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, www.purkeysfleetelectric.com.)