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I have a 2006 f350 superduty 6.0l diesel and if one of the batteries are bad will it drain the good battery...had them tested and one tested good but I keep having to charge them both
Yes. If you run a bad battery in parallel with a good one, the bad one will kill the good one every time. Replace the bad one, before you end up having to replace both at the same time.
You should replace both batteries at the same time even if one checks good. The only time you only replace one would be were one battery is under 6 months to one year old.