Having some issues with the boy's 7.3L

Recent batteries less than 7 months old

Cranking over real slow and getting worse

Voltage measures 12.5 at terminals

While cranking drops to under 6

Once it starts it reads 13.4 at terminals

I read somewhere on here earlier in the day about this but cant put the same combination of words into the search engine to find it again

No chips or mods

Any ideas???

Is the starter about shot?? 120K miles on it (factory)

 

Sounds like a bad cell in a battery. I'd pull them both and have them tested.

 

while your at it i would also have your alternator checked to!