Cheap 7 Inch Digital Photo Frame

Digital Photo Frames.battery life?
i have just bought a 7 inch linx d p f and charged up the inbuilt rechargeable battery as stated for 5 hours but it only lasted for 3 hours ! its not a cheap one at £80 is this a fault or do i need to keep it plugged in to the mains ??
i only used it a little bit for viewing photos and the rest of the time it was on the clock.calendar mode
the battery has a life of 1 month( allegedly) after it has been charged for 5 hours
I haven;t got a scooby what referrign to compact cameras has to the life of Photo Frames, but y’never know.
there should be soem reference in the manual as to what power the device draws.
I’d expect the rechargeable battery option to be used rareley.. I’d expect the device to be normally operated from an AC power supply that came with the dpf.
the 5 hours stated is probably achievable, but only if you select the settings that really optimise the battery life (that usually means turning of back lights as soon as possible).
7 inch android apad tablet PC only $72