Electrical Power

The electrical supply in RV parks can be notoriously bad. There are frequent outages, brown outs, power spikes. Sometimes electrical boxes are miswired or damaged. We tried to, and did a pretty good job of, getting an electrical system that protected our equipment, avoided crashing our computers, and gave us the option of operating even when an RV park's shore power was unavailable.

The major advantage of this electrical system, when operating in full-auto mode, is to provide graceful degradation of capabilities in the event of shore power failures. When shore power fails, the inverter picks up critical loads connected to it, but the capacity on that leg is cut from 50 amps down to around 30 amps. If power stays off long enough to drain the batteries to near 50% charge, then the generator comes on, but capacity on the inverter leg is cut futher from about 30 amps to about 20 amps.

This works out fine for maintaining business activities; we rarely drew more than 10 amps when operating only our computers, communications, printers, ventilation, etc. It means that you have time to respond to power changes. You have time to save files, finish a phone call, finish printing a document, etc. I have worked straight through power outages of an hour or two, just pausing long enough to make adjustments to what was running off the 120 VAC power.

Please note that this electrical system is NOT suitable for off-grid operation. The generator is a light-duty backup power supply. The solar panels are great for maintaining "life support" but will not operate much of anything else. Even when used strictly as backup power, care must be taken not to run out of propane by running the generator for too long; propane is critical for maintaing "life support" (heat, refrigeration, cooking, water).

The backup power features of the RV electrical system are intended for short power outages, which is the most common type of power outage. The longest RV park power outage we suffered in over three and a half years of full timing was about a day and a half. The next longest was four hours. Many outages were a couple of hours or less. The most common power outages were just a few minutes or seconds.

Brownouts were far more common than outages. They usually occurred in older parks, during hot weather, when the parks were crowded. Everyone turns on their air conditioners and the voltage sags badly. When this happens you can sometimes hear generators coming on in surrounding RV's.

50 Amp Service

The RV has a 50 amp shore power cord. Assuming the park provides 50 amp service, this provides two 120 VAC 50 amp circuits. Contrast this with 30 amp RV service which provides a single 30 amp circuit (2X50 versus 1x30 amps). The RV will operate on 30 amp service, but it requires a bit of care in what is powered up (can be tricky in hot weather with air conditioners).

The two 50 amp legs each supply different equipment (i.e., the circuit is unbalanced). There is a "high draw" subpanel for the air conditioners, refrigerator, hot water heater, and vacuum cleaner. The other leg feeds the inverter/charger to which the rest of the 120 VAC outlets are connected.

We use a SurgeGuard Model 34750 for added surge protection. It also protects equipment on the "high draw" subpanel from under and over voltage; the inverter provides the over/under voltage protection to equipment attached to it.

Inverter

The inverter is a Xantrex/Trace SW Series Inverter/Charger, model SW4024MC2. It is rated at 4000 watts, which means that it can provide a maximum of about 4000/120 amps (33 amps) for however long it takes the batteries to drain to near 50% charge. See Batteries, below.

The inverter provides near-sine wave output. The output is close enough to pure sine wave that we operated computers, printers, and almost all other equipment without problem. We had one clock radio that did not keep time correctly when on inverter power, but everything else has been fine (including other clocks).

The switch-over speed of the inverter is so fast that we never had any computerized equipment crash during a switch over. This includes transfers from shore power to inverter power to generator power and back again.

When shore power is available, the inverter "follows" it. That is, it stays in synchronization with it. If voltage sags a little, the inverter gives it a little boost to keep it within a safe range (generally set to about 104 VAC to 130 VAC). If power sags too low, the inverter takes over to supply power. The "following" feature reduces battery drain by only sipping a little battery power to boost the incoming shore power, rather than completely switching from shore power to inverting. It also means that the inverter is always synchronized and ready to take the load without any phase related damage to equipment.

Related to the shore power following feature is the inverter's ability to prevent over charging of the batteries by using any excess power to generate 120 VAC power. In the sunniest climates when the solar panels are producing a lot of power, the inverter supplies about one amp 120 VAC power, reducing the shore power draw by one amp for part of the day. If you are paying for your own electricity this can reduce your bill by a very small amount.

When the inverter is generating 120 VAC household power it is converting 24 VDC battery power to 120 VAC power. It is draining the batteries (you will see the 12 VDC RV lighting dim a bit). If the inverter is set up to automatically start the generator, when the batteries get close to the 50% level the generator starts and the inverter shifts AC loads to the generator. If generator auto-start is not turned on, when battery charge gets down to the 50% level the inverter cuts off 120 VAC household power to avoid damaging the batteries.

It is important to remember that the "high draw" panel is not connected to the inverter. This means that on loss of shore power and before the generator starts, equipment on the "high draw" subpanel will not have power. This means no air conditioning, in particular. You can press a button to start the generator at any time, if you need to.

Some RV parks prohibit running generators. In parks that permitted generators we usually set the inverter to automatically start the generator if necessary. Okay, I sometimes set it to run automatically even when parks prohibited generators if I did not want power outages to interrupt my work (if they don't want my generator to run, they should keep their shore power on!).

Almost all parks prohibit running generators during "quiet hours", generally from 22:00 (10 P.M.) to 08:00 (8 A.M.). The inverter has quiet time settings so that the inverter does not automatically start the generator during those hours. Note that the generator is located under the bedroom. It is not possible to sleep in the bedroom when the generator is running.

