charging issue?

[greeninbetween]

New member, new to me '83R65, new problem. Got the bike last week and everything checked out on inspection and ride. Previous owner installed new AGM battery recently. After getting bike home and checking fluids, valves, etc., took a few short rides. After about forty miles total, I went to start the bike after stopping at a local shop and the starter didn't spin fast enough to engage and the headlight was dim......depleted battery.  Bump started the bike and headed home. After a minute or two, the bike shut down, I coasted to a safe spot, and my ever patient wife picked us up with trailer. Checked battery with multimeter and it was reading a mere 9.5 volts. The red indicator light only lighted when the key was on and the engine not running. It never came on or flickered when the engine was idling or running. Removed battery to bench and put my 1.25 amp smart charger on it. The charger was not happy after a while- getting very hot- so I put my larger charger on the battery and it indicated an initial 6 amp charge and tapered down very slowly.. I left it on overnight, as the battery seemed reluctant to suck up a charge. Voltage in the morning was 12.2 volts, and I re-installed in the bike. Turn key to on and red light is bright- start bike and red charging light goes out, but voltage across battery terminals when running is less than 12 volts.
  Any help and suggestions would be welcome before I start ruining my eyes on the wiring diagram!

[Bob_Roller]

I would put your multimeter back on the battery, start the engine and increase the rpm's to 4500 rpm and see what you get for voltage .
If you can check this in low light conditions, check the GEN light, it may be coming on dimly, not readily noticeable in bright lighting conditions .

[mrclubike]

The good news is if the GEN light comes on with the key the rotor is not open (could be shorted but not likely they usually fail open causing the gen light not to turn on with the engine not running)
Like Bob said start it up and see what the voltage does when above 3500rpm
The voltage should start out above 12.8 and start climbing as the battery charges (the more discharged the battery is the slower it will increase)
When it gets fully charged you should see around 13.8
If you have a AGM battery i would get a adjustable reg and bump that full charge voltage to about 14.3
That is if you do not have any extra load beside the head lights and tail lights
If you do not see this happen
Your battery is either bad or you have a charging issue
you can full field the alternator by jumping out the reg (unplug reg and jump the blue and Black wire together)
and see if the voltage shoots up if so you reg is bad
if not chances are the diode board has some bad diodes
And of course you could have some bad wiring causing all of this

By the way the charging system is not that complicated  so looking at the schematics should not "ruin " your eyes

[mrclubike]

I would put your multimeter back on the battery, start the engine and increase the rpm's to 4500 rpm and see what you get for voltage .
If you can check this in low light conditions, check the GEN light, it may be coming on dimly, not readily noticeable in bright lighting conditions .

If the GEN light is staying on dimly as bob suggested you check
 this would indicate that  diodes  are failing (generally the 3 that feed the rotor) or of course bad wiring connections can cause all of this 

[mrclubike]

O and get a dash mounted Volt meter so you know if you are charging or not before the bike just quits
The gen light is not the best indicator

[greeninbetween]

So far, the alternator light does not even remotely light up in the dark at idle or at 4000 rpm. The battery was indicating 12.5 volts before starting engine, and dropped to a declining 11.5 volts at idle and higher rpms with lights on. Fully charged battery lost no voltage overnight on bench, and recovers to 12.5 after shutting engine down. Took front cover off and there appears to be some minor corrosion, so will try easy stuff first- clean all connections, before performing suggested tests. Something to do while waiting for Hurricane Florence to go away!

[Bob_Roller]

When you turn the key to the ON position, before starting the engine, does the red GEN light come on ??
A burned out light bulb, will prevent the charging system from working .
It could be associated wiring, or an open circuit in the alternator rotor .

http://bmwmotorcycletech.info/trbleshootalt.htm

Your brain may hurt after reading this, so take it slow !!!!