Talking about knowing your load, on the recommendation of a fellow observer I bought one of these inexpensive little gadgets and now keep it inline at all times:
http://www.ebay.com.au/itm/Digital-L...-/221399347636
I find it invaluable. Not only does it tell you the peak amps your rig was taking and the lowest voltage dips it experienced during an observing session, but it also counts the amp hours of charge drawn from your battery. This lets you make informed guesses as to the health of your battery, its maintenance requirements and for planning future purchases. Highly recommended.