This new inverter makes it possible to use external AC energy from shore power or a generator and DC current from a motorhome’s battery system simultaneously.
By Mark Quasius, F333630
October 2013
Motorhomes rely on electrical components and appliances to provide the comforts and amenities that today’s RVers enjoy. When parked at most campgrounds, plugging in to the facility’s shore power pedestal will provide the electricity needed to operate these accessories. When shore power is not available, the onboard generator can be run to provide power. Generators are great for powering large loads, but at times it’s not practical — or desirable — to run the generator, either because the electrical needs aren’t that significant or a quiet time is enforced. In those cases, an inverter is an ideal power provider.
An inverter changes 12-volt-DC battery power to 120-volt-AC power. Standard equipment in many motorhomes, the device makes it possible to operate a large number of electrical accessories without running the generator. The power needs of appliances such as a microwave oven, an entertainment system, a residential refrigerator, and various receptacles can be met by an inverter. An inverter is silent, so campground quiet times are not an issue. This makes it the perfect choice for supplying power to multiple 120-volt-AC accessories when dry camping or when driving.
Inverter Basics
In many instances, the circuits that are to be powered by the inverter are placed into a breaker subpanel that is connected to the inverter’s 120-volt-AC output. The main circuit breaker panel also supplies 120-volt-AC power to the inverter’s AC input terminals so that when shore power is available or the generator is running, AC current is present at the inverter’s input. This AC power from the shore pedestal or generator is used to operate the inverter’s battery charging mode so that the house batteries (and oftentimes the chassis batteries) can be recharged, providing clean, stable DC power to operate any of the coach’s 12-volt-DC accessories. This AC power is also passed through to the inverter’s AC output terminals via the inverter’s internal transfer switch to power 120-volt-AC accessories. When AC power is not present at the inverter’s AC input terminals, the transfer switch redirects the current to the inverter’s internal circuitry, which uses the 12-volt-DC battery power to output 120-volt-AC power to those accessories. The transfer switch operates in an “either/or” mode. The inverter’s 120-volt-AC output is either connected to the incoming AC power in pass-through mode or to the inverter’s internal circuitry to supply 120-volt-AC output via inversion from the 12-volt-DC battery bank.
When a motorhome is connected to a campground’s 50-amp shore power pedestal or when running an adequate-sized generator, plenty of power usually is available to operate the motorhome’s 120-volt-AC accessories. However, when the motorhome is plugged in to 20-amp or 30-amp shore power, or when the generator is not large enough to meet the coach’s energy needs, the available power may not be sufficient to operate the accessories. Load-shedding energy management systems, such as those made by Intellitec or Precision Circuits, can shut down certain circuits to prevent overloading the system and tripping the campground pedestal breaker. This is accomplished by temporarily shutting down high-amperage circuits, such as air conditioners or refrigerators, in order to keep the total amperage draw below what would trip the pedestal breaker. When plugged in to 30-amp shore power, you are still limited to the 30-amp total current that is available to power your coach’s electrical systems.
Introducing The Magnum Hybrid
Magnum Energy has been making inverters, automatic generator start systems, and other electrical components for a long time and is a popular choice with many motorhome manufacturers. The company’s latest innovation is the MSH3012M, a 3,000-watt true sine wave inverter/charger. What separates this unit from other true sine wave inverters/chargers is that it incorporates hybrid technology. Most inverters use only one source of energy to power the inverter’s output loads. The transfer switch selects the incoming AC power from a generator or shore power, or it uses the inverter’s battery-created 120-volt-AC power. The MSH3012M has the ability to combine the incoming AC power with the inverter-created power to deliver more power to the AC loads than what the external AC source has available. This Load Support mode parallels the inverter output with the incoming AC current, making it possible to use a smaller generator, which in turn improves fuel economy and reduces vehicle weight. When in Load Support mode, the inverter with the incoming AC power is capable of providing up to a total of 60 amps to power the inverter’s output loads.
The Magnum Hybrid inverter/charger also can be combined with an Automatic Generator Start (AGS) system and a Power Control System (PCS) load-shedding energy management system made by Precision Circuits Inc. that communicates with the Magnum inverter. Data communication between these components provides a comprehensive energy management system to handle a motorhome’s electrical needs. The PCS energy management system communicates with the inverter to allow load support operation and provide additional power when needed. If the motorhome’s total power requirements exceed what is available during load support operation, the PCS will automatically shed circuits as needed and re-energize them when the excess load has been removed. Should the battery voltage drop beneath the preset level, the AGS will start the generator to recharge the batteries. The AGS also will automatically control generator operation should the thermostat call for air-conditioning when shore power isn’t present.
