Because it's ur convert that not putting enough juice out and drains the btterries. I have a 460 ford engine and my batteries were NW and I charged thm and next day dead. Forget it! My mechanic told me 95% of time these symptoms are converter. He put new converter in. It doesn't drain anymore. Driving down road and conks out ended up having to replace in tank fuel pump. Presto no more stalling out.