Step 3: The Battery
The battery I chose for this is an AGM (Absorbed Glass Mat) Lead Acid type - 12V(volts) 7ah (amp hour). It's a maintenance free deep cycle battery that's designed to be used periodically. You'll find them in security systems, computer battery backups, etc.
A word (or two) about this kind of battery.
12V is perfect for this - the pump is designed for 12V.
It's not spillable - can be mounted in all positions.
Can put up with quite a range of temperatures.
Packs a punch - could run "safely" for a few hours at 1 amp drain - we only need this for a few mins here and there.
Small & cheap (~$26).
I could have gone with a slightly smaller size - 12V 3ah - but this was just a few dollars more.