With our purchase of a new 5th Wheel, we acquired as well a dose of new technology which we're planning to put to good use soon in the Southwest deserts. Our '96 Alpenlite came equipped with a 2,000 watt Heart Interface Inverter, and a single roof-mounted solar panel. Being justly accused of having a slight case of technophobia, I of course eagerly read all of the documentation which came with the new equipment. It didn't take long to discover that the Inverter is a very complex system; and that if we wanted to use the new inverter system to full advantage, we'd need acquire a bit more knowledge about inverters.
Fortunately I was invited by our local Alpenlite dealer to sit in on a seminar being provided by a manufacturer's representative for Heart Inverters. While the presentation was primarily to help the dealer's sales and service personnel understand the operation and capabilities of the system, it proved to be of considerable value as a consumer education session as well. While this writer would hold out no expertise in the nuances of electricity, it seemed useful to summarize some of the key items which were covered in this interesting class. Likely it will help provide other RVers with a better understanding of power inverter system, and at least provide a starting point for evaluating the advantage of such sytems.
Power inverters are becoming more common in RVs because of the changing demands of RV owners. Whether an inverter is "right for you" depends entirely upon one's RV lifestyle. For the RVer who always picks a site with full utility hookups, the inverter still offers some advantages -- but it would be far more difficult to justify. For most RVers, however, who either occasionally or mostly find themselves visiting sites which don't provide electrical hookups, a power inverter is definitely worth considering -- particularly in conjunction with one or more solar panels. Using the Alpenlite scheme as a model for illustrative purposes, let's explore the basics of what the system is and what if offers.
Our unit comes equipped with 4 "Group 27" deep cycle RV batteries. These offer 110 amp hours each, for a total capacity of 440 amp hours. The inverter's basic purpose is to efficiently convert DC power from the battery bank to usable alternating (AC) power for use onboard the coach. The most fundamental question, of course, is "how much is enough"? There is predictably no cookie-cutter answer. There are interrelated issues of RV lifestyle, solar power, generator availability, and individual "power consumption" needs. What is clear is that it's fairly easy to calculate precisely how much power (amp/hours) will be consumed by adding together the power requirements of all electric appliances which will be used on a typical day.
The power consumption of each appliance is stated somewhere on the appliance itself, and is expressed in terms of AC watts or AC amps. What the RVer really wants to know, however, is neither AC watts or AC amps. The RVer contemplating an inverter system wants to know what the equivalent is in DC amps, because that's what the battery bank is supplying. Where an appliance's power consumption is expressed in terms of AC watts, one can use a simple formula to calculate the equivalent power expressed in DC amps:
DC Amps = AC Watts / 12 volts
Example: A TV set with a 100 (AC) watt rating used for one hour would consume 9.17 DC amps.
Where the appliance is rated in AC Amps, use the formula:
DC Amps = AC Amps x 10 x 1.1 x (hours of use)
By adding together one's projected hours of appliance use (TV, coffeemaker, AC lamps, etc., it's fairly easy to calculate the projected DC amp needs on a daily basis. This represents the "draw down" on the battery bank. Naturally the bigger the "bank", the longer one can go without recharging the batteries.
In estimating how much battery power is needed, there are some interesting common sense considerations. It's fairly easy to visualize that if one uses a TV rated at 100 AC watts (which we've already calculated is approximately 9 DC amps), that watching two hours of TV daily will consume about 18 amp/hrs. But evaluating an electric coffee maker requires just a bit more thought. A typical coffee maker is rated somewhere around 1000 (AC) watts. Thus in 15 minutes, one can calculate that the coffee maker will take about 23 amp/hrs from the battery bank. But just leaving the coffee pot on its warming plate for the balance of a full hour could drain a much higher number of amp/hrs from the batteries. That's quite a bit of energy for that wakeup refreshment. When we made that calculation we discarded our old RV coffee maker, and purchased a new model which finishes its brew cycle well under 10 minutes (i.e., less than 16 DC amps), and then turns itself off! Meantime, the coffee is goes directly into the unit's well insulated carafe which will keep it piping hot for far longer than it takes us to consume it.
A note about solar power seems appropriate here, since it's the boondocker's only option (short of a generator) to put some amp/hrs back into the batteries. A good quality solar panel can deliver approximately 3 amps of power during daylight hours. A conservative estimate for sunny weather might be 6 "good" hours of full solar power daily -- and somewhat more in southerly lattitudes and/or during the longer summer days. Thus one panel would put roughly 18 amps back into the batteries; 2 panels 36 amps; and 3 panels 52 amps See our separate article about RV solar energy for more detail on this.
