Good point BA, a nicad charger wont charge lithium batteries, yes the adaptor will fit your old xrp charger but it needs to be a lithium compatible charger as the adaptor will have a lithium battery strapped to it, when I bought my/the very first lithium dewalt drill/impact combo the 'nano' the shop said although it was still an xrp fitment don't get your chargers mixed up, the nano charger would charge the batteries it came with i.e. the lithium and it would also charge the nicads too as it was multi chemistry BUT the nicad chargers for my other tools which I had plenty of will not charge the lithium as they were only dual chemistey, was told to mark the chargers as nicad only and lithium/nicad
Don't know which you have but the DE9135 is the multi chemistry charger that is ok with 7.2V - 18V NiCd/NiMH/Li-Ion batteries and the DE9116 is only for 7.2V - 18V NiCd/NiMH batteries
So if your lucky enough to have a 9135 XRP style charger it will accept your adaptor with a XR lithium strapped to it
If you only have the 9116 although still XRP and the adaptor will fit and with a lithium battery on it but the charger will not charge the battery as your asking a nicad charger to charge a lithium battery
You will need to invest in a dedicated xr charger I'm affraid or certainly ask a reputable dewalt outlet first
Give these guys a call, i use them for all my dewalt stuff, always got offers on, every time I order something from them I always seem to get something for free as there's an offer on, extra 5ah battery, tool bag, tough system box once, diamond discs, impact bits, dead helpful and got a cracking funny facebook page and sure to answer any questions, think they price match as well
http://www.thebigboystoyshop.com/dewalt-premier-centre-25-w.asp