Recently, the battery in my GT6 (Mk1, 1967) gave me some trouble. It ran empty twice soon after I was stuck in slow traffic, in the rain, and at night. Non-ideal conditions for an old dynamo, but I think it should in fact still perform well enough in this setting, at least as long as I don't feel like pushing my luck and turn the heater fan on! So I hooked up the multi-meter and drove around with it on the passenger seat for a couple of days: On normal/good days (no rain, no headlights) it does pretty well, at idle she sits somewhere around 12.5V and the voltage quickly rises to over 15V when I drive off, the cut-off should be set somewhere around 15.5V and I've not seen her go past that yet. However, on rainy days the performance is very different: once warmed up she idles at around 11.5V and she never reaches the cut-off. I took her out on the motorway today just to test and I couldn't get her past 13.2V even at ~4000 rpm. That might just be enough to charge the battery a little, but it's clearly too low.
Is it normal for a dynamo to perform worse in the wet? I can't really see a reason for why that should be, with the possible exception of the belt slipping, but I suppose that should show some audible symptoms as well then. I figure this one's probably more expensive than swapping out the dynamo brushes, right?