Assuming no other source of recharging the batteries (i.e. utility hookup or generator), the net result of a dry camping day's activity on the battery pack will be the difference between the total amp/hrs, computed as we've described above, and the total amp/hrs "in" from a solar source. If one consumed 50 amp/hrs during the day, and had two solar panels which added back approximately 36 amps, the net drain on the battery pack would be 14 amps for that day. If the total battery "pool" were 440 amps one can begin to get a handle on how many days away from hookukps are possible. However...
There is one important caveat: The literature suggests that a conservative rule of thumb is to count on getting only 50% of the rated amp hour rating for the battery bank -- and only up to 80% under perfect conditions. This is apparently due to various factors, including temperature, battery condition, and the degree to which the inverter is able to convert from DC to AC efficiently. "Efficiency" is defined as the loss of amps due to the process of changing it from DC to AC current. The unit which we have is rated between 84-93% "efficient", which we understand to be quite a good rating.
There seems to be another reason for "counting on" only 50% of your total battery bank capacity. Deep cycle batteries reportedly will perform considerably longer service if they are recharged considerably before they are almost fully depleted. Thus batteries which are regularly recharged after approximatey 50% depletion will last much longer than batteries which are almost completely discharged on each "cycle". This seems reason enough to consider 50% of one's rated capacity as a good rule of thumb in determining how many amp hours are "available".
The other function performed by the Heart Interface product we have is that it acts as a battery charger. But it is hardly a traditional battery charger. A conventional charger provides a "taper charge". This results in slow charging due to the continued tapering of the output current. A typical 60 amp taper charger is said to be capable of delivering only about 12 amps at 13.3 volts. If the battery bank is substantially discharged, it would take a long time to bring our bank of four batteries back to full charge. Moreover, once the battery bank is fully charged, the taper charge continues to charge at a voltage which is really too high for just "maintenance" purposes, and can cause excessive water loss.
The Heart Inverter provides three distinct charging rates, applying a "bulk" charge at 100 amps; then dropping to a lesser rate as the voltage increases in the battery bank; and finally applying a "float" charge just sufficient to maintain the batteries in a fully charged condition. The result is much faster recharging of the batteries, either from shore power or from the motorhome or tow vehicle alternator. What this means is that driving for just a few hours can sigificantly restore a depleted battery bank. There are nuances about alternator types and aftermarket products (including one from Heart) which can ensure top performance, but these topics are beyond the scope of this introductory article.
Perhaps the most interesting piece of the inverter/charger puzzle is the remote monitor and control mechanism which is mounted inside the coach. The unit which we have performs a wide range of functions pertaining to both the inverter and charger functions. It also has the basic "on/off" switch for the inverter. When "on", the control panel provides information about such things as: whether the the unit is operating in inverter or charger mode; if inverter mode, the present level of amps being consumed, and the present voltage under that load. If in the charger mode, it shows the present level of the amps in, along with the voltage at which it is being charged; and various warning functions pertaining to overheating and overloads.
In addition, the remote has 8 dip switches which provide special settings for particular needs. For instance, switch number 4 controls whether the inverter will act as a battery charger when the inverter is off. Its default setting contemplates it should do so. However, if a small generator were in use to run some electric appliances, an RVer might well want to turn the "charger" function off by turning the inverter off. This would preclude the small generator's output from being consumed entirely in attempting to deliver a high amp charge to the batteries -- likely forcing most small generators to shut down. Other switches provide for differences in outside temperatures, and for the use of different types of batteries -- e.g., wet cell versus gel cell batteries.
There is optionally available a digital remote panel (called "Link 2000") which provides more precise information (simply because it is digital, versus the Freedom Remote panel's LED lights which show, for instance amps in 10 amp increments). Reportedly it is also more accurate, because it takes its readings from additional information sources. It has the added ability to show the cumulative DC amps used since the last recharge -- which is probably the most important bit of information an RVer could have. A supplementary single purpose digital readout (the "Link 100") can be added to the "Freedom" remote panel, and provides the same "amps/hrs used" information.
The mechanics of how inverters operate are far too complex to cover in this short article. And we have not attempted to capture more than the basic information here. These systems are sufficiently complex that one needs to understand and consider the many features they offer when selecting a unit. Clearly this is not a "one size fits all" choice. Especially important is the need to consider the implications of one's RV lifestyle in determining whether an inverter would be a useful resource; and if so, what the needed characteristics and specifications of the system should be.
For further information about inverter/charger systems we think you would find the brochure prepared by Heart Interface to be of interest. You may wish to check their web site.
Post Script: We subsequently modified our own RV by adding an additional 3 amp hour solar panel, and adding the Link 10 monitor to give us precise information about amp/hours remaining in the battery bank. We also swapped out the simple "on-off" type voltage regulator for unit which uses the newer "pulse width modulation" technology. Both were significant improvements to the original system.