without distorting the lead terminals. This year I added dielectric grease to ward off corrosion. This year, like every year, the boat runs great for about one month and then refuses to start. The boat acts like the main breaker is off – no blower, no idiot light, no pump, etc. After wiggling the batter terminals and/or cleaning and reattaching them again, they all light up and the boat starts fine. There is no visible corrosion, dirt, grime, etc., that would explain the bad connection. If I tighten the terminals any more, it will distort and the open ends will meet.

Are there terminals with sturdier metal? Is it possible that the problem is in the cables? Any suggestions?