You may have a bad stator
As @paulster2 stated, you should have your battery tested. You could have a lead plate the broke free inside the batter and grounding out that cel to another cel. Your voltage test could be an indicator that it is not your battery though.
Here is a method you can use to test your stator.
Testing Your Stator
There are three yellow wires that come from your stator. They come out of the left side crankshaft cover and route into your sprocket cover and out along the cast bottom portion of your frame on their way to your rectifier. They connect directly to the rectifier. You can disconnect them from the rectifier and test them with a multimeter. Your first test will be static. The engine is not running. Be sure to disconnect from the rectifier and not just try and pierce the wires with the multimeter end. They are special wires and insulated a bit more, these are one of the few wires on the motorcycle that carry AC current.
Static Test Set multimeter to ohms and test between all the leads in pairs. The reading 0.1 to 1.0 max resistance.
No Load Test Dynamic Test Set your multimeter to AC Volts and start the bike. It should be cold. Have a friend hold the RPM's at 5,000. Test between the leads. You should see more than 70V. If it's below that, you may need a new stator. I would replace, but I'll get to that later.
Testing Your Rectifier
Your rectifier has diodes in it. Diodes are one way valves for electricity, think of a reed valve in a two stroke in-take. Since this is a three-phase charging system you need diodes to join the AC current into a single output and convert to DC. I could get more detailed but I want to keep it simple.
Rectifier Resistance Test Using your multimeter set to ohms connect the multimeter to the ends of each of the diodes and check the resistance in both directions. You should have low resistance in one direction and higher in the opposite direction. Generally, you will want to see 5 - 40 ohms of resistance in the forward bias direction, and infinite resistance in the reverse bias direction.
Procedure Attach the black probe (-) of the meter to the ground side of the rectifier (black wires) and the red probe of the meter to each of the three contacts for the stator. Record the numbers. Then swap around the meter leads (red and black are swapped) and take the readings again. You have thus measured the ground side of the rectifier.
If you have lower resistance in both directions (5+ ohms) then it's shot. If you have infinite in both directions, then it's shot.
IF you need to replace your stator or your rectifier I would consider going aftermarket. Your era of motorcycle has a bad track record for stator and rectifier life spans.
Link to stator replacement
Link to rectifier replacement
The aftermarket stator and rectifier replacements I have procured have been far more reliable for Suzuki and Honda motorcycles of the era.
Best of luck. Hope it's your battery.