So I have an extra battery wired up in my truck with a marine battery combiner. It's a yellow top under my rear bumper. It charges while I drive. I have a 1000w pure sine wave inverter wired off it in the bed. It worked fine for about 8 months. Now when I turn it on it beeps and immediately turns off. The extra battery is always reading 13V. If I haven't turned it on in awhile the connections at the inverter read 13V, as soon as I turn it on it drops to 5V and turns off. What am I missing? It's 6 gauge wire, somewhere (about 3' in length) between the battery and the inverter I'm losing 8 volts.