Batteries not charging

I have an 01 X-Star and I can't figure out why the batteries are not charging fully? I removed the alternator and had it tested and it was fine.I checked the ground wire and it was on tight. I can't rember what the voltage should be running but i thought it was around 13.5-14.5. I am only at 12-13 with occasional dips to 10. When I throttle down the voltage starts to pick up slightly but still doesn't go past 13. Any help would be appriciated I am at a loss. I would take it to the dealer but it is 5 hours away and I don't want them to take all summer to get to it.
THANKS, John

Are you testing it with a meter or just your gauge?

I have a capacitor for the stereo with a digital readout that I was using but I have a multimeter

Dont read the cap voltage. Read the voltage at the batteries and at the alternator.

When you had the alternator tested, did they test it for voltage and amperage? How many amps did it produce?

Like the others said, get yourself a multi-meter and measure the voltage right at the battery, then measure the voltage right at the alternator.

If you have the engine running at fast idle and minimum load (stereo, pumps and blower off) then you should get at least 13.5 volts at the alternator and essentialy the same voltage at the battery.

If there is a significant difference in voltage between the alternator and the battery then there is a problem in the wiring.

If the alternator is not putting out at least 13.5 volts with an engine RPM of 1000 or so then the alternator is weak.

Rod

Do you have a high output alt, sometimes when they wind them up for more output using a smaller case during idle it will be lower then 13.5 It wont achive 13.5 until it goes past idle.

Although you said that your ground wire was tight, check your cable ends for hidden corrosion. I can't stress enough how important good grounding is, especially on a boat.