The vehicle’s own engine-starting battery, kept separate from your house bank. A DC-DC charger tops up the house bank from it while you drive.
Keeping the engine-start battery separate from your house bank means a weekend of running the fridge can never leave you unable to start the engine. They have different jobs and very different discharge patterns.
It stays on the vehicle/engine side. A DC-DC charger bridges the two — topping up the house bank from the alternator while you drive, without the two banks fighting.
Design your van, boat, cabin or RV system in Wattonomy and it adds a DC-DC charger between the start and house banks when you opt into alternator charging, sized so it never overloads the alternator — from the appliances you actually run, sized to the recognized standard for your region. You see it on the wiring diagram, in the sized parts list, and in a plain-English build pack that explains the reasoning behind every choice. No account, no email — about a minute to a complete, validated design.
So house loads can never flatten the battery you need to start the engine. The two have different chemistries and duty cycles and should not share a single bank.
Through a DC-DC charger, which takes power from the alternator while the engine runs and delivers it to the house bank at the correct profile for lithium.
It takes about a minute. No account, no email.