The most effective way to check for battery drain, often called parasitic drain, is by using a multimeter to measure the electrical current flowing from the battery when the car is off and all systems are asleep. A normal reading is between 20-50 milliamps (0.02-0.05 amps). A significantly higher reading indicates something is drawing too much power.
Step-by-Step Diagnostic Process
First, ensure safety. Put on safety glasses, and make sure the vehicle is off, keys are out of the ignition, and all doors, trunk, and hood are closed. Many cars have courtesy lights that will skew your reading if a door is open.
- Set Up the Multimeter: Turn your multimeter to the DC current (A) setting, selecting a high amperage range (usually 10A or 20A) to start.
- Disconnect the Negative Terminal: Locate the car battery and loosen the nut on the negative (-) terminal. Carefully remove the cable.
- Connect the Multimeter: Connect the red multimeter lead to the now-disconnected negative battery cable. Connect the black multimeter lead to the negative terminal on the battery itself. You have now placed the multimeter in series with the battery, meaning all current must flow through it.
- Read the Initial Current Draw: The multimeter will display the total current drain. Wait a few minutes for the vehicle's modules to enter "sleep" mode. A healthy car should settle down to a drain of 20-50mA.
| Battery Drain Test Benchmarks | |
|---|
| Normal Parasitic Drain | 20 - 50 milliamps (mA) |
| Acceptable for Modern Cars w/ Keyless Entry | Up to 80 mA |
| Concerning Drain (Needs Investigation) | 100 mA or higher |
| Typical Drain from an Aftermarket Amplifier | 300 mA - 1 Amp |
| Drain from a Failing Module | 0.5 - 3 Amps |
| Healthy Battery Voltage (Engine Off) | 12.4 - 12.7 Volts |
| Voltage Indicating a Discharged Battery | 12.0 Volts or lower |
If the reading is too high, you need to find the source. The safest way is the fuse pull method. With the multimeter still connected, pull fuses from the interior and under-hood fuse boxes one at a time. When you pull a fuse and the amperage reading on the multimeter drops significantly, you’ve identified the circuit causing the problem. Consult your owner’s manual to see what components are on that circuit.
