- Measure quiescent current
- Detect clandestine consumers
- Common Causes
To prevent the battery from draining, the quiescent current with a parked vehicle should be as low as possible. Electrical systems such as the central door locking, possibly extended with an alarm system, must always be on standby for a key signal or detection in the event of a break-in. A quiescent current of less than 40 milliamps (0,04 A) is allowed. Quiescent current is also known as “leakage current”.
The moment the quiescent current is higher than 40 mA, there is a quiescent current disturbance. An electrical component or system remains on while the vehicle is parked. This results in a flat battery after the vehicle has been standing still for some time.
An electrical consumer that causes a quiescent current disturbance, but is not visible and audible, is called a “parasitic drain”. This is another word for “silent consumer”. When a battery is empty after a night of standing, it does not necessarily have to be a clandestine consumer. The internal resistance of a battery increases as the battery ages. At low temperatures, the internal resistance becomes even higher. An old battery can still function well in the summer, but cause starting problems in the winter. This does not include lighting that has been left on; one could easily have seen this.
When the battery is empty, the first thing to check is the charge status and capacity of the battery. By being deeply discharged several times, a battery may have become defective. A new battery will also suffer the same fate if the source of the problem is not solved. This page shows the steps that can be taken to perform a parasitic drain test.
Measure quiescent current:
To be sure that we are dealing with a clandestine consumer, and not with a malfunction in the car's battery and/or charging system, we hang the current clamp around the battery's ground cable. The Ampere setting on the multimeter can also be used to measure in series, but care must be taken to connect the measuring pins of the multimeter to the negative cable and the battery before removing the battery terminal. If you remove the battery terminal and then connect the multimeter, the vehicle has been de-energized. And that is absolutely not desirable!
In this example we use the simplest way to measure the quiescent current: with a current clamp. The following conditions must be met to obtain a reliable measurement:
- all consumers must be switched off;
- the car must be in rest mode (sleep mode);
- the doors must be locked. The alarm must not go off.
Since we have to access the fuse boxes in the interior (in the dashboard and sometimes in the trunk) all doors have to be opened and the locks locked by hand or screwdriver. When a car is in rest mode, you may not open a door: the car is woken up, all CAN and other networks become active, the interior lighting and the display in the instrument panel come on, so that it is no longer possible to read the resting current.
After locking all doors, it takes up to 30 minutes for the vehicle to enter rest mode. We hang the current clamp around the ground cable of the battery (see the following picture) and read the current.
It was stated in the previous section that the rest mode may be a maximum of 40 mA (0,04 A). In this example we measure a current of 1,90 A. The quiescent current is too high.
Instead of a clamp meter with a digital display, a clamp meter can also be hung on an oscilloscope. This allows us to analyze the quiescent current over a period of several minutes or even hours.
Parasitic drain test:
The previous section shows that the quiescent current is too high. The battery discharges because an electrical component in the car is left on. We can look in the fuse box for the fuse that protects this particular component from overcurrent. After all, the current of 1,90 A flows through one of the fuses in the fuse box.
A mistake many people make is that they pull the fuses out of the fuse box one by one. When the correct fuse is pulled, the quiescent current drops to almost 0 A. By locating the circuit the fuse is in, one can see what has been left on. However, in this way a malfunction can be temporarily remedied unintentionally. For example, if this interrupts the power supply of an ECU, a hard reset may leave it trouble-free for several weeks or months. So you are not sure whether a malfunction has been resolved temporarily or permanently.
The correct way to determine whether (too) much current is flowing through a fuse is to measure the voltage drop across the fuse.
A fuse has an internal resistance. The one through which no current flows has a voltage drop of 0 mV. The internal resistance causes a voltage drop when current flows through it. The following figure shows the voltage drop measurement across a 20 A fuse.
We look up the 6,4 mV voltage difference on both sides of the fuse in the table on the page: voltage drop across fuses. In the table we see that a voltage drop of 6,4 mV across the 20 A standard fuse is caused by a current of 1893 mA.
- the voltage drop of 6,4 mV corresponds in the table to a current through the fuse of 1893 mA. This is: 1,893 A
- the quiescent current measured with the clamp meter is: 1,9 A;
- the consumer behind the fuse provides a current of 1893 mA;
- the quiescent current without this consumer is 7 mA;
- in short: the current through the consumer + the quiescent current of the car without this consumer is: 1893 + 7 = 1900 mA = 1,9 A.
In the workshop documentation, we search the fuse overview to find out what the fuse is from. In this example, the fuse protects the retrofitted Bluetooth car kit against overload. When the plug of the bluetooth module is disconnected, the quiescent current immediately drops to 7 mA. This means that the bluetooth module remains on.
The following picture of the bluetooth module is used as an example to show the module hidden behind the dashboard, so it cannot be seen whether it is enabled or disabled. Perhaps the customer could have seen this because his phone remained connected to the bluetooth after switching off the car, but there was no other indication. The module will need to be repaired or replaced.
A clandestine consumer is often retrofitted or installed equipment with which the car was not delivered new. Consider car kits (the example above), retrofitted central door locking, equipment in the 12 volt connection (whether or not hidden in the trunk or center console) or a relay that sticks: connections 30 and 87 then remain connected, creating a ( main) current continues to flow without the relay being activated. Don't forget to check the light in the glove compartment or the trunk light; Especially in older cars, a bad switch can leave them on without being detected by an ECU, or they can be automatically switched off during rest mode.