To cut boards and other things in straight lines, a circular saw is just the thing. The voltage amounts to 18 volts and weighs 2.5 kg, so it's not that heavy to carry around.
From Amazon
DEWALT DCS571NT-XJ - Sierra Circular de mano sin escobillas XR 18V sin cargador/batería
Powertool World
DeWalt DCS571NT 18v XR Brushless 115mm Compact Circular Saw Body Only in TSTAK Case