An 8o being first registered in 79 isn't hard to under stand. Like most manufacturers, they start making a model year months before the year starts. On one list that has the year and model codes it, it tells the manufacturing year. This year starts August 1 and ends July 31. So anything built after Aug 1 of 79 are 80's. This lets them get the bikes in the show rooms in September or October. If an 80 gets bought as soon as it gets in the show room it will be first registered in 79.
Do you have a multi-meter and know how to use it? if so follow the steps in the Charging system guide. It's not hard to check the system.
Getting full battery voltage at the brown wire at the reg/rec and at the brown wire to the brushes is very important. Using a wiring diagram it's not hard to trace the power flow along the path from battery positive to the reg/rec and the brushes.
Once you establish this power flow is at or very near to battery voltage on the brown wires is acceptable, as in with in about .2 volts of the battery and still get no charge when doing the jumper from green brush to ground, then you can check the ohms of the rotor and stator.
I don't recall if the guide mentions it or not but when doing the stator ohm checks unplug the yellow wire to the safety relay. If not you will get a high reading when checking the white wire ohms, Some where around 20 ohms instead of .9 ohms The actual reading may not be .9, it may be a bit off. As long as all three white wires read the same is more important.
Just a thought, do you understand how the charging system is supposed to function? It helps to know this so you can better determine what's not functioning right. A run down on the function is battery power is supplied on a brown wire to the reg/rec and one brush. When turn the bike on the regulator part of the reg/rec reads the battery voltage on the brown wire. If it reads low which it will untill you start the engine, the reg turns on a transistor, this grounds the green wire. This sends power through the rotor and this creates a magnetic field in the rotor.
This magnetic field excites the stator to produce an AC current flow to the rectifier part of the reg/rec on the three white wires. The rectifier converts the AC current to DC current to charge the battery.
When the voltage at the brown wire reaches the regulators preset level of about 14.5 volts it turns the transistor off, stopping the power flow through the rotor stopping the charging till the voltage drops below the 14.5 volts.
This on/off cycle repeats itself very quickly. This maintains the battery at full charge.
If the brown wire doesn't have the proper voltage, as in it's low, the reg/rec will think the battery is low even when fully charged. This is not good for the battery.
Read the guide and follow the steps. You may find it's a low voltage issue on the brown wire or bad brushes. Only testing will determine the problems.
Leo