That awfully clever car battery charger is on offer again this Thursday for 12.99. It's just about small enough to fit in a pocket but punches well above its weight. About 4 amps output. Does pulse charging to try and recover a badly discharged battery. Goes to maintenance mode after charging. Similar costs over 40 quid elsewhere.
The max output seems low on paper but in practice it charges a battery fully as fast as many of over twice the nominal output.