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Installation
My motorhome originally came with a 3,000-watt modified sine wave inverter and no energy management system. I previously had replaced the modified sine wave inverter with a true sine wave inverter, but that unit was beginning to have issues, so it was the perfect time to upgrade to the Magnum hybrid inverter. I also installed the PCS energy management system from Precision Circuits to take full advantage of the benefits of the new inverter. This required replacing the original breaker panel with a similar unit that incorporated the load-shedding relays and control circuit board. A cat5 communications cable connected the PCS-equipped breaker panel to a remote panel that displays all of the voltage and current usage parameters as well as the status of any circuits being shed. A second communications cable was run to the Magnum inverter.
The original inverter’s location interfered with cargo storage, and it was always in the way. I installed the Magnum inverter in an area where it did not interfere with cargo, and I fabricated a new steel mounting cradle to secure it in place. Most times this won’t be necessary, because the Magnum inverter will bolt right in to any existing inverter location. I then rerouted the battery cables and AC electrical wiring to the new location and connected them to the inverter. I also added a 12-volt-DC disconnect switch and a class T-type fuse, such as Magnum’s ME-400F, in the battery positive circuit, because these were not installed when my motorhome was built. I replaced the existing battery temperature sensor with a new Magnum sensor; replaced my existing inverter’s remote control panel with the new Magnum control panel; and then connected the data cable from the remote to the inverter.
The Magnum remote control panel also is capable of controlling an automatic generator start system, but my motorhome came equipped with an Onan EC-30 stand-alone system, so I left that in place. I did add Magnum’s Battery Monitor Kit (BMK). This device utilizes a shunt in the negative battery cable to communicate to a module that displays detailed information about battery usage and state of charge. It is a great tool for keeping track of the batteries and 12-volt-DC system usage.
Operation
After the equipment was installed, I switched on the AC power and batteries and powered up the system. The PCS energy management system immediately booted up and displayed the voltage and current draw on each leg. The Magnum inverter’s remote control panel was fitted with a handy rotary knob with a “push to set” function. All I had to do was to press the setup button and use the knob to scroll through and select the various setup parameters. I identified my Lifeline batteries as AGM-1 and sized them at 880 amp-hours. I set the various battery monitor and charge parameters, and the entire system was operational.
The inverter immediately entered the battery charge mode. The batteries were fairly well charged, so it didn’t take long for the charge to taper off and enter float mode. The BMK information also was displayed on the Magnum remote control panel, and I was able to observe the rate of discharge or charge of the batteries as well as their charge status. The BMK monitors all incoming or outgoing power to the batteries and uses software calculations to display a true state of charge in percentage. It compensates for battery type, size, and temperature, so you no longer have to rely on voltmeter readings to determine just how much capacity remains in the batteries.
When the shore power was disconnected, the inverter automatically switched over to inverter mode. I then switched over to a smaller, 20-amp shore power feed and set the incoming power down to 15 amps on the remote control panel. The Magnum hybrid inverter automatically reduced the charging rate so that there was still adequate power to run the motorhome’s accessories. I loaded up the system to draw close to 15 amps and then fired up the microwave oven to test the load support capability of the hybrid inverter. The inverter seamlessly added the power required to operate the microwave oven without tripping the pedestal breaker.
When connected to 30-amp service, I ran two air conditioners and a few other items to bring the total load close to 30 amps. I then turned on the microwave oven, and the inverter once again went into load support mode and provided the additional boost in power required. I next switched on the electric heating element of the Aqua-Hot boiler, which is a circuit that is not fed by the inverter. The PCS energy management system determined that this would exceed the 30 amps of available shore power, so it shed one of the air-conditioner circuits to prevent the pedestal breaker from tripping. Once the load was removed, the energy management system restored power to the second air conditioner.
The combination of Magnum’s hybrid inverter, an automatic generator start module, and the Precision Circuits power control system created a coach-wide energy management system. Whenever incoming shore power cannot meet the motorhome’s power consumption needs, the inverter’s Load Support mode activates to provide additional power. A 30-amp shore power feed is equal to 3,600 watts, but since the inverter offers load support, this can increase the available power to more than 6,000 watts. This isn’t possible when using a traditional inverter, because of the inverter’s transfer switch design. But the hybrid design of Magnum’s MSH3012M now makes it possible. This Load Support also makes it possible to equip a motorhome with a smaller generator. An 8-kilowatt generator can be used instead of a 10-kilowatt unit, which will result in less weight and improved fuel economy. Look for systems like this to be offered in new motorhomes in the near future.
Sources and References:
Magnum Energy
(425) 353-8833
www.magnumenergy.com
Precision Circuits Inc.
(630) 515-9100
www.precisioncircuitsinc.com