Is it the ALT or the battery ,more than likely with the voltmeter dropping down with
each speed of the heater motor? My other truck with the same setup does not.
If the engine is running, the alternator is the primary source of electric current. The battery is just there to get the process started. Once the engine is running, the alternator takes over, supplies the current to run lights, ignition, heater blower, and any other accessories. Extra amperage capacity (if any) goes to recharge the battery. The battery also serves as a "shock absorber" to dampen out momentary voltage fluctuations as accessories are switched on and off.
Jerry
