I have a 1996 Ford F350 before I start it up and I turn the key forward the battery level is very low I just replaced the two batteries last night. We did an alternator test when I got the batteries and the test showed good. Once it starts up and sits running for around a minute it jumps to around 14 V where it should be. I'm at a loss I have no idea what is wrong with it I live in Michigan and the next coming weeks is going to be very cold around 10° and even lower, after it sits overnight or after I get out of school The battery level is so low when I turn it over it sounds like it's going to die, I just replaced the glow plugs and the glow plug relay so now I want to fix the battery problem. Please someone help soon

 

Sometimes the battery cables can look OK and be corroded internally. The better way to confirm condition is with a voltage drop test. And while doing that it would be a good move to have the starter draw checked. The connections from the alt output cable to the batteries should be checked also. If there is corrosion (seen or unseen) in that wiring, battery charging can be affected. Mileage and time have a way of allowing problems to grow in cold damp conditions.