If everything is okay it should not change, but that is not a valid test of anything. Start by seeing that you have about 13.5 to 14.5 volts across the battery when you rev the engine up. If you do and the battery is draining anyway, you need to put an ammeter in series with one of the battery cables and see if there is a drain. The ammeter should not read over about 100 milliAmps.

If you have charge voltage and no excessive drain, have the battery load tested. They will do this free most places that sell batteries like Sears.

If you did NOT have 13.5 to 14.5 volts with engine running the problem is probably the alternator. Pull and inspect the brush pack. If the brushes are worn down short then replace it.

Hope this helps,