External battery packs

I've complained that mobile computing devices have inadequate batteries (see my blog of 2 March 2012).  This is especially true of larger screen devices.  My  Thinkpad, for example, which I use to run Unix (openSolaris), weighs 2.35 kilos, of which only 0.3 kilos is the battery! The new external battery packs (like those for ipads) are a help although the small plugs are not very sturdy and worry me some.  I'd prefer that the auxiliary battery clip securely on to the tablet (or laptop).