How to test for a parasitic draw

Parasitic Draw Test

Sometimes, a battery may experience significant drain long after the engine has been shut off. It might not be a faulty battery causing this. Instead, this typically happens due to a short circuit that can be hard to pin down. It could be anything from a dimming dome light left on or a glove compartment switch sticking on, to a faulty aftermarket radio or alarm wiring. Also, any number of electronics systems that are installed on newer cars could be acting up, such as your GPS or proximity key system.

What you will need

A basic digital multimeter is best for this test as opposed to an analog one, which isn't as accurately read. Also, there are more advanced versions like the newly released DS200.

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Testing for a parasitic draw, which refers to an excessive electrical current draining from a vehicle's battery when the engine is off, can be done using a multimeter. Here's a step-by-step guide on how to use a multimeter to test for a parasitic draw:

  1. Prepare the vehicle:
    • Park the vehicle in a safe and well-ventilated area.
    • Ensure all lights, accessories, and electrical systems are turned off.
    • Wait for at least 20 minutes to allow the vehicle's modules to enter a sleep mode.2.
  2. Set up the multimeter:
    • Switch the multimeter to the DC ampere (A) mode.
    • Set the range to a suitable value higher than the expected current draw (usually in milliamperes or mA). Start with a range of 10A if available.
  3. Disconnect the negative battery cable:
    • Open the vehicle's hood and locate the battery.
    • Using a wrench or socket, loosen and remove the negative (-) battery cable from the battery terminal.
  4. Connect the multimeter:
    • Connect the multimeter leads in series between the negative battery terminal and the negative battery cable. Connect the red lead to the battery terminal and the black lead to the battery cable.
  5. Monitor the current draw:
    • With the multimeter connected, observe the reading on the multimeter.
    • It's normal to see some initial spikes as modules wake up, but within a few minutes, the current draw should stabilize.
    • Take note of the current draw reading. If it exceeds the manufacturer's recommended specification (usually around 20-50 mA), there might be a parasitic draw.
  6. Locate the source of the draw:
    • Begin narrowing down the source of the parasitic draw by removing and replacing fuses one at a time.
    • As you remove a fuse, monitor the multimeter reading. If the current draw drops significantly after removing a specific fuse, that circuit is likely the source of the draw.
    • Consult the vehicle's owner manual or wiring diagram to identify the systems or components connected to the circuit with the parasitic draw.
  7. Investigate and resolve the issue:
    • Once you've identified the circuit causing the parasitic draw, inspect the associated components or systems for any signs of malfunction or wiring issues.
    • Perform necessary repairs or seek professional assistance to address the problem.
  8. Reassemble and retest:
    • After resolving the issue, reinsert any removed fuses and reconnect the negative battery cable.
    • Double-check that all connections are secure.
    • Perform the same current draw test again to ensure that the parasitic draw has been eliminated or reduced to an acceptable level.

Remember, if you're unsure or uncomfortable with performing these steps, it's recommended to seek assistance from a qualified automotive technician.

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