Ugh yuck. Is this circuit frying something mentioned in the manual, or through bitter experience? Seems odd that having an additional load on the hub should cause problems down-stream in the charger/ voltage regulator.
I think it's something that Dahon perhaps didn't realise when they designed the product, because there is a warning sheet slipped into the package in addition to the instructions telling you not to disconnect the Reecharge battery unit and leave the modulator (which is supposed to be fork mounted and permanently fitted to the bike, connected.
Personally I wanted the option of leaving the spades at the hub permanently connected via piggyback spades along with my light, but removing the Reecharge from the bike or simply turning it off to reduce resistance in the hub whenever I liked.
As Kim mentions, this requires a little bodge involving some kind of connectors in-line (Tamiya connectors from Maplins are ideal) and also a switch in-line
before the current regulator.
I have got round this by keeping both the modulator and the Reecharge inside a small bar bag and having a tiny Maplins switch attached to the outside of the bag so I can turn things on and off at will. I'm not all that good at electronic fettling but I managed it with a cheapo soldering iron and a bit of spare dynamo lighting cable.
It is possible to run a dyno light and the Reecharge at the same time so long as you're doing a reasonable speed, but it isn't ideal and just isn't really necessary to since you can make sure your Reecharge is fully charged at dusk and the combo of a fully charged Garmin and a fully charged Reecharge battery pack can easily see you through the night. The Reecharge can charge anything with a USB cable, so handy for phones and ipod etc. if your touring. I can even charge my Kindle