I have a recurring issue with my batteries. After a few months, they are shot. I have put an upgraded hi torque starter on. The alternator is working. There is no phantom load on the electrical system. It starts fine until one day, it doesn't, and the batteries are shot. ???????? All connections are clean, no corrosion. The batteries show 12.6v, but it cranks too slow and runs down in seconds. Any thoughts?