Below is an article by Kurt Lesser from the G503 forum (reproduced here with his permission):

A common misconception is that there is an ammeter in the HMMWV.
The thing in the dash is actually a voltmeter and they work differently than an ammeter.
An ammeter actually measures the amount of current passing through it and indicates + current into the battery (charging) and - current being drawn from the battery. The voltmeter is simply monitoring the voltage in the system produced by the batterys with the engine off and the generator or alternator with the engine running.

If you have loose, dirty, or corroded terminals and connections in the system they can manifest themselves as resistance which can keep the voltage high, in the green, while never allowing enough current to flow to recharge your batterys. The small line on the voltmeter gauge is 28 volts, the place the needle should be in a healthy system with the engine running. Old worn out batterys can also have high internal resistance causeing them to show good voltage while not charging. If the system had an ammeter in it it would show - current or discharging and little or no + current recharging the batterys.

In a generator based system the regulator needs to be adjusted to change the voltage. Alternators have a small adjustment screw under a cover near the binding post for the output power that can be tweaked for proper voltage. You don't want to set this too high because it'll cook the water out of your batterys. Anywhere between the line and a needle width below it is OK.

You can also tell a lot about your batterys by disconnecting them and then charging them up. Fully charged they should read about 12.6 volts. A good battery will hold this voltage for weeks. A tired battery will start dropping it's voltage almost right away. If this happens don't rush out to the local parts store for that miracle battery rejuvenating juice, it's worthless as hair regrowing tonics were in the 1800s.