On most cars the alternator spins faster than the engine. The alternator 
should be supplying energy to the battery at idle speed, it makes no 
sense for it not to (as you imply). For the typical car idling at 1000 
RPM, the alternator might be going about 1.5k  - 2k  RPM. I have no idea 
where he got the 7k - 9k number, that's ridiculous for anything but a 
race car or a rotary engine.
--
Martin K

Gerhard Fiedler wrote:
> The post (and the reply) weren't talking about max rpm, they were talking
> about the rpm when the alternator starts to supply the energy rather than
> the battery. Which should be a bit lower than the max rpm or you're in
> trouble :)
>
> Gerhard
>
>   

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