I have seen many people purchasing that accessory for their Lenovo laptops.The problem is that many people are complaining about the fact that even though the OSs are able to see the extra battery "juice" from the Battery Bay,after the main battery rans out of energy the computer shuts down.Most of the cases,this happens because they haven't set up the Lenovo Power Manager.At least this is what they claim other users in support forums.I was wondering has anyone tested this under Ubuntu 12.04 LTS.I mean can it recognize/manipulate/use the extra battery energy from the Ultrabay or not?
I don't know for W520, but I am currently using a T430s with an ultrabay battery, under Quantal (just upgraded from 12.04). I had nothing to do to make it work, the laptop will first "consume" power from ultrabay, and then switch to main battery automatically.
All that is done at the bios level, so no special support is needed from Linux / Ubuntu.
Some people want to interact with the way the batteries are used (priority, etc...), to enhance the battery lifespan essentially. To do so, you either need a special driver, or use ACPI, depending on your model and bios. You should have a look at http://www.thinkwiki.org/wiki/How_to_use_UltraBay_batteries if not already done.