Vehicle Quiescent Current Testing

VEHICLE QUIESCENT CURRENT TESTING

If an open circuit voltage test gives a result in the region of 12.45 to 12.49 Volts it is recommended that you ensure the battery isn't slowly being discharged by an electrical consumer that is remaining on after the vehicles shut down period. Like wise if a customer complains of a vehicle battery that discharges continuously or when left for a prolonged period of time it is recommended that you perform a quiescent drain test as described below.

Method of Measurement
In order to correctly measure quiescent drain, it is preferable to use WDS with the current probe attachment. If this is not possible, then an ammeter may be used. Whichever method is used, technicians MUST adhere to the following procedure:-

Using WDS with current probe:-
- Switch off all electrical loads & remove the ignition key.
- Connect current probe to WDS.
- Calibrate the probe.
- Clamp around the battery lead / fuse box lead.
Go to test results.

Using a multimeter rather than WDS:-
- Disconnect the battery negative lead.
- Connect the ammeter between the battery negative post and negative lead using clips.
- Select high meter range (i.e. equal or more than 10 amps) then switch the multi meter on.
Goto test results.

Test results (both methods):
- Switch ignition 'on' then 'off' (DO NOT CRANK) and remove the key.
- Close or latch all doors and remove any other potential electrical consumers.
- Observe / compare the readings obtained over the time scales identified on.
- Enter the reading obtained on to the battery report form. Battery Report Form (Diagnosis)

NOTE: The preferred method of testing following an excessive current consumption figure is to use the clamp of the ammeter around individual fuse box leads to the various suspected circuits to identify a potential cause. Rather than the old method of pulling fuses out, in a one after the other fashion.

The main reasons for using the current probe to determine which fuse box/circuit the drain is going through is that:
a) Many modules take a considerable time to power down, every time the fuse is removed and refitted, the quiescent drain figures will need 45 minutes to reset.
b) Sometimes the drain is caused by a module "holding on" to something that has been switched off. Sometimes it is only a relay winding that is activated. Pulling the fuse can allow this to "reset" and the drain will be lost and undiagnosed. Similarly, if for example, the headlights of an S-Type are switched on, removing the switch input will not turn them off. Therefore the reading obtained would be incorrect and could mislead.