Howdy all,
Here comes a comment from one who is currently running on an 8 volt battery, bought before I read all the better advice here. The generator did not charge the battery before I tweaked the regulator. After doing that, it has been performing fine for 2 years. The voltage regulator is set at about 9.3 volts. In those 2 years I have had to change one headlight, which was over 25 years old. Also, here in Sweden the law says I must drive with the headlights always on, so they are getting plenty of use. I think the concerns about bulbs not tolerating the higher voltage are exagerated.
That said, my next battery will be 6 volt.
Karl