There is a reason that you have desktops and laptops: power consumption. I mac chugs power, I'm not sure about exact numbers but the PSU should gibe you a good idea of the power rating. If you want to power it from lipo cells for more than 10 minutes, you need a huge pack of them, plus some buck converters to convert to voltages like 12, 5 and 3.3 (I'm again not sure about exact different power lines of a mac, you should measure from PSU). All in all, forget about making it portable. The amount of weight and space the batteries add make that impossible.