Parasitic draw on a car battery is caused by a short circuit that can be hard to find. A certain amount of draw is normal – in the region of 25mA (milliamps) – but if you have 100mA or more being lost, it is definitely a cause for concern.
The parasitic draw on your car battery could be anything from a dimming dome light staying awake or glove compartment switch sticking on, to a faulty aftermarket radio or alarm wiring. In addition, any number of electronic systems that are installed on newer cars could cause the parasitic draw, such as your GPS or the proximity key system.
Generally, if your car battery was drained overnight and prior to this battery drainage situation you were using the car normally, you might call for roadside assistance, but the technician responding to the call will only be able to boost your car and get it started, test your car battery to tell you if it's still good or it needs to be replaced, or even test the vehicle for parasitic drain, but that is where it ends. Keep in mind that narrowing down the parasitic draw cause in your vehicle is a time consuming process, especially in the newer vehicles overloaded with electronics and circuits. After your vehicle was boosted, you will have to wait until the battery was fully charge to start the parasitic draw testing procedure assuming your battery is in good shape and it charges. Even if you replace the car battery, the parasitic draw malfunction will drain the new battery the same way.
What You Need To Test Your Vehicle For Parasitic Draw
A digital multimeter (DMM) is best for testing your vehicle for parasitic draw as opposed to an analog one, which is not as accurately read. A DMM with the ability to measure continuous current to 10A will be fine in most cases, though one rated 20A would be less prone to blowing a fuse. You also need a milliamp range for higher resolution to pick up minimal current draw as most faults tend to be of the trickle variety.
You can buy fairly cheap, half-decent multimeters, but keep in mind in mind that the cheapest digital multimeters are not exactly known for their quality or input protection. It is best to stick with a decent brand name multimeter.
Parasitic Draw Testing Procedure (Pulling Fuses Out)- Old Method
Before you start the parasitic draw testing procedure:
- You should have a fully charged car battery before starting – voltage would be around 12.6V. Close all doors, turn off the radio and disconnect accessories, such as phones plugged into the USB charger and add-on GPS devices. Keep in mind that even a key in the ignition can cause your vehicle's electronic systems to wake up on newer cars, which will initiate parasitic draw.
- In order to test for current, the multimeter must be connected in series with the circuit as it needs to gauge the electron flow. Voltage, on the other hand, is measured in parallel (see step 2 below).
- CAUTION: when connected, avoid starting the engine or even turning on the headlights as this will likely blow a fuse in the multimeter. Vehicle batteries put out a lot of amps due to being low voltage.
Here are the parasitic draw testing procedure steps:
- Disconnect the battery. Pop the hood and disconnect the negative cable from the negative battery terminal. It is best to do a parasitic draw test from the negative side so there is no chance of accidentally shorting (if you test from the positive side).
- Connect the multimeter. Plug the black jack into the common (COM) jack input of the multimeter and the red probe into the amps (usually A) jack input. Then connect the red probe to the previously disconnected negative cable of the car loom, and the black cable onto the battery terminal.
- Give the car time to sleep. Modern car systems may wake up when you connect a multimeter in such a fashion as it picks up the power of its batteries. So wait a few minutes before conducting the draw test.
- Reading the meter: Newer, auto ranging multimeters will automatically select the correct range of a signal they pick up. On manual ranging meters, you will need to select a range, usually either 2A or 200mA. If you get ‘OL’ on the screen, it means over limit and you should select a higher range. If you see a reading of around 50mA and above on a standard car, it generally means something is drawing power and you likely have parasitic draw. Just note that if you have a big, modern, luxury car its added computerized systems will likely increase the natural current draw. In this instance, the threshold might be 100mA.
- It is now time to see what is causing the draw. The most common method is to start pulling fuses to see which circuit is the cause of the parasitic draw in your vehicle: Locate the fuse panel(s) and start pulling out fuses one after the other, taking notice of any drop in the reading on the multimeter. If there is no success with pulling fuses, start removing the relays. For those fuse boxes that are inside the vehicle, it would be best to place the digital multimeter on the windshield facedown so you can see the display.
- Identifying the cause: When the display drops to a nominal mA draw, note the fuse/relay position number and check the vehicle’s manual. There is often more than one circuit on a given fuse, so you would then need to eliminate each one, such as remove a bulb or unplug a heater.
- Repair and reconnect: When the meter drops down to 25mA or less after removing the offending fuse, bulb or unplugging a circuit, you likely have your cause for the parasitic draw. Repair the fault/replace the item and run another test to be sure everything is ok, and if you’re confident there are no other faults, reconnect the battery.
Faulty Alternator?
if you’ve gone through all fuses and relays (and checked all other fuse boxes), many times the alternator could be at fault. If one of the diodes may have shorted out it will cause current to drain back to the battery. Test it with the multimeter before and after unplugging.
Parasitic Draw Test on Newer Cars
Some of newer vehicles can be a real difficult to diagnose parasitic draw test with a multimeter due to all the added electrical circuits, computers and control modules. From electric mirrors, heated memory seats, GPS, trackers, to a multitude of gadgets, some of these systems are interlinked, complicating the testing procedure. Even car keys can indirectly cause current drain. Proximity keys are designed to wake up a car’s locking mechanism when they are within close proximity so that you can unlock and open the door without the key. This is great in normal conditions, say, at your house, but can be an issue in practice. If you park in an area where lots of other cars have these systems, for example, the computer may wake up every time their drivers walk past your car. In this situation, you may also need to disable the sensor.
Old Method vs New Method
The old method detailed above – disconnecting the battery, connecting the multimeter in series and pulling fuses until you find the draw – might not work as effectively on newer cars. This is because the very disconnecting of the battery may temporarily fix the issue by sending the system to sleep along with the error. You don’t want this; you want the fault to keep drawing current so you can diagnose it on your meter.
An alternative is to the above diagnose parasitic draw by measuring voltage drop across the fuses. You can do this right on the fuses as auto fuses have open test points on them, so no need to remove them or disconnect the battery. What you are looking for is a voltage drop in the millivolts range. It is caused by an increase in resistance brought about by the heating of the fuse through movement of current.
Different size fuses will show a different voltage drop, but all of the same rating will have roughly the same resistance, so it is easy to get a reference by testing one 10A fuse against the next. As an example, a 10A fuse with 1A flowing through it might show a voltage drop of 5mV. If you were to measure the same fuse with no current flow, it would be more like 0.1mV or 0.2mV. Quite significant and easy to spot.
Parasitic Draw Testing Procedure (Testing Fuses) - New Method
- Drive the car and operate all circuits: wipers, heater, seats, windows, GPS, lights, sat-nav, etc. Return and remove the key.
- Consult the manual on how long it takes for the various systems to sleep.
- Tape down the driver door switch that operates the dome light and leave the car unlocked so you don’t end up switching on any systems upon re-entering.
- Carry out the testing of each fuse and note any serious voltage drop.
Other vehicle types: this method also works for motorcycles and other vehicles, with a similar process.
How To Extend The Car Battery Life
Along with being a headache and potentially leaving you stranded, even minimal draw can wear down a vehicle’s battery over time, often leading to performance issues and possibly a slow death.
The key to good battery life is to eliminate any parasitic draws in their early stages and also ensure the battery keeps charge above 12.4V. Vehicles that only used occasionally, are more prone to losing charge, where sulfation further compromises performance and longevity.
Those vehicles driven daily have the benefit of the alternator to maintain charge, and if you test the voltage and current frequently, you can pick up any potential issues before it's too late.