Ultrabay as secondary battery. I had two batteries in the machine always. One 9-cell in the machine, one 9-cell in my bag and a 3-cell in the ultra bay. I could get 9 hours out of that without too much trouble and both the 9-cell packs were knackered.

Now you can get 22 hours apparently on a X220 with battery base and a 9 cell!

I use my ThinkPad as a charger too. You have to turn it on in the BIOS but it charges literally anything.