Some dozen or so articles back, I mentioned that charging your house batteries from the vehicle's engine is the most difficult of all methods of replacing your household consumption. This issue, we'll examine some of the methods available and suggest some alternatives
By far, the most popular methods of recharging house batteries in a vehicle are solenoids and isolators. We will examine these systems individually, after noting that each has the same effect, that is to connect the house batteries to the engine starting battery charging system. Each has disadvantages, which, for your purposes, may or may not be offset by their individual advantages.
First, solenoids. A solenoid is an electrical device that can connect the house and engine batteries directly together, effectively making them one battery bank. The solenoid is usually controlled by either the vehicle's ignition switch, or by a switch that is operated by the driver. Sometimes, both controls are used, that is the solenoid will only connect the batteries together when the ignition switch is on and the driver manually calls for the batteries to connect by operating the switch, pulling in the solenoid.
What is supposed to happen then is that the alternator (or generator on older vehicles) charges both batteries until full. What actually happens most often is that the engine battery drains into the discharged house battery and the alternator has to recharge two half-dead batteries. On a long trip, this probably will happen eventually, but at a cost, which we will observe later.
Advantages of this type of system are that it is quite inexpensive to install, may be completely operator transparent (ignition switch operation only), and in a pinch, the house batteries can act as a jump start for a dead engine battery, getting your rig back on the road in an emergency.
Disadvantages include the possibility that a very dead house battery will drag the engine battery system down enough to cause electrical and starting problems. Leaving a manually operated switch in the wrong position can result in both dead house and engine batteries, so care is suggested in this system.
Isolator-based systems overcome some of the shortcomings of solenoid charging systems. Basically a battery isolator is a dual high-current diode bridge that completely hides one battery from another as far as loads are concerned. Think of it as a couple of one-way valves that only allow the flow of current to go one way, into the batteries. In this manner, draining either battery completely dead will not affect the other battery's state of charge. When the engine of the vehicle is started, the current from the alternator can flow into both batteries, replacing any charge that was used in house lighting or engine starting.
Advantages of the isolator are that operation is completely transparent, the driver has no switches or other controls to remember to operate. Engine starting and running should be unaffected by the state of charge of the house battery.
Disadvantages include a higher initial cost and installation into the vehicle's charging system requires that modifications be made to the factory wiring.
While literally thousands upon thousands of recreational vehicles use one of the two systems described above, the both have one very serious disadvantage that cannot be overlooked by anyone who is serious about installing an electrical system in their house truck or bus that is as efficient as possible, and provides that best possible charge for longest battery life. The fact is that engine charging systems are optimised for charging the engine starting battery and running the vehicle's electrical accessories, NOT recharging deep-discharge storage batteries that are the proper type for a renewable energy system. Neither the charge rate (amount of available charging current) nor terminal voltage are correct for deep-discharge battery health.
Engine battery charging systems are optimized to replace a very small amount of discharge very quickly. Starting your vehicle's engine takes only a small portion of a battery's capacity, so in an isolator-based system, once the engine battery is full, the alternator stops putting out much current, and the house batteries don't recharge fully. In a solenoid-based system, the alternator sees two badly discharged batteries, and tries to really cram a lot of current into them all at once, possibly damaging the house batteries, and likely doing the engine battery some damage as well.
Properly, what is needed is a separate charging system for each battery system, which is exactly what we will take up as a subject next issue!