So I'm trying to make my 2.0 petrol AWD into the perfect multipurpose vehicle. And a car which is supposed to be perfectly prepared for any situation obviously needs a huge power supply. So I got a cheap 100Ah deep cycle AGM battery, and a proper 12V-12V smart battery charger. The engine bay had a spot barely big enough, with one of the engine mounts with an extended bracket right at the bottom of the space, which was perfect for mounting the heavy battery. 3D printed a panel with some plugs that allow me to swap between which battery is the input and output of the charger.
Most secondary battery systems have the auxiliary battery constantly charging alongside the starter battery, and then hooking up the power hungry units to the auxiliary battery directly. But I already have an absolute ton of stuff connected to the starter battery, and there was no way I was going to spend an entire week rewiring everything. And I also wanted the car to still have power to all it's regular systems. So the solution was to use the auxiliary battery as a reservoir which I could drain and recharge on command as I wanted. So everything still draws power from the starter battery, but I can put the aux. battery into the charger intput and the starter battery into the output when the car is off. Then it will use the aux. battery to charge the starter battery constantly. Keeping everything powered on, and the starter battery topped up and happy. When I'm driving and want to charge the aux. battery again, I just swap over the wires so the starter battery is connected to the charger input, and the aux. battery to the output. The smart charger regulates charging sequence automatically to maintain maximum battery life for both batteries. Normally, when the aux. battery is full and I don't need to charge it, I just pull out the input side of the charger, which puts the whole system to rest. So that's how the system stands 90% of the time with daily driving.
Ignore the hot air intake. It was purely temporary due to having to move everything around to fit the battery. I've taken the cone filter off now and using a normal filter in the airbox, and a short hose to get some colder air from between the aux. battery and the radiator fan.