Don't forget, if the device doesn't come with a battery access door it is considered "NON user replaceable".
The units that have no battery access doors are usually designed as a one time use device.
Why?
The device can be easily damaged on opening for those with no experience and the proper tools.
The battery may not be easily available and of a poorer quality than the OEM supplied battery.
By the time the battery runs out of life the unit is very obsolete.
If the device is beyond it's warranty coverage - the cost of sending it away, service charge for replacement + battery and return postage may exceed the replacement cost of a newer up-to-date device.
The best thing you can do is follow "best practices" to try and get the most of your battery's life before failure. Most of to-days batteries do not suffer from "memory" issues due to charging before full depletion as the Ni-Cads did but there are still things you can do to try and achieve the most from your battery. Check out
http://batteryuniversity.com/ for more info.
I am still using an automitive GPSr from 2007 that still has a very good battery life when off car power even though I can't get anymore maps for it.
John