Work trucks, boats, recreational vehicles, long haul trucks and fleet vehicles all have a battery isolator if the alternator is being used to charge both the house and vehicle batteries. The vehicle (chassis) battery is typically an automotive type of battery that provides high cranking amps to start the engine. This battery relies on the alternator to keep it charged up for the next cranking cycle. Automotive type batteries do not like to be discharged to a low level. If this battery is drained down because the car won't start or the lights were left on, the battery life decreases rapidly and will probably remain dead the next time it is drained down.
The batteries used to provide power operation of DC and AC appliances in a work truck, recreational vehicle, or boat are called the house batteries. These batteries are typically deep cycle batteries that prefer to be partially discharged and recharged on a regular basis. Some sort of battery isolator is required if the alternator is going to be used to charge more than one battery.
A battery isolator is a one-way electrical device that allows an electrical current to flow in one direction but not the other. A battery isolator typically is used in situations where multiple batteries are required, and its primary purpose is to ensure that the failure of a single battery will not incapacitate an entire electrical system. Battery isolators are used in large trucks, RV's, boats, utility vehicles and other vehicles that require multiple batteries and backup sources of power.
A battery isolator separates multiple batteries from one another to ensure that a vehicle has access to a backup power source. In addition, a battery isolator ensures that a vehicle's electrical system does not pull power from multiple batteries at the same time, which would rapidly drain all the batteries and cause the electrical system to fail. A simple setup is one in which a vehicle contains two batteries, both of which are attached to the isolator. The battery isolator is attached to the vehicle's alternator, which is the device responsible for providing the vehicle with electrical power and for charging the vehicle's battery. The battery isolator uses sensors to determine the number of batteries it is connected to and to draw power from the strongest battery. The idle batteries are charged by the alternator, and the battery isolator can switch power sources if the battery in use suddenly fails.
Diode Versus Solenoid Battery Isolators
There are two main types of battery isolators: the diode isolator and the solenoid isolator. A diode isolator uses two high-current diodes to direct the flow of current from the battery to the battery isolator. A diode isolator is not as complex as a solenoid isolator, is easier to maintain and repair and generally lasts longer. A solenoid isolator uses several electrical relays to control the flow of electrical current. One of the relays receives the electrical current from the battery, while the other relay (known as the central relay) monitors the power levels of the idle batteries. Additional relays transmit the charging current from the alternator to the idle batteries. A solenoid battery isolator does not have as much current degradation as a diode isolator and is more efficient.
Inexpensive 12 volt Smart Battery Isolator with 150+ amps pass-through and solid-state control
This hybrid device uses a solid state microprocessor to control the charging and isolation functions, and uses a solid tungsten points contactor relay to control the big currents.
This battery-isolator is used to control extra battery systems that charge off the vehicle's alternator. The isolator won't let the auxiliary batteries be discharged by the vehicle's gadgets. Neither will the house battery be used to start the car. It will allow the extra battery to be charged at whatever rate the alternator can put out, so it needs big enough cables to do that. Big lead acid batteries when they are empty can accept 150+ amps, so rate the wires at the maximum current of the alternator.
The following diagram shows the proper wiring of a smart battery isolator to the alternator and batteries.