Battery class driver.

[Roland Dreier]

 > No, I think the distinction between batteries and ac_adapter is large
 > enough to have different classes of devices. You may have many
 > batteries, but you'll only ever have one ac_adapter. I'm not sure it's
 > an obvious abstraction to make.

Speaking from ignorance here, but what about (big) systems that have
multiple power supplies and multiple rails of AC power?  Would it make
sense to use the same abstraction for that as well as laptop AC adaptors?