Truck starts, I drive a few miles (daytime) and then battery goes flat. The headlights appear bright when I check for power. Terminals and posts have been cleaned and connections are good
Make?
Model?
Model year?
Age of battery?
Odometer mileage?
Has alternator been checked?
You may not be driving enough to charge your battery. Have you brought the charge on the battery up to complete charge status by trickle charging overnight? Also, I would do a load test on the battery (free at most parts stores) to make sure it isn’t weak. If the battery is more than about four years old, it may be at the end of its useful life.
I agree with jayhawkroy. It’s time to have the battery and charging system tested. The headlights will be bright even if the battery is too weak to start the engine, so this is not a valid test. You need a “load test.”