1.  Field of the Invention
The present invention relates, in general, to electrical systems for vehicles of the type having a first storage battery associated with the engine, horn, running lights or the like, and a second storage battery associated with auxiliaries such as interior lighting. 
2.  Description of the Related Art
Motor homes, power yachts and similar vehicles commonly use two storage batteries: a first battery (the engine battery) associated with the engine, horn, running lights or the like, and a second battery (the auxiliary battery) associated with auxiliaries such as interior lighting.  While the engine is running, its associated generator means maintains the charge in both batteries.  When the engine is stopped, the auxiliary battery exclusively powers the interior lighting, etc.,  to avoid depleting the engine battery since the engine battery is essential to starting the vehicle's engine.  Vehicles at rest for extended periods of time are commonly powered by a separate on-board independently powered generator means or by connection to utility electric power.  When so powered, the charge in the auxiliary battery is maintained and its load is powered.  However, the engine battery and its load have not heretofore been so powered and, over time, the engine battery can become too depleted to start the vehicle.  It is common practice to provide an "emergency start switch" in the electrical systems of such vehicles to momentarily connect both the engine battery and the auxiliary battery to the engine starter in the event that the engine batter is too depleted to start the engine.  This does not, however, remove the prime dependence placed on the engine battery for engine starting. 
To help avoid engine battery depletion, it is common practice to use an isolator means, which may consist of a solid state isolator or a system of electrical relays, etc.,  to electrically isolate the engine battery from the auxiliary battery and the auxiliary battery's load, and to prevent electric current either from the engine battery or from the auxiliary battery from flowing to the vehicle's engine generator means.  A typical prior art electrical system of such a vehicle is shown in the diagram of FIG.  1 with the auxiliary battery identified by the letter A; with the engine battery identified by the letter E; with the engine generator means, isolator means, independently powered generator means, AC/DC convertor means, electric utility source, auxiliary battery load and engine battery load identified by blocks or boxes; and with a switching means identified by the letter S providing automatic selection between the independently powered generator means and the electrical utility source. 
With respect to their electrical needs, such vehicles (i.e., motor home recreational vehicles, pleasure boats, and similar vehicles referred to herein as "the vehicle" or the like) commonly have five primary modes of operation:
(a) underway with the vehicle engine operating and the independently powered electric generator means either stopped or unavailable, PA1 (b) underway with the vehicle engine operating and the independently powered electric generator means operating, PA1 (c) at rest or under sail with the vehicle engine stopped, and the independently powered electric generator means either stopped or unavailable and no connection to an electric utility service, PA1 (d) parked, berthed or anchored with the vehicle engine stopped and the independently powered electrical generator means operating, or PA1 (e) parked, berthed or anchored with the vehicle engine stopped and a connection between the vehicle and an electrical utility service. 
With the vehicle engine operating, i.e., in modes (a) or (b), the electric generator driven by the vehicle engine is commonly used to provide direct electric current to supply both the engine battery load and the auxiliary battery load, and to charge both the engine battery and the auxiliary battery. 
When a connection has been established between the vehicle and electric utility service, i.e., in mode (e), the vehicle may be said to be utility powered and will be s defined in the following disclosure.  An AC/DC convertor means is commonly used while the vehicle is utility powered to convert utility electric service to suitable direct electric current to supply the auxiliary battery load, and to recharge the auxiliary battery. 
When an independently powered electric generator is in operation, i.e., in modes (b) or (d), the vehicle may be said to be generator powered and will so defined in the following disclosure.  While the vehicle is generator powered, the generator means may provide direct current to supply the auxiliary battery load, and to recharge the auxiliary battery.  Alternatively, the generator means may provide alternating current to power the same convertor means as is used when the vehicle is utility powered and thus supply the auxiliary battery load and recharge the auxiliary battery.  Such generator means often provides both direct current and alternating current simultaneously, but this does not alter their function with respect to supplying the auxiliary battery load and recharging the auxiliary battery. 
When the vehicle engine is stopped, electrical utility service is unavailable, and an independently powered electric generator means is stopped or unavailable, i.e., in mode (c), the vehicle may be said to be battery powered, commonly by means of rechargeable storage batteries, and will be so defined in the following disclosure.  Under this condition, and any other condition in which only direct current electric power is available, a device known as an invertor may be used to convert direct current to alternating current, but this does not constitute an additional operating mode, simply a variation on modes (a) and (c) and possibly (b) and (d), depending on the type of independently powered generator available.  When battery powered, the only sources of electrical energy are the engine battery and the auxiliary battery.  To assure sufficient electric energy to restart the vehicle engine, it is essential that the engine battery not be depleted during battery powered operation and it is common practice to incorporate an isolator means in the electrical system to, inter alia, electrically isolate the engine battery from the auxiliary battery and the auxiliary battery load. 
