My Valk won't charge at 1000 RPM. It got to be above 2500.
At idle [900 rpm] and with over 200 watts of accessories running my alternator [OEM @ 85k New brushes @ 60k] is still putting out 14 volts. At 1500 rpm it's doing 14.2. When I park it the battery is at 13.2+, so it is charging.
However, at 2 weeks of not riding it won't start. This is due to the memory drain of the car stereo, CB memory and the alarm. So, if it sits for a week I put the Tender on it.