I got Smudge to knock me up a piggy-back battery for my Maxx-D – just plugged into the back, was simpler, cheaper (and a WHOLE lot quicker) than going to Exposure themselves. My first and second Maxx-D’s both failed within a few months and turn-around from their customer services was too poor to go through again (2 weeks + of daily chasing and broken promises “yes, it’s in the post” / “oh, sorry forgot” etc – each time not good enough when you’re then faced with borrowing lights for a 3 hour daily commute).
I’ve gone with a Trout Liberator instead as my main light has a lot of use and external batteries will save me money in the long run / outweigh the hassle of a battery bag.
(that said, my Enduro Maxx which is older than my Maxx-D was but has seen less use / fewer charging cycles, has run-times not far off new)