Both when utility powered and generator powered, the isolator means prevents either the electric utility service or the independently powered generator means from recharging the engine battery and from supplying the engine battery load.  Thus, the engine battery charge may become too depleted over time to start the vehicle's engine because it is not receiving a charging current and also because it continues to supply any engine battery load. 
To avoid engine battery depletion when the vehicle is idle for an extended period of time, owners commonly run the vehicle engine periodically for short periods of time to keep the engine battery charged.  This practice requires owner attention, increases fuel costs, and may shorten the vehicle engine's life.  Alternatively, owners may periodically or continuously connect a battery charger to the engine battery to restore or maintain its charge.  Unless the battery charger is specifically designed for continuous charging, this practice could result in battery overcharge and failure.  Convertors of the type described previously are commonly designed to be continually connected to rechargeable batteries and are not likely to overcharge them. 
A preliminary patentability search conducted in class 307, subclasses 10.7 and 16, and class 320, subclass 15 produced the following patents:
Gebhard U.S. Pat.  No.  2,866,907, issued Dec. 30, 1958, discloses the use of a "reserve" battery whose function is to provide backup power to the engine starting battery.  In this patent, the only source of energy for charging the reserve battery is the engine generator.  It provides for the reserve battery to be charged only during the interval between the time the engine starts and the time the engine reaches operating temperature.  Neither the engine starting battery nor the reserve battery is charged when the vehicle is idle.  In this patent, the reserve battery is cut in or out of the charging circuit by use of : (a) a vacuum operated switch mounted on the intake manifold, (b) a thermostatically operated switch mounted on the exhaust system, and (c) a manually operated switch.  The manually operated switch is described as being of the type which will bridge the contacts from the two batteries when it is operated.  When the contacts are so bridged, the two batteries are connected directly together and current may flow freely from the engine starting battery to the reserve battery. 
Follmer U.S. Pat.  No.  4,044,293, issued Aug. 23, 1977, describes the use of a "second starting battery" whose purpose is to provide, in conjunction with the normal engine starting battery, twice the normal voltage to the engine's starter.  In this patent, the only source of energy for charging the second starting battery is the engine generator.  In this patent, the second starting battery is always used when the vehicle is started, not just when extra power is needed. 
Ghibaudo et al.  U.S. Pat.  No.  4,264,855, issued Apr. 28, 1981, describes an electrical system for recreational vehicles having one main or engine battery, one auxiliary battery, one engine generator, a main or engine battery load, an auxiliary battery load, and a supply device or convertor to supply power when the engine is stopped.  Three operating stages are disclosed: vehicle running, vehicle stopped and convertor in operation, and vehicle stopped and convertor not in operation.  While the vehicle is running, both engine battery and auxiliary battery loads are directly connected to each other and to the engine generator.  The engine generator is also connected to one of the two batteries, the choice being made by the vehicle operator via a manual switch.  Two ammeters are described to assist the operator in making this choice.  While the vehicle is stopped and the convertor is connected, the two loads and both batteries are powered by the convertor.  While the vehicle is stopped and the convertor is not connected, the engine generator load is connected either to the engine battery or the auxiliary battery (depending on the switch position selected by the operator), and the auxiliary battery load is connected only to the auxiliary battery. 
Iwaki et al.  U.S. Pat.  No.  4,564,799, issued Jan. 14, 1986, provides a second battery whose sole purpose is to power the vehicle's ignition system.  Since this second battery is independent of the engine battery, its output voltage does not drop when the engine is being started.  The claimed purpose of this invention is improving the operation of the ignition system during engine starts particularly when the engine battery is not fully charged and during cold weather, either of which conditions result in lower-than-normal engine battery voltage.  This patent does not provide means for maintaining a charge in the engine battery during periods when the vehicle is idle.  This patent describes several means for charging the second battery: solar cell, thermoelectric transducer, thermionic generator, or a piezoelectric transducer. 
Farber et al.  U.S. Pat.  No.  4,757,249, issued Jul. 12, 1988, describes a dual battery and dual engine generator system wherein each battery is charged only by its associated engine generator.  To provide extra power to the engine starter, both batteries are always connected in parallel when starting the engine.  Otherwise, each battery powers its own load, and shares in supplying "critical loads."  Enhanced capability for engine starting is provided by redundancy in both generators and batteries. 
Nothing in the above patents or prior art discloses or suggests the present invention.  More specifically, nothing in the above patents or prior art discloses or suggests an improved vehicle electrical system for use in a vehicle having an engine and including a rechargeable engine battery, an engine battery load, a rechargeable auxiliary battery, an auxiliary battery load, an engine generator means powered by the engine for generating electric current, and a supplemental electric current source capable of providing electric current to the auxiliary battery and to the auxiliary battery load, wherein the improvement comprises link circuit means for electrically linking the engine battery, the auxiliary battery, and the supplemental electric current source; and control means for allowing electric current to flow through the link circuit means from the auxiliary battery and/or the supplemental electric current source to the engine battery and the engine battery load without allowing the flow of significant electric current from the engine battery to the auxiliary battery or the auxiliary battery load.