An alternator generates AC current, whereas batteries (as far as I know) provide/are charged with DC current. So I'm wondering, how is this apparent mismatch explained? Do car manufacturers employ a rectifier downstream the alternator in order to convert AC to DC? Wouldn't it be simpler to use a dynamo instead, to directly generate DC current? Also considering that, when present, regenerative braking systems provide DC power (so are they connected to the battery pack via a different power transmission line?).

P.S. I apologize in advance if the question does not meet some community requirements, I am new to Stack Overflow, plus it regards a discipline I am still trying to learn about.

  • $\begingroup$ Dynamos were used at least as late as the 1970s. See this one on eBay for an Austin 1300 (as driven by Basil Fawlty). I know because it failed on my girlfriend's car and we had to tow it home. (Forty years married now but we divorced the car.) $\endgroup$
    – Transistor
    Apr 23, 2023 at 17:08
  • $\begingroup$ @Transistor and on many cars until the mid 80’s - a common and easy job was to remove the dynamo and control box and replace with an alternator. The Dino-start was a different beast though. $\endgroup$
    – Solar Mike
    Apr 23, 2023 at 21:32
  • $\begingroup$ a car alternator has DC output ... the rectifier is built into it $\endgroup$
    – jsotola
    Apr 24, 2023 at 3:07
  • $\begingroup$ Best answers came from EE's on their SE. electronics.stackexchange.com/q/290059/297693 $\endgroup$
    – Tiger Guy
    Apr 24, 2023 at 6:53

1 Answer 1


The brush arrangement in a DC generator wears out with use and when it stops working, the driver is stranded with a car that can't start and won't run. There are no brushes in an alternator to wear out and it is therefore less expensive to build and more reliable in use.

The diodes that do the job of the commutator brushes do not wear out and are less expensive than the parts they replace.

  • $\begingroup$ Many alternators on cars DO have brushes and the centre brush would “drill” a hole through the slip ring of some alternators. Those with experience could tell the exact fault of the alternator by the behaviour of the warning light… $\endgroup$
    – Solar Mike
    Apr 23, 2023 at 21:27
  • $\begingroup$ @solarmike, this is interesting. I would like to know more... -NN $\endgroup$ Apr 24, 2023 at 2:04
  • $\begingroup$ @nielsnielsen boschautoparts.com/en/w/…. $\endgroup$
    – Tiger Guy
    Apr 24, 2023 at 6:50

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