I have a 93 f250 with the 7.3 and for the first time it wouldn't start. I jumped the battery, went to school and at lunch I went to move it and it started up no problem. At the end of the day the battery was dead again so I got a friend to jump me and I went to advanced where they tested the batteries and one had 750 cca and the other had 430cca, I replaced the one putting out 430 cca. The rest of the day it started up fine but this morning it was dead again and I had to jump start it. Im assuming something is draining it but I don't know what.

 

I had a similar problem with my 92. Turned out to be the under hood light. It would stay on with the hood closed all night and day. I just disconnected it and solved the problem.

 

So I skipped 3rd period and drove to advanced (truck started fine) and had them test the batteries and they tested fine and then 10 minutes later it wouldn't start and each battery tested 100 cca less then when I got to advanced. Something is definitely draining the battery. Does anyone know the best way of going about finding what's draining it?

 

Dual batteries are a matched pair, or two "cells" technically connected in parallel. You wouldn't replace just one cell in a flashlight, because it will flicker and start to leak. It will work, just not very well.

Jump starts will get you going but at the earliest opportunity charge them up to 100% with a dedicated charger of the correct size.