What Causes a Car Battery to Go Dead Overnight?

Waking up to a dead car battery is no fun. Let’s quickly diagnose the issue and get to work. These are the common causes for a car battery to go dead overnight:
  • An electrical problem in the charging system
  • A parasitic drain
  • An old battery

In order to understand fully what is going on in the system, it is also important to read up on the three components of the car.

How Does a Car Battery Charging System Work?

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Lots of mechanics will over complicate the due process of diagnosing a car battery and finding out what causes a car battery to go dead overnight. In most cases, it’s a lot more simpler to just start with the charging system itself.

In the most simplest of explanations, there are three components to the car battery charging system:

  • Battery
  • Alternator
  • Diodes


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Traditionally, car batteries are lead-acid batteries — composed of a sulfuric acid solution.

When fully charged, car batteries are usually at 12.6 Volts. When the engine is running, this measurement should be between 13.7 Volts to 14.7 Volts.

Obviously, the battery is the component of the system that holds the energy for the vehicle to use. When this component is not working correctly, the instantaneous energy is used instead (the alternator produces energy as long as it is turning).


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The alternator, another component of the charging system, works by turning mechanical energy into electrical energy.

Without a working alternator, the car runs on the battery’s stored energy and doesn’t top off its stores. The car will, in this case, only run for a few minutes before it ultimately fails.

Taking care of your alternator ensures you take care of your car’s charging system.


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Although the diodes are a part of the Alternator, I find it necessary to treat the diodes as a separate topic because they serve a few important purposes:

  • Converts AC (alternating current) to DC (direct current). AC is generated via the alternator. AC is the diodes’s input. DC is the diodes’s output. These diodes, more technically, are rectifier diodes.
  • The diodes block current from flowing backwards. The current runs from the alternator to the battery. There shouldn’t be any significant amounts of current running backwards.

Voltage drops and other technicalities of the rectifier diodes can be omitted for simplicity here.

The Electrical Problem in the Charging System that Causes the Car Battery to Die Overnight

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Usually, there are two issues that cause dead batteries overnight related to the charging system:

  • Diodes have failed and are allowing current to flow backwards. In this case, the current is flowing from the alternator to the battery. There could be numerous places for voltage drops to occur (one is the diodes if they are allowing current to flow backwards).
  • Battery terminals are shorting. When your battery terminals are shorting, due to leaked battery acid or other substances shorting both terminals, expect the battery to rapidly lose its stored energy.

The Parasitic Drain: Why Your Car Battery Goes Dead Overnight

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Sometimes, your car battery goes dead overnight for reasons beyond failure. Commonly, this is caused by electrical circuits that are active overnight. These can include:

  • Newly added electrical or electronic features improperly installed. Imagine installing new interior or exterior lightings on a circuit that doesn’t turn off — ever. This can happen quite frequently when a technician or hobbyist is not careful and mindful. It is crucial to ensure all customizations only run when the key is on — unless you know what you’re doing.
  • Failed switching components. Relays are electromagnetic components that short two bodies at command. Due to corrosion, it is possible for relays to short circuit between turns. This would bypass the “normally open-circuited” functionality of that particular circuit.
  • Accidental grounding. Any live wire that is hot but is accidentally grounded will thus drain the battery overnight. It is important to ensure all wires are insulated to avoid this situation.

Any electrical issue labeled as parasitic drain is just as it sounds — an electrical drainage that is unwanted and only achieves damage, unintentional harm and stress to your system.

The Old Battery: A Dead Battery that Causes the Car Battery to Die Overnight

Other times, it’s not the other electrical components that are causing your car battery to go dead overnight. But rather, the car battery itself.

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Failures That Causes a Car Battery to Go Dead Overnight

In hot climates, these types of damages may cause your battery to fail:

  • positive grid growth
  • positive grid metal corrosion
  • negative grid shrinkage
  • buckling of plates
  • loss of water

These accelerate aging:

  • Deep discharges
  • Heat
  • Vibration
  • Fast Charging (High current (Amps))
  • Overcharging (like topping off your gas)

Most cases of premature car battery failures happen due to loss of water by:

  • Lack of maintenance
  • Evaporation from high heat (under the hood)
  • Overcharging

If your battery has failed, or you believe your battery has failed, this is usually the first consideration to account for.

How to test for a failed battery:

  • Load test: maintain 9.6 volts at 15 seconds when tested at one-half the CCA rating and 70 degrees F (or above).
  • Alternatively: Use an OBDII Scan Tool and use the “test starting / charging system” option. This will do the above load test for you — while also checking the other components of the system, including the starter (part of the starting system).
  • Another cheaper alternative: Go to your nearest NAPA, O’Reilly’s, AutoZone, etc. They have scan tools and other types of diagnostic tools that they can use to let you know whether or not your battery is at fault or if you need something else.

Further Readings

Car Battery Testing & Voltage. Autobatteries.com. https://www.autobatteries.com/en-us/battery-testing-and-maintenance/car-battery-voltage-and-testing

Dan Ferrell. “Why Does My Car Battery Discharge Overnight or When Parked?” Sep. 2, 2020. https://axleaddict.com/auto-repair/Why-My-Car-Battery-Discharges-Overnight-or-When-Parked

“What does an Alternator Do?” Firestone. https://www.firestonecompleteautocare.com/blog/maintenance/what-does-an-alternator-do/#

“What are the common faults of relays?” Quisure. https://www.quisure.com/blog/faq/what-are-the-common-faults-of-relay

BatteryStuff Tech. “How Do Batteries Die? Well, Let Me Tell You…”. Batterystuff.com. https://www.batterystuff.com/kb/articles/battery-articles/how-do-batteries-die.html

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