When the bikes are brand new, they 'just about' cope - the headlight is still crap though, as it's just fed directly from a coil, so dims as you slow. There's no regulation at all for the charging, so I suspect it's relatively low to avoid frying the battery.If, as Nigel said in Post #9, there is simply not enough energy provided by the magneto, then taking more from the battery than you put in will inevitably result in the battery going flat, no matter how you tinker with the coil voltage.
Always worth checking.Still say to check the grounds. While not the same bikes over the years of working on old iron(aluminum?) some charging and lighting issues have found to be a bad ground connection. Acheck from battery terminal to the fram , not the bolt holding that ground terminal to the frame, but the frame itself should be done. High resistance will mess with the battery charge.
An earlier road bike I had (a Suzuki T350 2-stroke twin) had no problems with the electrics, but it was 12V with the headlights fed off the battery - I've no idea what kind of regulation was done?. Just had a google, and apparently none according to the wiring diagram, just the alternator and a silicon rectifier. However, there's a connection to it from the switch, so perhaps the current is increased when you turn the headlight on?.Very likely. Guy on another thread had the opposite problem: his batteries kept getting cooked.
This is in stark contrast to an experience a friend of mine had about 35 years ago:including getting harassed by the Police wanting to know what I was doing walking around at that time of night - I explained, and suggested he might like to give me a lift home - but he declined!.