I think I've found the correct tip for Ep121 for use with the XP18000 external battery. I haven't received it yet, but I should be getting it before next weekend and I'll post if it works. From Heatlesssun's and Edwood's measurements, I googled 1.1 mm x 3.0 mm dc plug and found this website listing various dc plugs. Plug size 1130 "rs size g" matches both of their measurements exactly. However, I wasn't sure about the barrel length or what "rs size" meant. A google search for "rs size dc plug" brought up radio shack in several posts and this wikipedia article on coaxial plugs confirms that g happens to be the arbitrary letter radioshack assigns to this particular plug (I'm pretty sure the length is 9.5mm, which is essentially what heatlesssun measured, because that standard for these plugs). Finally, therealmav posted about batterygeek.net having the right plug with adapters that would work with the xp18000. On the site, I was able to find a plug of "size g" and a picture showing all of plugs side by side. From this picture and the dc plugs link earlier in the post, you can confirm for yourself that the letters on the batterygeeks website appear to correspond to the radio shack size letters. I ended up ordering the set of 10 dc plugs from batterygeeks since it was only 15 dollars, vs 1o dollars for just the size g plug. Also, shipping is just 6.60 for priority mail which I figured would be quicker than their ups ground option which said 4-5 business days. I know this will be more expensive than energizer's official plug, but I didn't feel like waiting any longer and I don't have a soldering iron to make my own plug. Hope this helps!!