02 E350 7.3TD battery issue
 

Batteries <1 yr old> were at 12.1V and the turck would not start. I charged them to 100%. Checked the alternator output, OK. Mechanic did a battery load test and said they were bad. When he removed them from the cables both batteries tested ok. Cleaned all terminals and again they checked bad. He's looking to replace the battery cable at $297 for the cable plus labor. Any ideas before I proceed with this?

batteries
 

If your batteries are showing bad, they should warranty both of them.