Well it all works a beauty. At least 8 hours service life from what were apparently totally shagged batteries. This is the main external battery I am talking about, not exactly the nominated thread topic! Even though the "external" battery is really internal. Oh heck if a moderator has a problem please feel free to try your darndest to separate out these topics!
To help others diagnose if they are likely to get their main battery back to life - when I first realised I had a problem a while back I measured the volts, didn't know which terminal was what but loosely remember measuring about 1.5V. So, now that I know what is going on, I measured the volts again.
I left the unit on last night to self discharge and this morning it had 7.19V across +'ve to -ve and 4.82V across +ve to data (also a -ve). I then turned it on to self discharge 3 more times just to see how low it could go. After that we were at 7.04V and 4.68V for -ve and data respectively.
So, somewhere between 7V and and 1.5V the charging circuit refuses to latch in. Which is pretty poor as a time discharged battery is not necessarily cactus, mine certainly seems to have revived to the point that I will still get a useful service life from it. I didn't check the voltage after I gave the batteries the heart starter treatment but I would guess it was around the 6V mark. Interestingly when connected to the power after repair the display showed a full battery but charged it for ages anyway before declaring that it was full. Now that I have self discharged it the display shows the correct empty battery picture and is showing it progressively being topped up, as it should.
I have the latest firmware on this unit (5.4) and I also note with wry amusement that it was to improve the battery life designation. When I was doing the triple discharge I had remaining life showing of 1.6 hours initially, 42 mins on the second one and I didn't look for the others, but the whole triple discharge thing didn't take more than an hour, and some of that was slack time when I didn't notice it was off - so this may still need a little work, or it may simply be that my battery is operating outside of normal parameters given that I have (unwittingly) abused it quite severely.
See the below thread for help with that oh-so-important first step - getting the bloody thing open!viewtopic.php?f=4&t=7664
And, just a tip from me for your patience in reading this far, the display on the 276C can be set up to be much more user friendly than what you get out of the box. I don't remember exactly where the menu items are, but I have full screen map showing with speed and altitude at the top and distance to next and destination at the bottom. When tooling around country roads on the bike this is all you are interested in and not wasting screen space on the side menu and getting mapping instead is really useful. And I find in the car it is also just brilliant, plus I get the audio for extra (usually banal) info to help with navigating. Enjoy............