Generator

The backup generator is a Cummins Onan Marquis Gold 5500 LP. It is rated at 5,500 watts (5.5K watts).

The generator is designed as backup power, not full time power. Cummins recommends running the generator no more than eight hours a day.

It runs off low pressure (LP) propane, the same propane tanks that power the furnace, hot water heater, and refrigerator. Cummins documents say that this generator will consume about 4.6 pounds of propane per hour at full load. This means that the generator can consume most of a 40 pound propane bottle in eight hours (the RV has two 40 pound bottles).

The output of the generator is divided into two circuits: one has a 30 amp breaker; the other has a 20 amp breaker. The 30 amp breaker is connected to the "high draw" subpanel. The 20 amp breaker is connected to the inverter.

At 120 VAC, the maximum total amperage that could be delivered by this 5.5K watt generator is approximately 5500/120 amps: 45.8 amps. Compare this to the shore power which can deliver 50 amps on each of the two legs, for a total of 100 amps (12K watts). When operating on generator you are operating on less than half power. This requires some care in what is operating on 120 VAC. Upon a shift from shore power to inverter to generator it may be necessary make adjustments.

Note that you can manually start and stop the generator at any time, regardless of the inverter's auto-start or quiet time settings.

Solar Panels

The solar panels are (I believe) Shell SP75's. There are four of them connected in series/parallel to provide 24 VDC for charging batteries via a Morningstar ProStar PS-30 controller.

These panels can supply a theoretical maximum of 75x4 watts (300 watts). This is about 12.5 amps at 24 VDC or about 2.5 amps at 120 VAC via the inverter.

In practice the most I have ever seen them deliver in the Southwestern USA with the panels mounted flat on the top of the fifth wheel is about 5.5 amps at 24 VDC. This is about 132 watts, or about 1 amp at 120 VAC through the inverter.

The solar panels are most useful in maintaining battery charge when the inverter is off. On a sunny but cold day we have run the refrigerator, lighting, water pump, and furnace "off grid" for several hours without measureably discharging the batteries.

The irony of the solar panels is that when we have needed them the most they have generated no power at all. During storms that knocked out the power, the panels could not generate power because the sun was blocked by clouds, rain, snow, sleet, etc. We had to run the generator to keep the heat on once the batteries got low.

Batteries

The battery bank contains four Trojan 6V-AGM 200 amp hour 6 volt batteries. They are wired in series to provide the 24 VDC needed by the inverter (above). There is also a tap for the 12 VDC that the DC trailer wiring requires. To support the mix of 12 VDC and 24 VDC usages, there is also a battery equalizer to prevent the two batteries supplying the 12 VDC from becoming more drained than the remaining two batteries.

The batteries are charged by four sources:

  1. The inverter (main charging source).
  2. The solar panels.
  3. A trickle from the truck during towing.
  4. The generator.

The major loads on the batteries are:

  1. Inverter.
  2. Water pump.
  3. Refrigerator (if not on 120 VAC).
  4. Furnace.
  5. Lighting.

It is important to note that the batteries should never be drained to less than 50% charge. This means that the 200 amp hour batteries can really only safely supply 100 amp hours.

The load that will drain the batteries fastest is the inverter when it is producing 120 VAC from 24 VDC supplied by the batteries. To supply one amp of 120 VAC power, the inverter must draw approximately 120/24 = five amps of 24 VDC power.

So, if the inverter is supplying 10 amps at 120 VAC, it must draw 50 amps at 24 VDC from the batteries. At 50 amps/hour, the 100 amp hours of battery capacity will be used up in two hours, right? Wrong.

The amp hour rating assumes an even discharge rate over a period of 20 hours. Thus, the 100 amp hour batteries can really only supply 100/20 = 5 amps and still provide the rated 100 amp hours. You can draw more than five amps from these batteries, but the number of amp hours you actually get drops drastically as the current draw goes over five amps.

Our experience is that when the batteries are fully charged it is possible to get 10 amps of 120VAC for 15-20 minutes. Drawing five amps of 120 VAC, we have gotten 30-40 minutes.

This is why we have a generator, and this is why the generator auto-start features of the inverter are useful.

Battery Equalizer

The inverter requires a 24 VDC battery bank. As mentioned above, the four 6 VDC batteries are wired in series to provide 24 VDC to the inverter.

However, the trailer's DC systems require 12 VDC. The 12 VDC systems include lighting, furnace, refrigerator, and the TV antenna amplifier. Most of the DC load on the batteries come from these 12 VDC systems. The "bottom" two in-series batteries provide the 12 VDC power. This means that these two batteries would be drained faster than the "top" two, which could cause a huge imbalance in battery charges pretty quickly.

The battery equalizer takes care of this problem by converting 24 VDC to 12 VDC on an as-needed basis to keep the two sets of batteries charged to the same level. Thus, the inverter sees four 6 VDC batteries wired in series, while at the same time the trailer sees the same four batteries wired in series-parallel to provide 12 VDC.

The equalizer is a 75 amp Sure Power Industries model 5210.

Trickle Charge DC-DC Converter

When towing a 12 VDC trickle charge is provided by the truck to the trailer. However, the batteries require a 24 VDC charge. The truck's 12 VDC is converted to a 24 VDC charging voltage by a Vanner Model 91-10A DC-DC converter.