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Battery Dies Overnight. Why?

Fernando the electronics guy is an electronics engineer. He earned his Bachelor of Science in Electronics Engineering from UC Riverside.


Battery dies overnight. Why?

Understanding why your battery is discharging overnight starts with understanding why alternators are needed and how they work.

The Alternator: Three-phase AC Production


In order to make sure alternating current is produced in a way that means constant current, there are coils and goodies inside the alternator that are out of phase by 120 degrees. This is great, as you’ll be able to charge your battery with no headache, as there is current being offered consistently (no drops in current that are too drastic).

This means your battery is charging. Great!



Diodes aren’t hard to explain. They’re essentially components that, simply put, don’t allow current to flow backwards.

Diodes also convert alternating current (AC) to direct current (DC). This process is known as rectification. The reverse process is what inverters are made for! Anyways, there are diodes, called rectifier diodes, that simplify this AC to DC conversion.

It’s important here to note that your car battery needs to be fed DC current in order to charge correctly.

Why it happens: Overnight Discharging


If you haven’t figured it out by now, a discharging battery is a symptom of a bad diode. While you are sleeping, the diodes are failing at blocking current from going backwards. As a result, you are feeding current towards the direction of the alternator. Your battery is discharging! Oh no.

However, it is also possible that your car battery simply can't hold a charge. This should be ruled out first.


Your battery is discharging due to:

  • A bad set of diodes (bad alternator)
  • A bad battery that won't hold a charge.

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