Voltage Inside a Car

I previously wrote a post with some calculations about the power supplied to laptops from a car battery [1]. A comment on the post suggested that I might have made a mistake in testing the Voltage because leaving the door open (and thus the internal lights on) will cause a Voltage drop.

So I’ve done some more tests:

Test Voltage
battery terminals 12.69
front power socket with doors closed 12.64
front power socket with doors open OR ignition switch on 12.37
cigarette lighter socket with ignition switch on 12.32
front power socket with doors closed and headlights on 11.96
front power socket with engine running 14.38
front power socket with engine running and headlights on 14.29

In my previous tests I recorded 12.85V inside my car (from the front power socket which although having the same connector as a cigarette lighter isn’t designed for lighting cigarettes) and 13.02V from the battery terminals – a 0.17V difference. In my tests today I was unable to reproduce that but I think that my biggest mistake was to take the reading too quickly. Today I noticed that it took up to a minute for the Voltage to stabilise after opening a door (the Voltage dips after any current draw and takes time to recover) so a quick reading isn’t going to be accurate.

My car is a Kia Carnival which has two sockets in the front for power and for actually lighting cigarettes. The one for lighting cigarettes has a slightly lower Voltage and only works when the ignition is turned on. The car also has a power socket in the boot (the trunk for US readers) which delivers the same Voltage as the power socket in the front.

Also one thing to note is that today is a reasonably cold day (16.5C outside right now) and my car hasn’t been driven since last night so the battery would be quite cold (maybe 12C or less). My previous measurements were taken in summer so the battery would have been a lot warmer and therefore working more effectively.


The Voltage drop from turning on the internal lights surprised me, I had expected that a car battery which is designed to supply high current wouldn’t be affected by such things. Certainly not to give a 2% Voltage drop! The Voltage difference from reading inside the car and at the battery terminals might be partly due to the apparent lead coating on the terminals, I pushed the probes of my multimeter beneath the surface of the metal and got a really good connection.

The 14% Voltage increase when the engine was running was also a surprise. It seems to me that if you are running a power hungry device (such as a laptop) it would be a good idea to disconnect it when the engine is turned off. A 14% higher voltage will give a 14% lower current if the PSU is efficient and therefore less problems with heat in the wiring and less risk of blowing a fuse.

Also it’s a good idea to be more methodical about performing tests than I was before my last post. There are lots of other tests I could run (such as testing after the engine has been running for a while) but at the moment I don’t have enough interest in this topic to do more tests. Please leave a comment if there’s something interesting that you think I missed.

6 comments to Voltage Inside a Car

  • PiRX

    Voltage increase with running engine shouldn’t come as surprise. That’s because alternator is running and supplying additional voltage to circuit.

  • Mathieu

    The 14% Voltage increase can be explained by the fact even though your battery is spec’ed for 12V, it gets charged when receiving something between 13.5V to 14.5V: that’s what your alternator delivers.

  • Have you considered that you might have a bad car battery?

    I expect higher voltage with the engine running because the alternator has to charge the battery. I would have expected around 13.5 volts from the battery terminals. In my rather non expert experience, a 12 volt reading at the battery terminal is a precursor to death. The sharp drop under even trivial battery loads suggests impending death to me.

    Your mileage — ALL puns intended — might vary.


  • Let a friend of mine (hi Henk) read it and he said (translated with google translate):
    “not so strange, the wires are thin and you also have the internal resistance of the battery.
    If the engine is running the alternator ensures that the voltage remains at 13.8 volts and the internal resistance does not matter anymore”

  • Joey Hess

    For some years I’ve been doing ad-hoc experimentation with using cheap car laptop power adapters and USB chargers in an environment ranging between 6v and 15v.

    The USB stuff has held up well. The laptop chargers tend to die after about a year, although there’s a lot of variation between models.

    The best way to prevent them melting is to avoid charging the laptop when the voltage is low. That’s the danger area. Running a fully charged laptop with significantly less voltage than the charger is rated for is unlikely to cause enough heat to cause a problem,
    assuming that the laptop is not particularly hungry for watts. Mine use 15 watts.

    Adding a fuse to the adapter, if it doesn’t come with one, can also extend its useful life.

    I’ve never had any damage to the laptop, no matter what happens to the chargers.

  • Anonymous

    One other major thing to test: what about the voltage while cranking the starter? That voltage drop is often enough to cause a power failure condition in sensitive devices.