Before we move on to other enjoyable subjects, there is another type of power processing equipment to consider, Converters. These devices convert utility grid power into 12 volt direct current to run lights and appliances in your bus or truck. "Wait a minute" I hear you say "Isn't that backwards? Just last issue you were telling us all about the wonders of inverters, which change 12 volts DC into household utility power!". This is true, but there are times when you may want to run your low voltage loads directly off of the utility power when it is available, and not drain your batteries. A converter is the way to do this.
Most converters are essentially heavy-duty transformers and rectifiers, not unlike a common battery charger. In fact most converters also charge the battery while they are powering the vehicle's loads. The difference is that converters, unlike chargers, have an internal relay which disconnects the lights and other loads from the house batteries so that the converter takes the entire load itself, allowing the batteries to experience only the charge current.
To be effective, a converter must be capable of supporting a load of 30-40 amperes, while simultaneously supplying a charge current to the batteries of 10 or more amps. In practical terms, this means that the converter must contain a large and heavy transformer. An appreciable quantity of heat can also be expected to be generated, so converters should be mounted where they can receive a free flow of cooling air. It is not unusual for an operating converter to make an audible humming noise, and I have heard some that were positively loud, so take care to install such a device where you won't be bothered by the sound it makes.
The relay inside the converter is a single-pole-double-throw device, which allows the batteries to power the loads when utility power is not being used, and automatically switches the loads over to the transformer when the grid is available.
Keep in mind that most converters supply voltage and current that is neither filtered, nor regulated. Sensitive electronic devices such as audio equipment, two-way radios, cell phones and the like should not be powered from converter power, and instead should be connected in such a manner to assure that they receive only battery power. A dedicated regulated and filtered power supply can be installed to run these devices from the utility power if desired.
The benefits of using a converter are realized when one is switching between utility power and battery power frequently. Many converted buses and trucks have lighting fixtures which can be powered by either type of current, and the bulbs are changed to whichever voltage is being used at the time. Other conversions have light fixtures which contain two lamps each, low and line voltage. Use of a converter allows the use of low voltage lamps in every fixture, simplifying installation and operation of the same. Of course, if you have switched over to using compact florescent lamps, this argument is invalid. Other significant loads such as pumps, fans, and motorized devices are easily energized by use of a converter.
A sufficiently strong battery charger, whether part of an inverter, or as a separate device, can take the place of a dedicated converter in your electrical system. There is nothing particularly complicated about a basic converter, and someone with a working knowledge of electronic principles should be able to construct their own from discrete components.