I had the bike in the garage for a week a short while ago because of persistent warm, muggy, stormy weather in the upper Midwest. On starting the bike the morning after that spell, the headlight and panel lights pulsed in a regular fashion for a while, and the battery voltage started climbing up toward 15V. Not wanting to replace the battery or headlamp bulb I switched the bright + driving lights + heated grips on to pull the voltage down. It was successful, and it also stopped the pulsing. After about a minute everything settled down, and the electrical system has behaved as normal ever since.

My guess for the cause would be moisture in the alternator which rapidly dried out with the heat of operation. Would this be a good guess? Has anybody experienced this before? Do I need to worry?

Purely guessing here.  I think you mean you checked the voltage at the battery while the engine was running and got 15?  This would be somewhat normal after a start-up from a weak battery as the alternator is trying to charge it back up an can put out about 14.5 volts.  I think the battery at rest should be about 12.5 volts.

Your voltage regulator (inside the alt) maybe acting up.  If it ultimately fails, you can usually find a replacement alt for about $50.  You could just replace the regulator too.

At a minimum, I would check the fluid levels (if it is not sealed) in the battery and place it on a normal battery charger overnight to bring it back up.  Then, just keep an eye on it for a few weeks to see if the issue comes back.

not only would I check the electrolyte level in the bat, But I'd pull both cables off, scrub them good with sandpaper, and the terminals also, and re-install, might even do the ground cable to engine junction also...
That pulse sounds like you were putting out more amps than the bat could absorb, common on a bat that is low, and has a resistive coating on the terminals... Turning on all the electrical stuff to drain off extra load didn't help the battery at all, likely just made the generator work double duty... not wise.

Yes.  Exactly right.  I always forget to add the simple stuff I do automatically.  That connector at the alternator has a habit of burning up as it gets older too (unmaintained) and can leave you stranded with a dead battery.

Thanks for the replies...

Hmmm, not to dispute the very wisdom I asked for, but I have never, ever seen my voltmeter rise up to 15V within seconds after starting the engine, and certainly not after a week of sitting around: it settles nicely at around 14.2 to 14.3 with no load and street-cruising RPM. Secondly, above around 2800RPM the alternator has always managed to drive the two extra lights and the heated grips, with the battery voltage drooping to around 13 or so, so I don't worry about overloading it...

But I do like the comments about the battery terminals: if the connection to the battery isn't good one won't get good charge current into the battery which should, by all accounts, be a little lower than normal after sitting for a week.

I'll get under the seat and check electrolyte level and terminals.

Thanks again for your input, gents!

Thanks for the replies...
....... I have never, ever seen my voltmeter rise up to 15V within seconds after starting the engine, and certainly not after a week of sitting around: it settles nicely at around 14.2 to 14.3 with no load and street-cruising RPM. .....
But I do like the comments about the battery terminals: if the connection to the battery isn't good one won't get good charge current into the battery which should, by all accounts, be a little lower than normal after sitting for a week.

I'll get under the seat and check electrolyte level and terminals.

Thanks again for your input, gents!

you DO(?) understand your voltmeter is giving you the reading of what is going to the battery.... not what is going from the cable, THRU the battery and out the other cable.... don't you?

a resistive connection will do this....and the battery will not charge.

Finally got around to checking this response...

I have to go look at the schematic and trace the actual wiring now, M.O.B., to see how current actually flows around the bike.

But while I'm doing that...
In principle, the alternator and voltage regulator provide charge current to the battery and operating current to the electrical loads. If the loads are low, then the vast majority of available power will charge the battery. If the battery is fully charged and properly connected (low terminal resistance) then it will provide voltage stabilization because of its impedance. One shouldn't see that flickering, much less 15V, if the battery is connected properly.

Secondly: voltage is potential ("pressure", to the hydraulically inclined), not flow. It is a measure of what is "at" a point, not of what is passing through or past a point. Applying a voltage to a point doesn't mean that a current is flowing past it. I think this is what you are alluding to, M.O.B.

When I said "I liked the comment", I meant that I felt it was a likely diagnosis of the problem, not that it was a good thing :-)  I totally agree with you that the high voltage indicated by the meter is indicative of a dirty battery connection.

But what was interesting to me was that the problem seemed to resolve itself within a minute or so, so I was speculating that the regulator was driving too hard (in agreement with the other part of your comment), and that poor battery connections merely showed up the problem.

Anyway, I've cleaned the battery connections, and haven't seen the problem reoccur.

Thanks for your reply!