My bike has died on me again, and this time I am stumped to the cause.
It seems to be related to running with the headlights on. I haven't had any issues with running it without lights on.

It would sometimes cut out when stopped when running with the lights on, so I replaced the Rectifier/Regulator, and was getting a far better voltage across the battery at idle with Lights.

I took it out for a quick spin last night, about 20 mins. Then, when I stopped for the first time at a set of Traffic Lights, the bike cut out. Totally dead. No ignition lights. Nothing. I initially though it was a fuse, so check the fusebox under the pillion seat. The 15A headlight fuse was melted. The plastic was melted, but the filiment looked intact.

Took off the seat unit to check the main 30A fuse. Fuse again looked intact, but the whole connector block above the starter relay was melted!

The electrics on the bike are standard, except a new Regultor/Rectifier, and the relay mod on the headlights so I can run UK spec H4s without blowing my switchgear.

Does anyone know where I should even start with this one?

The bike is a '89 NC30.

When you changed the reg/rec why didn't you change the battery.reg/rec fail one of two ways. Normally if the reg goes it over changes the battery and that over charge gas to go somewhere, melting plastic sounds like its this fault and not the other where the reg/rec fails and the battery gets run down to where it doesn't do much.

I'd say the reg/rec has failed and mullered the battery too, you've fixed the reg/rec but the damage has been done.

So what's the best plan? New battery and fix the melty bits, or is it going to be a lot worse?