Do yourself a favor, find out the rating of all of your equipment at full power.
ie, water pump 3.0 amps, Mychron 800, mah, etc. That will determine what size battery you need. In the above example you have a total of 3.8 amps draw, meaning at full power a 3.8 amp battery would last one hour. I would use a 4 amp battery, all the races I run are about 30 minutes, that gives me another 30 minutes reserve.
Also, they make Lipo batteries that are the correct voltage rating.. Not cheap, but check out Ballistic Batteries…