[paulgato]

Thanks Redpoint. Using after-market HiD's for the dipped headlights WOULD solve a few problems at once - you're right - and it is an elegant solution in that regard. (No headlight dimming without the alternator, and less power consumed.) However when I looked into it I found it wasn't that straightforward for my car, ...mostly for legal reasons but also for technical/cost reasons. But I will look at it again another time.

[EDIT: I think I decided to try first upping the headlight voltage by running power to them direct from the battery via a pair of relays. The stock wiring is rather thin and goes the long way round via the headlight switch on the dash. Fitting those relays would be cheap (£10?), easy, legal, and would increase voltage by at least a volt, so when my battery is low (11.5v) the lights will be as bright as they are now when the battery is fully charged (12.5v). At 12.5v the lights are definitely bright enough, at 11.5v they are only just OK, but when the battery gets really low (11v?) the headlights do dim alarmingly and that's what compels me to stop and reconnect the alternator. The engine will still start at 11v, and everything else seems to work fine, so fitting the relays should increase my night-time alternator-free driving range by a margin. I'll do that first and see how it goes.]

I tend to use headlights only when necessary, and having good, strong, LED sidelights means I often can get away with just using those. Around town at night I will typically use sidelights only (all LED's on my car) and only switch headlights on when approaching a junction. (Sidelights-only-running is legal in street-lit areas at night in UK.)