One common failing on the 5300 machines is a weak power jack - it's too small and snaps/breaks off the motherboard within time. Jiggle this around/hold the charging pin firmly in position and see if you get more life that way. Pretty much all "junked" 5300 machines I've come across have had broken power jacks, a popular mod in the past was to remove it and put a Powerbook Duo power connector in it's place.

The 5300 isn't too bad a machine, the active matrix TFT is much nicer compared to the display on a PB 540c (it's predescessor), and you can overclock it to 117Mhz (move a few resistors) happily.

JB