[allotmenteer]

Just seen these on ebay:

ADJUSTABLE TRANSFORMER - 12-28V IN TO 3-15V OUT - BMW - eBay Battery Chargers, Batteries Battery Chargers, Consumer Electronics (end time 04-Nov-07 15:51:46 GMT)

They look just the trick for powering lights from a spare 24V battery. My li-ion is about 28.4V fulled charged so it should be ok.