|Depending on the make up of the battery [Lithium Ion or Nickle Metal Hydride], constantly using the navigator via tha battery and never flattening it completely is detrimental. Especially when you use 50% then recharge it all the time, the battery ends up with what is know as a "Memory Effect".
Try to fully discharge the battery within the unit every so often prior to recharging, say every third use, if you do not have the unit constantly powered when not in use.
Conversely if you have the unit constantly powered when in use, exercise the battery to flat by unplugged from power and using by battery alone.
Hope this helps.