I seem to remember reading somewhere that the GS500 needs to achieve four to five thousand rpm to begin charging the battery while in operation. Can anyone of you vets confirm or deny such numbers?

As a general rule, the charging begins at a lower RPM, probably around 3K, but IIRC, the alternator doesn't get to MAX output until about 5K. The EASY way to test the charging system is to watch the headlight as you rev the engine; if it gets brighter when you increase the revs, the charging system is working. It might have some problems (weak output, for examole), but if the headlight gets brighter, the basic system is intact and functioning.

Yep!It starts charging around 3500 rpm give or take a couple 100 rpm.If I'm in stop and go traffic, I usually keep the rpm up around 5000 when I'm moving.It's embarassing enough to stall a bike,but stall and a dead battery in cause for suicide.J/K  Out on the high way,I stay in high gear.If the speed limit is low and I can't keep the rpm up around 5000.I either drop down a gear or if there is little to no traffic,I take a quick 2 or 3 mile spirited ride. 

Its about 3k but there is no really exact figure, it depends on the condition of the battery, its state of charge and the demands being put on it at the time. Put a voltmeter across the battery and bring the revs up, as soon as the voltage across the terminals starts to increase its starting to charge. Discharged batteries will begin to accept charge at lower engine revs than partialy charged ones.

around 3500-4000 depending on humidity and temperature for me. 5000 is the normal cruising rpm, reguardless of the actual mph. mine just sounds nice right there

Thanks guys!

To test the charging system, you need to be at 5000 rpms.

That's good to know. I generally also never let the bike fall below about 5k when riding except for coming to a light or cruising at a higher speed without a lot of traffic, so I don't have to worry about someone changing lanes into me.
In traffic when it's still cold for the first 3 miles or so I keep the revs up a little too, for the same reason as ben2go. I don't trust it not to stall.

I'm hoping that will change some with a rejet.   

my gauges dim with lower rpms(idle) and they're freaking LEDs  the headligh also dims with the rear brake light activated.

About 60 years ago when I first started checking auto regulators and generators (DC) we would hook up an ammeter in series between the + battery cable and the + battery terminal and a voltmeter in parrallel from the cable to ground.  Gave a much better picture of what's going on as far as when the charge starts flowing to the battery and at what voltage and net amperage of the charge as well as max charge at higher rpm's.

I just dug out a Clymers for 250-450cc Honda twins and they show the same setup and I actually did it that way on my old CM400A that had alternator problems.  It would start charging at fast idle somewhere between 1500-2000 rpm, didn't have a tach on that Hondamatic, but it had a kickstarter. 

I'm going to check my GS500 that way one of these days because I'm pretty sure it charges much sooner and much more than indicated here.  I've run the battery down in freezing weather, jump started it, and taken a short ride of only 15-20 miles of putting around at 4000-6000 rpm in freezing weather on winter streets and it would then start again the next day in freezing weather.

I could hook up the meters on the Hondamatic and then kick start it so I wouldn't blow the ammeter starting it with the starter load amperage flowing thru the meter.  On the GS I will have to start bike and then hook multimeter to battery cable end and to battery post with alligator clips and then seperate the cable from the + post.  Have done that before on one of my bikes, don't remember which one.  Blew the multimeter I had back then doing that on JP's GS1100E and just have a little one now so I'll need a better one to do it.

I was searching around and found this GS500 charging system test PDF from Electrexusa.com who make mc electrical componets that was saved by our John Bates.  It calls for a voltage reading of higher than 13.5V at 2500 rpm and lower than 15.5V at 5000 rpm with a fully charged battery.  I'm guessing it must start charging at approx 2000 rpm particularly with a low battery.  I recall my old 82 CB750K started charging at about 1800-2000 rpm. Very easy to see on the ammeter, when it goes from - to + the baterry is going from discharging to charging.

http://www.bbburma.net/Documents/JohnBates_ChargingCircuitTests3.pdf

I just got a brand new battery for my GS, and darn ... starts like a beauty even in cold weather. I'm a student, so I use my GS mainly to commute really tiny distances. If I'm not going on a ride, then I'm just normally putting on 5-10 minutes (15 max) of engine time one way to classes/grocery store, and the same going back.

How long should I run the gs for to ensure I don't have to pull the battery and put it back on the tender? (a pain in the a**, since I've got a hard moped case on the back, so the seat doesn't pop off unless I remove the case). I've been in the habit of taking a spirited longer ride every two or three days, but recently I've been swamped. Any ideas?

Depends on where you live/use the bike, I wouldn't expect a problem with your short trips here in NE OH in the summer but it might be a problem in the winter.  Seems bikes get hard starting after they sit unused a week or so in the winter, doesn't take but a single turn of the engine to start it in the summer.

I took my little multi-meter over to the garage to get the bike about noon today when it was in the mid 30F's.  Checked the battery and got 13V reading before starting it up.  Started it on full choke and it idled at about 1300-1400 rpm.  Checked the voltage again and it was 15V.

Shut it off and checked and got 13V again.  Restarted and the speed on choke was picking up as it warmed so I set choke to give 2500 rpm and voltage was reading just under 15V.  Put the seat back on and went for my ride.   

Seems it is charging quite low when the battery needs a little charge after startup.  I still have to see it on an ammeter to believe it all.  I worked with hydraulics way back when I worked and I want to see the flow (amps) as well as the pressure (volts).