Why do batteries keep dying????

[Stamey711]

Ok guys I've read and read and idk what's going on I have a 2004 hd edition f250 6.0 and the batteries keep dying if the truck sits for a day or two I put a brand new alternator on it 2 weeks ago and it cranked fine even after sitting 3 days... I can drive the truck everyday and it won't die bit I it sits it won't do anything but click click click.... Help please? I tested the batteries but this was after I jumped it off and for it to my buddy's shop and on the digital tester one of them tested bad so we used another tester and they tested good and charging system was working fine I'm positive I messed the ficm up letting this happen a few times Bc truck runs like crap until warm and that's the code it throws but does this have any effect on the batteries? What should I do?

[Snake]

Start with the simple stuff first. Did you replace both batteries at the same time? What kind of alternator did you put on?

[Stamey711]

I haven't replaced the batteries yet that was my next thing to do... The alternator is from O'reilieys it was under warranty Bc I put one on it 2 years ago

[Snake]

Get the batteries load tested and replace both if one fails the test.

[dirty outlaw]

Mine did the same thing actually went threw three alternators and the last time the alternator took a crap it took out one battery so I replaced both of them been good ever since

[ZMANN]

one bad batt will definitely kill the other

[Stamey711]

Ok I'm going to take the batteries out and have them tested.... I'm leaning more toward something sucking power while the truck is off I've pulled every fuse I know of and checked... Bad ground maybe?

[Stamey711]

Weird thing too when the truck is off I can hear a little buzzzz coming from the alternator?

[Stamey711]

Ok guys I had the batteries tested and they were 95% and under load tester great too...? I still hear that buzzing sound from alternator when the truck is off but it's new alternator, could it still be bad? Also could it be starter going bad sucking power? Thanks

[Snake]

It's entirely possible that the new alternator is bad. I bought an aftermarket 220a alternator and it was dead right out of the box. The buzzing sound you're talking about.....I don't think I've ever heard of someone with a similar issue.

Do you have a mulitmeter or other way of checking battery volts?