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Overview of Inverters and Chargers


Battery chargers do just what they say - they charge batteries. An RV is a bit more complex in that when camping with shore power, there are a number of 12 volt accessories that need to be powered such as lights, the fresh water tank's water pump, ceiling fans, and other devices. To do this, RVs use what is called a converter. Unlike a pure battery charger, a converter is designed to actually put out a consistent DC power level to run these accessories. At the same time the converter also handles battery charging tasks. Today's converters have excellent 3 stage battery charging capabilities to help prevent out-gassing of the batteries while still maximizing the charge rate. The converter is connected to a master output post someplace in the RV that then feeds a 12 volt fuse box to distribute this power to the needed accessories. A separate circuit also feeds the batteries to handle battery charging needs.

A converter basically "converts" 120 VAC to 12 VDC current. But, what if we want to run 120 volt accessories from our batteries without needing to run the generator set or use shore power? That's where inverters come in. An inverter will "invert" DC battery power into 120 volt AC power. It basically just the opposite of the converter. Small inverters are typically found in RVs as a device to run the entertainment gear. The TV, VCR, and any other devices such as a satellite receiver are plugged into a set of electrical outlets at the front of the RV. A small inverter is located close to that equipment and it can be turned on with it's own on-off switch. It then converts (inverts, actually) battery power to 120 volt AC power to run these items. These inverters vary in size from 250 watts to 900 watts.

Larger RVs tend to have an all-inclusive inverter in the basement. Technically this is an inverter/charger because it performs both functions. This inverter is of a larger capacity, generally 2,000 watts or greater. The inverter output goes to a number of receptacles throughout the coach, which are generally labeled as inverter outlets. It also feeds the microwave oven. Because this inverter is larger in both size and capacity it is located in the basement area. It is located close to the batteries in order to minimize any voltage drop due to long cables when the larger amp draws are used. This inverter is fed 120 volt AC power. When running on either shore power or the generator set is running, there will be 120 volts present at the inverter's AC inputs. In this case the inverter's built in transfer switch will simply pass the existing power through without processing it. When the power is removed, the transfer switch will actuate and the inverter will begin to invert battery power to the output circuits. As soon as input power is restored, the transfer switch will revert back to the bypass mode. When running in the bypass mode the inverter is also turned into a battery charger. It will provide 3 stage battery charging power to the batteries - pretty much like an inverter running in reverse. It will also provide clean regulated power to feed all of the 12 volt accessories, much like a converter would.

Notice that so far all we've talked about is coach batteries. The reason that we haven't talked about chassis batteries is because none of the above systems have anything to do with the chassis batteries. When the RV is running the engine's alternator is charging the chassis batteries. At the same time it closes a charge solenoid, which will tie both the coach and chassis battery banks together. Now the alternator can charge both sets of batteries while you drive. The minute you switch off the ignition key, the charge solenoid opens and the connection between these two systems is broken.

If you have shore power and want to maintain the charge state of your chassis batteries you will have to add an auxiliary charger to accomplish this. There are a number of ways to do this. You can add a small pulse type trickle charger, such as the BatteryMinder or you can go with a system such as the Trik-L-Start which ties both batteries together and allows your converter or inverter/charger to safely charge both sets of batteries. These battery maintainers are small in output and won't damage your batteries from overcharging.


Submitted by Mark Quasius - 2/24/06

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