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Let me get this straight, you measured battery voltage with the engine off and got 17 volts? If so I would say your new meter is FUBARd. Radio Shack will exchange it won't they?
I once destroyed a meter by accidentally measuring battery voltage with the leads plugged into the current ports. Something in the meter blew to protect the fuse. The fuse in the Fluke I bought will interupt 2000 amps and costs $8 so I never use the current function any more because I am a forgetful, ham fisted idiot.
Most meters use Mean Average Deviation to get AC readings. The AC is rectified and then averaged (low pass filter). You can then multiply by the factor that would convert the full wave rectified sin wave average value to what the Root Mean Squared value would be. Only expensive meters (>$400) have the signal processing capability to do true RMS measurements and the are limited to like 10 kHz.
The peak function on most moderately priced meters holds the peak sample that comes out of the A/D converter. Since most meters use a dual slope A/d (slow but accurate) this is sampled at a pretty low rate. It would hold the peak average AC reading not the 60 Hz peak.
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