Why Does My Car Battery Die Overnight? (And How to Fix It)

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Fix the battery issue or parasitic drain causing your woes
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There’s nothing more frustrating than going to start your car to get to work or run an errand only to find out that your car is unresponsive. It’s especially frustrating if that keeps happening after you’ve jumped the car once! The good news is that there are really only two likely explanations here, and narrowing the issue down is definitely something anyone can do. In this article, we’ll show you how so that you can get back to knowing your car will start after leaving it overnight.

Why Your Car Battery Drains Overnight

In most cases, your battery drains overnight because the battery itself is bad, or because there’s a parasitic drain somewhere that’s pulling an electrical current after the car has been turned off. In rare cases, the alternator, voltage resistance regulator, or wiring can cause a battery to die overnight.

Section 1 of 4:

Why would a car battery die overnight?

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  1. When your car is off, the battery stores the charge your vehicle needs to start again. But if you leave an interior light, headlight, or radio on after shutting the vehicle off, it will use all of your battery’s juice and your car won’t turn on the next day. To fix this, simply jump your battery and be more careful about shutting everything off in the future. [1]
  2. If your battery keeps dying overnight, you almost certainly have one of two conditions impacting your car:
    • Bad battery: If the car battery itself is no good, you may be able to jump the vehicle just fine because the alternator takes over the power duties after the car has started. Replacing the battery will solve this problem.
    • Parasitic Drain: A parasitic drain refers to some component or accessory that remains on after you’ve shut the car off. In this case, locating the parasitic drain and fixing the underlying issue will keep your car from dying overnight.
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Section 2 of 4:

Ruling Out the Battery

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  1. A swollen battery is on the verge of explosion and must be replaced immediately by a professional mechanic. If you have corrosion, it’s a sign your battery is on the verge of failing and needs to be replaced soon. In either case, a new battery should solve your battery-dying woes. [2]
    • Corrosion usually appears as a chalky buildup atop the battery’s terminals. You can clean the corrosion off to see if that solves the problem, but corrosion is almost always a sign you’ve got a leak, which means you need a new battery.
    • In the future, know that your battery needs to be replaced every 3-5 years.
  2. If you want to check for a draw before you break out the multimeter, perform a load test with your headlight. With a fully charged battery, put the car in the accessory position (electrical on, no engine running) and turn your headlights on. Wait 10-15 minutes and then start your car. Here are what the results mean: [3]
    • If your headlights dim as the engine starts… Your battery is not stable and it needs to be replaced. The battery shouldn’t be able to hold a steady charge for 10-15 minutes without draining, so if it can’t stay stable, it’s time for a new battery.
    • If your headlights don’t dim or flicker… your battery is fine. It’s storing its charge perfectly well and is unlikely to be the problem. You may still have a parasitic draw, though.
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Section 3 of 4:

Diagnosing Parasitic Draws

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  1. Set the multimeter to milliamps. Keep the vehicle off and disconnect the negative battery terminal. Plug the black (negative) lead into the neutral common (COM) port on the multimeter. Set the red (positive) probe in the amps input. Connect the red probe on the multimeter to the disconnected negative battery cable. Then, hook the black probe of the multimeter up to the negative battery terminal. If you have a parasitic drain, the multimeter dial will react. [4]
    • The car must be completely off and the battery must be fully charged for this to work.
    • Key note on the dial moving: If the dial pops up a little then drops below 25mA, you do not have a parasitic draw. Some vehicles maintain a tiny charge while the car is off, so a tiny reading is no big deal.
    • If your multimeter doesn’t react, your batter is not draining when the car is off and you can safely rule out parasitic draws as the problem. If your battery continues to die overnight anyway, your battery is bad and needs to be replaced.
  2. Keep the multimeter attached to the negative battery cable and negative terminal. Locate the fuse box in your vehicle and open the cover. If you cannot find your fuse box, refer to your instruction manual. [5]
    • Your fuse box is typically located either inside of the car, below the steering wheel, or inside of the engine bay just behind the steering wheel.
  3. You’ve located the source of the draw once the multimeter drops below 25mA . Start with any fuse in the box. Remove the fuses one by one until your battery stops drawing power. [6]
  4. The fuse that you removed corresponds to a specific electrical component or accessory. You have one fuse for your headlights, one for the radio, and one for every other single electrical system in the vehicle. Refer to your manual’s fuse diagram to see which component is to blame and either replace or repair the issue. [7]
    • If you cannot personally fix the problem (which is likely, since electrical components are a serious pain to work on if you aren’t a licensed mechanic), just see a professional and explain what’s going on. They’ll help you out.
    • Just to clarify, the fuse you removed that caused the multimeter to drop below 25mA is the fuse that corresponds to the component responsible for the parasitic drain. The fuse itself is perfectly fine.
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Section 4 of 4:

Other Potential Causes

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  1. If your battery is dying overnight when the vehicle isn’t running, it’s extremely unlikely that the alternator is to blame. Most of the time, a faulty alternator will cause the battery to die after the car starts—not when it’s off. Still, a buggy alternator may not recharge a battery properly and cause the battery to drain after turning the car off. [8]
    • Prevention tip: Your alternator is likely to be to blame if you notice weird electrical surges when you’re driving (i.e. the headlights dim and brighten or the radio gets quiet and then loud) or if your car stalls out and dies while you’re driving. If you see any of these symptoms, take your car in to the mechanic.
  2. The voltage regulator is responsible for measuring the input and output of all electrical systems in your vehicle. If the voltage regulator is damaged or failing, your vehicle’s electrical systems won’t function properly and you’re likely to encounter a dead battery every time you go to start it. [9]
    • A bad voltage regulator can cause all kinds of odd electrical issues, so if your vehicle seems to have a mind of its own lately, this is very likely the source of your problems.
  3. Damaged wires can cause short circuits, which may ultimately keep your battery from charging properly before you shut the vehicle off. If you’ve ruled out every other potential issue and your battery is still dying overnight, see a mechanic to have them inspect all of your vehicle’s wiring.
    • Prevention tip: Ask the mechanic to take a look at your car’s wiring every time you take it in for basic maintenance. This way, you can catch damaged cables before they become a serious problem.
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      • Extreme weather (either heat or cold) can make it more difficult for a battery to function correctly.
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      • Parasitic draws often don’t make themselves known because they’re malfunctions. However, you can occasionally spot a parasitic draw by looking for accessories that stay on after you shut the car off.
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      References

      1. ↑ https://www.kbb.com/car-advice/how-to-jump-start-battery/
      2. ↑ https://www.aaa.com/autorepair/articles/when-to-replace-your-cars-battery
      3. ↑ https://www.firestonecompleteautocare.com/blog/batteries/how-to-test-car-battery-at-home/
      4. ↑ https://www.motortrend.com/how-to/battery-drawing-down-check-old-jeeps/
      5. ↑ https://testmeterpro.com/parasitic-draw-test-with-a-multimeter/
      6. ↑ https://testmeterpro.com/parasitic-draw-test-with-a-multimeter/
      7. ↑ https://testmeterpro.com/parasitic-draw-test-with-a-multimeter/
      8. ↑ https://www.kbb.com/car-advice/dead-battery/
      9. ↑ https://www.goldeagle.com/tips-tools/bad-voltage-regulator-symptoms/

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