A recurring question on some of the forums I monitor is, how do I connect a "house battery" or auxiliary battery to the charging system in my vehicle, so it can be charged as I drive?
I've addressed this issue before, but I recently saw the question surface again, and I had an idea I think is worthy of mention. I would like to point out that this is not just a theoretical exercise for me, because as an avid Jeeper, ham radio operator
and RV boondocker, I have built and used several auxiliary vehicle battery systems.
Setting up one of these systems would seem to be simple: just run a hot wire, suitably fuse-protected, from the alternator to the house battery, then run a ground strap from the negative terminal of the house battery to the chassis of the vehicle. There are several problems with that approach, the first and simplest of which is that, if left connected like that while the engine is not running and hence not charging, any current used by the house battery system will draw current starting battery as well, so that after camping and using battery power for awhile, you may find that you are stranded with a vehicle that won't start. Another problem is that, even if you don't use the house battery when the engine is not running (as may be the case with a car sound system aficionado or ham radio operator) the two batteries will "fight" when the vehicle is parked, eventually draining both batteries. This is usually attributed to the two batteries not being matched as to type, age and capacity, leading some to use a deep cycle battery for the house battery, while at the same time replacing the starting battery with a second, identical deep-cycle battery. This is a bad idea, and doesn't work anyway unless the two batteries are physically close and connected with a heavy gauge battery cable that is as short as possible, in which case it is no longer a dual battery but simply a single, larger battery. If the two batteries are separated and connected by a longer wire, they actually do become electrically separated by the resistance of the wire connecting them, and then you have the mutual disadvantages of a house battery that is limited in size and capacity to that which is appropriate for the starting battery, and a starting battery that is not really designed to be a starting battery. Also, they quickly cease to be matched, because the starting battery is subjected to heavier loads (the starter motor) and higher charging voltage due to closer proximity to the alternator.
A much better and more popular way to go, regardless of the end use, is to separate the batteries with some type of isolator which allows both batteries to be connected to the alternator while the engine is running, and disconnected from each other when the vehicle is parked. That way one can use a house battery or battery bank that is designed for the load it will be used for, and a standard starting battery in the normal location for starting the engine.
There are two schools of thought concerning how to isolate the battery systems. The first is to use a passive isolator, and the second is to use some type of switch. Actually there is also a third, ultimate solution, which I will briefly describe later, but is beyond the scope of most needs.
A passive isolator is simply a pair of diodes, one going to the house battery and the other going to the starting battery. A diode is a semiconductor which allows current flow in one direction, but not the other. This is good because it allows each battery to receive charging current from the alternator, but does not allow current to reverse flow from one battery into the other system. The passive isolator doesn't require any input from the operator; it just sits there quietly doing its job. However, diodes are not perfect; they have a voltage drop of approximately one volt. Guess how much difference there is between the voltage of a lead-acid battery, such as a starting battery or deep-cycle battery, in the fully charged vs. fully discharged state? One volt, in a 12 volt system; 11.7 volts (at rest) discharged and 12.7 volts (also at rest) charged.
For this reason a lot of people, previously including myself, use a switch arrangement in the line to the house battery. This switch must, like a passive isolator, be rated to handle the highest current that will be passed through it. In use, the switch must be opened (turned off) when the engine is off and closed (turned on) when the engine is running. This can be accomplished manually by throwing the switch at the proper times, or automatically (best for most folks) by using a relay instead of a manual switch, and connecting the control circuit to a point in the vehicle's electrical system that is powered only when the engine is running. The most commonly used relay is popularly known as a "continuous duty solenoid" and is usually rated at 100 amperes. I have used these, and they generally do a good job. This is a fairly good choice for most people.
There is a problem, though, that all auxiliary battery systems have; and that is voltage drop on the long wire from the alternator to the house battery. It wouldn't be that big of a problem in and of itself, because the alternator has a voltage-sense circuit in its regulator, that senses battery voltage and tells the regulator how much voltage to supply. I already discussed at-rest battery voltage; lets go a little further now with charging voltages. Because of internal losses, a 12V battery needs 13.5 to 13.8 volts from the alternator just to maintain its charged state if there is any load on the system. Actually charging the battery requires 14.1-14.5 volts. This is on top of any losses, such as a long section of wire or a diode. Switches and relays aren't perfect either; they all have some loss. You won't be able to measure that loss with a voltmeter when the system is at rest, but start passing current through the wire and/or switch and then you can measure the voltage drop at a given current. The way around that is to run a second, smaller, sense wire from the regulator directly to the house battery. That way the regulator will raise the voltage output of the alternator to compensate. The problem now is, what is happening to the starting battery? It is being overcharged. Now, lead-acid batteries are fairly forgiving creatures, so that running at 15 volts for awhile probably won't damage a typical starting battery (stay away from the sealed, gelled-electrolyte, super deluxe batteries though; they are not so forgiving). The thing to do is make sure the electrical path to your house battery is a good one: use a good switch or relay, large-gauge wire (I generally use 6 gauge) and run a ground wire of the same size if not even larger, directly to the case of the alternator. Not to the starting battery, and not just to the chassis (although, do make that connection as well). The ground side carries exactly as much current as the positive side does, and expecting that current to find its own way back through the steel chassis guarantees problems. The losses in that path add to the losses in the positive wire, so give it a good, copper return path. The ground losses through the engine block from the starting battery back to the alternator actually help here, as they bring the losses in the starting battery charging system up closer to the losses of the long wire to the house battery. If the difference in voltage loss is 1/2 volt or less, the system will be quite workable.
With a passive isolator, the same is true as long as the sense wire goes to the house battery; not through the isolator but bypassing it.
I was given an isolator once that was burned up on one side, so that one of the diodes would not pass current. I just jumpered over that side, figuring that actually, only one system needed a diode to isolate the systems. It worked, too. However, the side I used the diode on was the house battery, which exacerbated the losses I already had from the long wire; but I made up the difference by plugging a charger into the house battery whenever I had a chance (a good idea with any unbalanced system, BTW). The sense wire was in its stock configuration, so that is the only way it would work without some rewiring; and I was getting ready for a trip, so I made the best of it.
That brings me to my idea: connect a large-gauge, low resistance wire directly from the alternator output to the positive terminal of the house battery, and a ground wire from the negative terminal back to the alternator case, as well as a chassis ground connection near the battery. Now, cut the wire that goes from the alternator to the starting battery, and insert a passive isolator, using only one side. The other side is unneeded and becomes a backup, in case the diode in use ever fails (it probably won't). The voltage drop across the diode offsets the drop across the long wire to the back, making for a well-balanced, trouble-free system. The sense wire should go to the house battery.
But what about the third option, the ultimate system I mentioned? Simple. Two alternators, one connected to each system. The stock alternator can remain connected exclusively to the starting battery, while a second, high-output alternator is added, connected exclusively to the house battery. The two systems are therefore totally separate and cannot affect each other. This is the best way to go for several reasons, but the expense and difficulty is not warranted in most systems.
Note:Any wire that is connected to your auxiliary battery MUST be fused at the battery, and the charging wire from the engine compartment must be fused at BOTH ends. That long wire is exposed to enough hazards to present a very real chance of developing a short circuit, and if not fused, it can burn your rig to the ground, perhaps with you in it.
Just a word to the wise.