The “Charge Booster” is a charging converter between the regular car starting battery (usually lead-acid – SLA) and the auxiliary board battery (often the LiFePO4 – LFP).
It also works as a separation between these two batteries. If the starting battery gets deeply discharged, the board battery will not be influenced by this. The “Charge Booster” is an ideal solution when you need to join the SLA battery (starting) and the LFP battery (board).
This product is readily available from various suppliers on-line. (like Schaudt Booster WA 1208, VOTRONIC VCC 1212-30 Lade-Booster B2B, Dometic PerfectCharge DC 20, etc.)