I want to install a new stereo for my car which requires 12V. I tested the battery first it and shows 12.29V but when I started the car the battery became 6V. Is this normal? What am i doing wrong?

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2 Answers 2


If you are using an auto-range / auto mode digital voltmeter which automatically measures either AC or DC voltage depending on what it detects, it is erroneously trying to measure AC voltage because it is detecting the AC ripple voltage produced by the alternator. If your meter has a manual range and mode setting, set it to Voltage, DC, 20V to get a true reading.

Or try a less "intelligent" voltmeter!


You car has 12V system. Go on with your stereo.

Rare exceptions:

  • VERY old cars (pre-1970). Some of them are 6V. If you happen to own one, you would know better.
  • European trucks of more than ~5 ton payload (24V).
  • European busses of more than ~15 passengers (24V).
  • Some US trucks and busses (may be 24V, but majority of them are 12V no matter how big)
  • NATO-standard military wheeled vehicles and machines of almost any size (24V).
  • Beware of anything with 2 batteries (high chance of 24V, but not 100%).
  • EV and hybrid cars (they have 2 systems - one 12V, the other something between 48V and 480V)

  • a 12V car system will measure between 12V and 13V with engine off, down to 9V when cranking and between 13V and 15V with engine running. This is normal.

On the other hand, from your question one can deduce that your electrical skills are somewhat insufficient for installing the stereo yourself.

For the sake of fire safety, get someone knowledgeable to help.

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