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.
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.