| Inverters |
 Power Inverter |
return
to Rental Rates page Caravan / RV Manual page |
Power inverters are typically
found in the ultra luxury motorhomes i.e. pusher diesels. 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:
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.
NOTE: This online manual is a work in progress and is subject to change at any given time.
It is also a generic peace of work and is meant only as a general guide. Each
Caravan/RV is distinct and individual in it's own right, do not take anything
written here literally as it may or may not apply to the exact Caravan/RV you
will be using.