You need a battery load tester. A meter tells you nothing when it comes to battery condition. You can have 12 volts, but no amperage carrying capacity.
The load tester applies a load to the battery, by heating a coil and you can watch the needle drop on the tester. This is the only sure way to test a battery.
Ps: when load testing the batteries, make sure you remove the cables first so you are checking only one battery at a time.
If only one battery is bad/weak, ALWAYS replace them as a set. The one that is now ok will surely fail in the near future.
If you want to know if your alt is charging, put the meter on the battery when it is running. You should see between 13 and 15 volts. If lower than 13 your alt is not charging. Above 15 your alt is overcharging.
I assume you cleaned/checked your battery connections?
Sent from my iPhone using Tapatalk