How is the best way to use Battery?
Nintendo DS, cellphone, or computer.
keep charge? use with power?
empty fully before charging?
does it depends on battery type?
Li-Ion batteries don't have the "memory effect" Ni-Cd batteries had. That means that you can charge them no matter how charged they are, however, a battery has a limited number of charge cycles. I'm waiting for the battery to be almost fully drained before I recharge it.
As for the MAC vs PC issue, There are some Windows laptop which can get to approx. 5 hours of use, it depends on the battery, specifications of the laptop and the power configuration of the OS, and of course if your running a DVD movie don't expect to get more than approx. 1.5 hours.