GNOME Power Manager supports merging multiple laptop batteries into one virtual battery for the time remaining calculation and also the status area icon. Unfortunately my new statistical driven approach breaks with primary batteries being removed and inserted, as we'll need some sort of profiles for "one battery" or "one battery plus extra battery" just to make the new code work. Unfortunately, my Lenovo only has the one battery, with no option to extend it.
Now, long shot: Does anybody have an old laptop that they do not want that they could send to me? It would need to have two batteries expressed in ACPI (with at least 10 minutes of charge in both), with the ability to remove one at run-time. It doesn't matter if the machine is 400MHz with a smashed screen, any old test machine would be great.
If I get my hands on a multi-battery laptop, then the new profile code work will work really well with two batteries. Sorry if this sounds cheeky, but it's really hard to code for a situation you can't test for.