Jeff Brakley, senior business portfolio manager for trucks at Maxwell Technologies, told FE his method for determining the parasitic load on a truck:
This can be accomplished with a digital multimeter (DMM). Prior to running a parasitic load test, the truck must be off, all doors closed and lights turned off. Many onboard computers and other electronics will go into a sleep mode 20 to 30 minutes after the vehicle is shut off, so it’s a good idea to wait 30 minutes before conducting the parasitic load test.
The DMM must be placed in current mode, and it should be set to the highest amp range prior to conducting the test. Many modern DMMs are auto-ranging, so this may not be an issue, but the best practice is to start in the 10-amp range setting. Most DMMs have an internal 10-amp fuse to protect the meter, so it’s always a good idea to wire in a second 10-amp fuse in parallel with the meter connections before connecting the meter.
The parasitic load test requires removing the ground (black) cable from the batteries then putting the DMM in series between the removed ground cable and the battery’s negative terminal. The test leads on the DMM need to be in the correct current measurement inputs, which are different from the voltage measurement inputs. If there are additional negative cables supplying the loads on the truck, then additional tests will need to be conducted for each cable and the results added together. It gets a little complicated, but with the right training it can be done successfully.
As a general rule of thumb, the parasitic loads should be less than 0.1 amps, or 100 milliamps. This number may vary between manufacturers and truck models. If the readings are higher, each of the loads at the battery connections should be removed one at a time to determine which load is causing the high parasitic current draw.
Once that is determined, it may be possible to rewire that load, depending upon the equipment it is powering, through a low voltage disconnect switch or a cutoff switch to keep the load from draining the batteries when parked for extended periods.