VEVOR Dual Battery Isolator Kit, 12V 140 Amp 20 ft 6AWG, Manual and Automatic VSR Voltage Sensitive Relay with Voltmeter, LCD Screen, Smart Battery Isolator, Compatible with Lithium Batteries, for UTV

You need an isolator somewhere between the start battery & where it connects to the MPPT charger. This will basically disconnect your new accessory circuit from the car's main power when the ignition is turned off so you don't accidentally drain your start battery, but then when you start the engine, the alternator will charge both batteries. Here's an example of a battery isolator kit for lithium secondary: https://www.amazon.com/VEVOR-Automatic-Isolator-Compatible-Workshop/dp/B0FYFDRGJG/