If you plug your smartphone into your cars usb cable which is part of the stereo system I believe, I guess it’s charging itself whilst plugged in.

Obviously you should not leave it in accessory mode as it will drain overtime.

But what about when running? Is the smartphone draining to the point the alternator can’t replenish etc and so one should not attach phone to car, or is the alternator recharging the battery when running regardless of iPhone use.


1 Answer 1


Yes, the smartphone takes power from the socket when it is powered.

However if you check the demand of the smartphone in W and AH then you will find it is very small compared to the car battery capacity and the alternator output. Check the units: car battery in AH and smartphone mAH...

If you have a car battery going flat it is due to one, or both, of two things:

  1. something is left on (lights etc, even interior or boot light)

  2. a failing battery - common with older batteries.

So, in short, if the battery is going flat, you have a problem to find but it won’t be the smartphone.

  • I think you are mixing up two very different units of measure ... AH and Amps are way too different things. While the car is running (as long as the alternator is working correctly), the thing you should be looking at are Amps. Amp Hours (AH) determine how long a source can provide power for given a temperature and a certain amount of draw. Amps (or ampere) is a unit of electrical current, which it appears is what you are trying to describe. If the car was not running, it would be a completely different story and AH would rule the day. Oct 1, 2020 at 16:41
  • @Pᴀᴜʟsᴛᴇʀ2 perhaps you are mixing energy and power...
    – Solar Mike
    Oct 1, 2020 at 16:50
  • Nope. Don't think so. Oct 1, 2020 at 16:55
  • @Pᴀᴜʟsᴛᴇʀ2 car batteries have CCA and AH ratings... phones, having smaller batteries (also a different technology, tend to have mAH ratings.
    – Solar Mike
    Oct 1, 2020 at 16:58
  • Which all comes into play ... if the car isn't running. Since the car IS running, Amps and the means by which that power is supplied is ... I'm saying your answer is barking up the wrong tree. Oct 1, 2020 at 17:03

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .