Battery Drained Overnight? How to Fix Parasitic Power Loss

You head out to your vehicle one morning, only to find it won't start. On checking it out, you find the battery is dead. What the heck? It drove fine just last night, so how did it lose charge overnight when it was off? Unfortunately, you may have a slow battery drain somewhere. Here's a look at why this happens, and how to fix it.

Vehicle System
Ignition, Charging, & Electrical Systems
Skill Level
Intermediate

This is a project that needs some know-how

Time to Complete
1-2 hours
    Why the Battery Drains Overnight

    A slow battery drain when everything is off is called a parasitic power loss. This happens when something – a glovebox light, an amplifier, any electrical component – is pulling power from the battery when it shouldn't. Since the alternator isn't generating vehicle electricity, the problem component pulls from the battery, slowly draining it overnight until your vehicle won't start.

    This probably sounds familiar for some 2004–2008 Ford F-150 owners, as that generation had parasitic battery drain issues caused by a failed park sensor not allowing the body control module to turn off interior components.

    Slow power drains are inconvenient and annoying, and result in a shorter battery life.

    Step one, after getting a jump or charging your battery, is to test your battery or visit your favorite auto parts store for a free battery test. Batteries need to be replaced every few years as part of regular maintenance. Your battery could be near the end of its service life and it should be recycled and replaced with a new one. If the battery test shows that it is still good, then it's time to start looking at a parasitic power draw. While you're under the hood (or at Advance Auto Parts), test the alternator. A failed diode in the alternator creates a closed circuit that drains your battery even when the engine is off. If you're too busy during the workweek and can't get to it until you're off, some short term fixes are to disconnect the battery at night, and carry a portable jump starter in your vehicle.

    Keep in mind that you'll lose the radio, power seat, and other presets stored in memory, and the engine could be difficult to start due to a lack of recent fuel trims.

    Narrowing Down the Problem

    Any electronic device with memory settings, like a clock and radio station presets, draws power from the battery while the vehicle ignition is off. This drain is so small, you can leave a modern truck sitting for a week or more and it will still start up and run fine.

    For some perspective, 0.05 amps should be the usual power draw when a vehicle is off. A clock in the radio draws as little as 0.01 amps, whereas the combined interior lights can exceed 1 amp. That seems like a tiny amount, but a single incandescent dome light pulls enough power to drain a battery in one day. Remember that the drain doesn't have to take your battery to zero overnight, just low enough for it to not start. If the battery drains when the vehicle sits for three or four days, that's a lower amp draw than one that drains the battery overnight.

    Discovering the source of an electrical draw is a process of elimination. First, check the easy stuff. Make sure the dome light or other accessory lighting isn't staying on in a closed and locked vehicle, which would mean you just need to flip the switch to off. New aftermarket modifications or changed equipment may also be the culprit. If you have recently upgraded the stereo, installed a new alarm system or aftermarket fog lights, you should start with a hard look there.

    Note that the multimeter listed below should be able to measure the same amperage of the fuses, up to 20 amps, and as low as the hundredths range. The repair manual is a must for this one, as it will list amps pulled by each component, how to test them, and the wiring diagrams showing everything on a fuse.

    Step-by-Step
  1. Start with a fully charged battery. If yours is low, use a battery charger, or have a friend assist with jumper cables from a good battery.

  2. Recreate the same configuration as when the battery drains overnight. Meaning, shut everything off, lock the doors, and take a key with proximity sensor away from the vehicle. Your vehicle has fuse panels in the interior that you will likely need to access later, but opening the door will trigger a “wake up."

    A workaround is to open the door and use a clamp to depress the door sensor on the door jamb so the body control module registers the door as closed. If interior fuse panels need to be checked, you can enter since the door is already open and the power stays off.

  3. Pro Tip

    Wait for the vehicle to go into sleep mode. Modern vehicles (especially with keyless entry) prime the vehicle for driving when you unlock the door or open it. That action is a signal to pull power to operate the PCM, fuel pump, and accessories, creating a large power draw. Instead, let the vehicle sit for at least 10 minutes after step two so it can go into sleep mode.

  4. On the amperage side of the multimeter dial, set it to 20 amps. Disconnect the negative battery cable.

    multimeter set to read amps
  5. On the amperage side of the multimeter dial, set it to 20 amps. Disconnect the negative battery cable.

  6. Touch a lead to the negative battery cable terminal. Touch the other lead to the negative battery post, completing the circuit within the multimeter, which will display the amp draw.

  7. Power draw should be under 50 milliamps. This will display as 0.05 amps. If it's higher, that is your parasitic power draw. Well under 50 milliamps, and you'll need to be shopping for a battery. Note: older vehicles will likely have less power draw.

  8. Pro Tip

    Note that there should always be some amperage. A total zero reading on every measurement likely means the meter is malfunctioning.

  9. Have an assistant hold the leads in place (or use plastic clamps if going at it alone) and open the nearest fuse panel. Use the fuse puller to remove suspect fuses one at a time, starting with any aftermarket electrical accessories.

    fuse panel under dash

    Most vehicles have more than one fuse panel. This one, on a Toyota Corolla, is located under the dash on the driver's side.

  10. After pulling each fuse, observe the multimeter display to see if the amps drop. The fuse that reduces the amps is where the parasitic draw is located. The wiring diagrams in a repair manual will show you everything that is on that fuse.

    fuse panel with labels inside cover

    Fuse panels usually have covers that need to be removed. Look for a labeled diagram on the inside of the cover for fuse identification. 

  11. Check each component on that fuse. For example, a wiring diagram might show that the stereo, amplifier, and instrument cluster are all on same fuse.

  12. The failed component may have a bad ground, power connection, or wiring. Remove power from the component to see where the power draw drops. If the accessory has a remote switch, such as a rear window defrost button, also check the switch.

  13. Replace failed components with new parts. For expensive items like an aftermarket head unit, first disconnect, inspect the wiring, and retry with solid connections. If it still draws power when the ignition is off, the component has an issue and needs replacing.

  14. How have you diagnosed a power drain in the past? Let us know in the comments.

Last updated August 9, 2019

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