Checking battery terminal voltage only works when you're running a good generator against a fully-charged battery, particularly on a 6-volt system. If the battery is low, the voltage required to charge it will also be low. A generator is regulated for current outpuit, as well as voltage, and most 6-volt current regulators are set at 25 amps or so. It will take a long time to recharge a low 6-volt using the generator, many miles of highway-speed driving.
If you've got a low-amperage battery charger, run it overnight every few weeks until you are able to drive the truck on a regular basis. Just be sure to check the water level in the cells before you attach the charger. NEVER allow the water level to get below the tops of the cell plates. Add only clean water, never acid, to a battery. It doesn't have to be distilled water, but the bottled water sold for steam irons or deionozed drinking water is great for battery filling.
Jerry
