Look on top of the maxifuse relay center on both cars. That'* the little one to maintain power to the car during the swap.
The big one also has an 80-amp starting circuit, battery and alternator diagnostics, and a buncha other stuff. Maintenance mode is automatic after charge is complete (trickle). Enough options and features that you don't dare lose the manual.
• 80 Amp Engine Start – for emergency starting.
• 12<>30 Amp Fast Charge - self-adjusting, variable charge rates for everyday charging needs and maintaining your battery in peak condition. Charges and automatically desulfates and restores car, truck, RV, farm equipment, AGM, gel cell and deep-cycle batteries.
• 2 Amp Slow Charge – for charging and maintaining small batteries; charges motorcycle, ATV, snowmobile and lawn tractor batteries.
• 125 Amp, Sure-Grip Clamps – for top and side-mounted battery posts.
• Electronic Touch-Pad Controls – easy-to-select switches for Digital Display, Battery Type or Charge/Start. Choose from 12V regular automotive, 12V deep-cycle, 12V AGM or gel cell and 6V regular batteries.
• Built-in Digital Voltmeter/Tester – helps diagnose problems.
• Retractable Handle – for easy portability. Cord wrap on back.
• 1 amp trickle charge rate – charges in 2 to 12 hours.
• Reverse-hookup protection.
•Quick-Disconnect Harness – Snap either connector into place in seconds – Use the 50 amp clamps or permanently attach the ring connectors…great for hard-to-reach motorcycle batteries.
Not much point to a rear battery other than hauling extra weight around. Just more to replace later. Get an Optima and be done with it. I'm not real keen on large-gauge primary feed wires running back and forth in my cars.