Hey Friends!
    Thanks to all your help, my '85 K100rs is running beautifully!
    Now I want to help my friend get his K75 running so he can come ride with me. I think it's a 1991 or 92, but I'm not sure. I can check if it matters.
    He's having charging/battery issues. The bike was running (sort of) until about six months ago. He had to jump-start it pretty much every time, but then it would run. He replaced the battery, but it didn't help much. So he let the bike sit for six months and is now working on getting it back on the road.
    He replaced the voltage regulator (not sure that was necessary), but is still having issues.
    Just now, he had the bike running and was measuring voltage across the battery terminals. At 1500 rpm, he was reading 13.7 volts (good, right?). Drop to idle and he initially measured 12.8 volts, but it dropped pretty quickly to 11.6.
   After turning the bike off, the reading went back up to 12.5 volts. Turned the ignition on and the voltage dropped back to 11.
   This sounds to me like a massive current leak somewhere. Do you agree? Do you think the battery is shot, too? Or could a good battery plugged into a bad bike give these symptoms?
   Thanks as always for your input!
   -Jon

Ks don't do much at idle, when you fire up first the red charge light normally stays on and you need a quick rev to get the alternator to kick in.

BUT, if he put a good battery in and left it six months its quite possible the battery is toast at this stage. The 12.5 volts dropping back to 11 volts could be a sign of a bad battery if it's not capable of holding much charge.

If you have the ignition on you can check which circuit is your problem by taking fuses out one at a time to see where the current is going. This might be an indication of what direction to look.

Does or did the bike have heated grips?

Dropping to 11 volts when the ignition is turned on means the battery is shagged.
Get it load tested to be sure but I will bet on it.

Yeah, the one-fuse-at-a-time idea seems like a great way to isolate a current issue. Any idea what the current draw would be on a properly functioning K75 with the ignition on but not running?
     I don't know about the heated grips. I'll ask my friend. Are those particularly suspect for a battery drain?
   -Jon

Rick, I was thinking the same thing myself, but I don't want to put a new battery into a bike with wonky electrics that will just fry the new battery, too. So, I was thinking to get as much info as possible with the existing battery. Is it possible that the battery is okay, and there's just such a substantial short that it drops the voltage that quickly?
   -Jon

At 2500 to 3000 rpm the charging voltage should be 14.2 or very close and even with a current draw of around 8 amps the voltage should not drop below 12.2 with the key on and not running.
Check the voltage when cranking (unplug the fuel tank connector so it wont start) should not drop below 10 volts.

If your friend has a U.S. model, the headlight, taillight and instruments come on when the ignition is turned on.  This is going to put a load of about 10 amps on the battery which is enough to pull the voltage as low as you are seeing when the battery is shagged, which it probably is, having sat for 6 months with little charge on it.

One check that comes to mind is to disconnect the battery cables from the battery and connect an ohm meter to them.  With the key off the only load should be the clock, and I would guess that it might read as 10k ohms or higher.  If you read something a lot lower, like in the 1000 ohm or less range then pull the fuses one at a time to see when the resistance goes up.  That should tell you what circuit has the problem.

One thing is that if the ABS is playing up it can drain the battery when the bike is sitting. If it has ABS the problem will show as a 7 code

I'm another for the failed battery vote.

Battery is showing 12.5V with ignition off, suggesting the ignition and circuits are in fact good and unlikely to be a parasitic power drain.

Definitely a case of sort/eliminate battery before looking anywhere else.

Don't get tempted to try to start with a bad battery, you may end up destroying/welding the starter relay.