Calculating Vest Heat Loss From Harness Resistance

You can use this procedure to compare a vest's heat output with different wiring harnesses or controllers. Ideally there would be no loss in the harness, but since wire, switches, connectors, and circuitry all have resistance, there will always be some loss. Harnesses and controllers with large gauge wire and few, high current connectors will have less loss than harnesses with small wire and many connectors or low current connectors.

Measure and record the resistance of the vest. Be sure to compensate for the meter lead resistance.
Connect the vest to the battery through the harness/controller under test.
Turn the vest on (fully on if testing an adjustable controller).
Measure and record the voltage across the battery.
Measure and record the voltage across the leads at the vest.
Calculate the vest current at the voltage recorded in Step 5: Current = Step 5 voltage / Step 1 resistance
Calculate the vest wattage at the voltage recorded in Step 5: Wattage = Step 5 voltage x Step 6 current
Calculate vest current if directly connected to the battery: Current = Step 4 voltage / Step 1 resistance
Calculate vest wattage at the battery voltage recorded in Step 4: Wattage = Step 4 voltage x Step 8 current
Subtract Step 7 wattage from Step 9 wattage to get wattage loss of the harness.
Divide the Step 10 wattage by the Step 9 wattage and multiply the result by 100. This gives the percentage heat loss of the harness.

For example, if the vest resistance is 4.0 Ohms, the battery voltage is 13.8 volts, and the voltage at the vest is 12.8 volts:

Step 6 -- Vest current is 12.8 volts / 4 Ohms = 3.2 Amps
Step 7 -- Vest wattage is 12.8 volts x 3.2 Amps = 42 Watts
Step 8 -- Vest current is 13.8 volts x 4 Ohms = 3.45 Amps
Step 9 -- Vest wattage is 13.8 volts x 3.45 Amps = 47.6 Watts
Step 10 -- Wattage loss is 47.6 Watts - 42 Watts = 5.6 Watts
Step 11 -- Loss percentage is 5.6 Watts / 47.6 Watts = .1176, or 11.76% heat loss caused by the resistance in the harness.

Last updated 3 Nov 1997