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reply to fartness

Re: Do I need a new battery?

This is quite common in winter. It's cold out, so the heater is on full blast. It's dark, so the headlights are on. It's snowing or misting, so the wipers are running. Many high current things running at the same time. The alternator can't keep up with demand at idle so the voltage drops a bit. I see this on our Suburban, even though it has a 105-amp alternator to help deal with dual heat/AC's. The output curve on an alternator falls off a cliff as the engine approaches idle speed. As long as you aren't idling for long periods of time (like a cop car, ambulance, etc) then don't worry about it. As soon as engine RPM is above 1K or so then the alternator will keep the battery charged.

There are alternators designed for high output at low RPM, but they're expensive. They're used in emergency vehicles and taxis. They're also quite popular in the hard core off-road crowd to help keep up with heavy winching, lots of high-draw off-road lights, etc.