[brucepick]

Many Hondas - our Gen 6 ones included - have an alternator cutout circuit that activates under certain conditions. The purpose of this is to increase fuel economy. When the computer cuts out the alternator, the unassisted battery voltage is what you will see on your gauge or meter Typically it drops fairly rapidly to 12.5V, and then drops slowly towards 12.0 as you drive. In my experience, I'll end up bump-starting the car to regain speed before it gets down to 12.0. After an engine restart it will run at 13.x for a few seconds, and then goes back into alt cut mode as long as all the conditions are met. I never learned yet, at what reduced battery voltage it goes back into full charging mode.

Conditions required for alternator cutout:
Battery not absorbing significant charge from the running alternator
- - That is, after an overnight sit and running the starter motor, the battery will absorb charge due to some charge depletion. The computer will detect the load on alternator and will NOT cut the alternator till the battery charging load reduces, due to the battery now having been recharged.
Car not in fuel cut mode. If you go into a coast with engine engaged, it will run the alt. at full voltage. At least you get something out of that coast.
Speed not above 50 mph. Engine rpms don't matter, just road speed.
Very low electrical load. Headlights on will take you out of alt-cut mode. Small corner lights only, fan on 1 or 2 (maybe), or radio (assuming no moster amp +/or subwoofers) will not take you out of alt-cut mode. Power windows will take it out of alt-cut mode. In all cases, when you stop using the device, alt-cut mode will come back within a second or two, as long as all other conditions are met.
Engine idling? I can't recall whether idling kicks it out of fuel cut mode or not. Easy enough to test that though, now that you know the rest of the logic map for this.

This is managed by a device known as the "ELD", the Electrical Load Detector. It's in the under hood fuse box, a low flat panel that doesn't look like a removable relay or fuse. Not really intended to be messed with, I'm told. I think if it goes bad you replace the whole fuse box. Failure mode is that voltage goes haywire, high and low voltage levels. not under true control.