Renogy DC to DC Charger 12V 40A - Multi-Stage Charger for LiFePO4, AGM, Gel, Flooded Batteries | Car, RV, Marine & Boat Battery Charger with Bluetooth Compatibility

Never pull from the car battery directly! Look into using a DC-to-DC charger like this Renogy one. You'll need to wire it yourself or hire somebody, so additional materials will be required, but this is by far the most efficient way to use an alternator to charge a power station without major risks, afaik. There's good resources online (youtube) about how to hook these up.