From SATMAP's website:-
"The older Active 12 with a built-in rechargeable battery, weighs 210 grams, it has a width of 7.6 centimetres, a height of 13 centimetres and it has a depth of 3.2 centimetres. The screen size is 5 centimetres wide and 7.1 centimetres tall. That is the viewable screen size.
The new Active 20 GPS is 310 grams, so it's a little bit heavier, but it does have a much bigger, higher capacity rechargeable battery in the unit. So that weight does include the battery. It's 7.1 centimetres wide and the height of it is 14.6 centimetres and the depth is 3.7 centimetres. The viewable screen size is 5 centimetres wide by 7.4 centimetres tall.
The Lithium Polymer rechargeable battery can last between 12 and 16 hours."
My experience is that it lasts all day without a problem with reasonable use but presumably it will depend upon how you use it i.e. if you constantly keep "playing" with it the battery will run down quicker rather than just keeping it switched on to allow it to do it's job of tracing your route/tell you where you are. If you wild camp then you would presumably need to carry either a spare Lithium battery/AA batteries or solar power pad (if that is what it's called).