Batteries. When you are reving above 1600 rpm as in normal driving, the alternatore should be hitting the battery with about 14.4 volts, +/- temperature which automatically adjusts the voltage.
An "autmatic charger," of which the battery maintainer and charger should be if its the one made by Deltran, or equivalent. Heres what It does: It starts out ar 15 volts to push as much current as itscabible of, to help with sulfation. Then it switches to about 14.2 until the battery is charged 90%. This is called the "bulk charge" mode. Then it switches to about 13.3 volta which is the maintenance mode. You can leave the charger on at this voltage forever without damaging the battery. If this is a vehicle which gets infrequent usage, leave it on all the time you are nit using it.
Heres a (welcome) glitch in at least my Expy. Normal is when you start it, about a secnd later the gauges come up to the readings. If the battery is sick, the gauges will start at left side,and flop over to the right then read. This happens because the batterys voltage goes below a certain amount during cranking, and then insterment microprocessor i sresetting itself. Its very soon when it has a problem. ANother thing, If you say, leave the headlights on and the battery goes dead, Charge it right now. If you wait the battery is damaged , called sulfation, and it WILL fail soon.
How do I know all this? I was in the design engineering area of a battery charger company before retirement.