2002 Mecury Optimax starting problem

[Whalerfisher]

The port outboard only starts if the battery switch is set at "all", even if the other starter battery is put on the port side. Both starter batteries have 13.2 volts at rest. Any thoughts?

 

[jerryjerry05]

Load test the battery.
A battery with a dead cell or weak battery will show 13v
Then when starting it drops way down.

Harbor Freight load tester. I paid 24$