As Jim suggested above, there’s more to choosing a battery than its cranking ability and price tag. Consider a few questions:
- Are you using an engine with one alternator, or two?
- Is your engine self-sustaining, or does it require electricity to run?
- How do you plan to use the airplane (VFR or IFR, back-country, cross country or local)?
- What is your ship’s running airframe electrical load?
- What are you willing to turn off to shed electrical load if all power generators fail?
- How much battery-only endurance do you want to achieve?
- How hyper-focused are you on minimizing empty weight?
Your battery capacity and chemistry preferences will be different depending on how you answer those questions.
For a Rotax engine, check the electrical section (chapter 24) of the engine installation manual. For all of the 900-series engines Rotax recommends a battery with at least 16Ah capacity and 350CCA, but the capacity will obviously be more critical for an electrically dependent engine like the 912iS or 915iS, for which a dead battery = no thrust. The same consideration would apply to any automotive conversion with high pressure fuel pumps, electronic ignition and an ECU.
As an example, I’m using a 912iS in a plane that will be used for day/night VFR and some back-country flying. I plan to use an EarthX ETX-900 (15.6Ah, 400CCA). My plane will be configured to aggressively load shed if I lose both alternators and have to run the engine on battery only. The entire Dynon suite, transponder, ADS-B and most lights will be shut off. I’ll finish the flight using a standby instrument (combined altimeter, airspeed and VSI), comm radio, landing and cabin lights (only as needed) and pitch trim. All of that adds up to a few hundred miiliamps of average load. Finally, I’ll test the battery at least annually to verify its capacity.