It used to be when you turned the vehicle off, all electrical loads went away. Now that today's trucks use electronics full time, including the extra hotel loads for driver comfort, these electrical loads (parasitic loads) increase the demands on the vehicle electrical system.
Testing for parasitic loads: A multi-meter is connected to a battery and to the vehicle wiring....
Testing for parasitic loads: A multi-meter is connected to a battery and to the vehicle wiring. There is an inline fuse along the negative cable that is used to prevent current from the electrical system from blowing the multi-meter fuse.

If the engine is not running, the alternator has no output and the batteries supply all the energy to power these parasitic loads. The extent of the total load is determined by two factors:

1. Length of time the parasitic load is on.

2. The amount of current the parasitic load is drawing.

For example, a small cooler plugged into a 12V power plug could draw four amps per hourand be left on for the entire weekend (72 hours). Four amps multiplied by 72 hours is 288 Amp/hours. However a dc-ac power inverter can pull 150 amps to power an 800-watt microwave oven. It takes five minutes to cook the item. Five minutes equals 0.0833 hour. 150 amps multiplied by 0.0833 is 12.495 Amp/hours.

As you can see by the examples above, a small load over an extended period of time requires much more energy from the battery pack than a heavy load over a short period of time. Most four-battery packs contain approximately 400 Amp/hours if they are in good condition and at 100% state of charge. As a rule, the depth of discharge must be kept less than 30% to improve battery life.

The deeper the depth of discharge, the shorter the life will be for all batteries regardless of the type or manufacturer. The small cooler left on over the weekend results in a 72% depth of discharge -- much more than the 30% rule of thumb.

(Information courtesy Purkeys Fleet Electric,