It holds enough for a full charge for my phone and extra. If they seem a bit steep keep a watch around special occasions as they (or identical devices) have been half price both last Christmas and the one before (when I bought mine).
While more hassle than a hot swap battery (while charging you've got a device hanging off your phone) they have the advantage of not being phone specific so can be used to charge a variety of devices rather than just being a spare for one.
It holds enough for a full charge for my phone and extra. If they seem a bit steep keep a watch around special occasions as they (or identical devices) have been half price both last Christmas and the one before (when I bought mine).
There are models with larger capacity too, like http://direct.tesco.com/q/R.208-7092.aspx?utm_source=GoogleS...
While more hassle than a hot swap battery (while charging you've got a device hanging off your phone) they have the advantage of not being phone specific so can be used to charge a variety of devices rather than just being a spare for one.