Battery overcharging.

[Reckless Rat]

I would guess that you definitely need to back off the 3rd brush - you really don't need or want to be shoving that amount of current into the battery. For the summer/winter to function you need a resistor across those two terminals of about 3 ohms. (assuming the rest of the wiring is correct.) - I'd also treat myself to a new bottom hose before it disintegrates as well!

I run with the metal dust covering strip removed as it helps to cool the dynamo. You can also adjust the third brush easily.