HTC Desire :: Battery Level Indicator Went From Green To Orange

I was using satnav at the weekend; the battery level indicator on my Desire went from green to orange (20%) and the existent data connection dropped off straightaway. I was unable to re-establish a data connection even after switching the phone off and on again. I was in an area of reasonable/good network signal strength. The GPS continued to function however so I could still see my progress on the map but a data connection message appeared.

When I got home the phone continued to function on WiFi (with GPS on too) for nearly another two hours before it turned itself off, so there was plenty of "juice" left. My question is as to whether there is anything I can adjust to ensure that the battery continues to function fully (ie maintain a data connection) until it runs out? Or whether at 20% the phone is somehow set up to end a data connection? I have now got myself an HTC car charger to make sure the battery will not drop that low again when I am using satnav.