Charging House Batteries from Alternator
The simplest house battery circuits simply connect the house battery in parallel with the starter battery when the engine is running so the alternator charges both.
But the house battery often is a deep-cycle battery with different charging requirements from the simple starting battery. The highly variable and relatively low-voltage alternator output isn't the best for charging a deep-cycle battery. Worse, the long wire runs from the alternator to the starting battery, forward to the dashboard, and back to the usual location of the house battery under the driver's seat often result in a substantial voltage drop, further compromising the quality of the charge on the house battery.
A good solution is a DC-to-DC smart charger such as the PowerStream PST-BC1212-15. This device takes a 10- to 15-volt input, like that provided by the alternator, and up-converts it to as much as 14.4 volts, applying a smart charging algorithm to ensure the house battery is re-charged and conditioned optimally.