| We came across this excellent article on
a USA board. We liked the good clear explanations and the reasoning behind
each suggestion. We are very fortunate in Australia that both A'van and
Chalet fit as standard at least 10 amp converters (battery chargers).
A'vans come ready set up for the easy addition of solar at some point down
the track - great stuff A'van! The three stage inverter/chargers cost in
excess of $2,500.00 in Australia so are not really an option. We have
replaced our solar regulator with a far better unit that indicates battery
voltage digitally and also displays amps going into the system. This unit
does three stage charging and cost an horrific $215.00 BUT
our solar system is now far more efficient. We will be covering this is
detail shortly. In reading the story below it makes one realise how lucky
we are in Australia in not having to mess with furnaces and the like as we
are able to camp all year round in our wonderful climate. Makes you think
doesn't it?
There have been discussions about using solar to run A-frames and recharge batteries. Many people talk
about the off-the-shelf RV solar set ups. These have their place and work for some people. But how do
you know if it's right for you or will meet your needs? How do you size a system for your needs and
tie it into the trailer? Here's some stuff I've gleaned from over the past couple years of
researching solar.
In order to get a proper solar set up, one needs to understand: 1) how to size their individual power
requirements, 2) have an idea of how to size panels for their needs, 3) know how batteries are charged,
and 3) be familiar with options on how to tie solar into the trailer.
1) POWER USAGE REQUIREMENTS:
The first thing you should do is estimate your worst case scenario power draws for the seasons you camp.
I suggest worst case be winter, or a blistering summer, because you want a solar
set up to meet *all* your needs. Surplus is good.
Figure out how many amps you consume over a 24 hour period. That is the number of amps you will need
per day. At a *minimum* your battery should be twice this amount because batteries that are
repeatedly drained beyond 50% have shortened lives.
Most of the appliances in our trailers have Amp ratings listed in the owners manuals. This is the
number of amps the appliance uses in one hour. For example, the 16,000 BTU furnace in the A'liner uses
3.4 amps. A Fantastic Fan on high uses 3 amps. For other items, such as bulbs, you can use the
following formula: wattage / 12 (volts) = amps. So
a 15 watt bulb running off the trailer battery will consume 1.25 amps per hour.
Solar panels are always rated in watts. Once we have the total amps per 24 hours, we need to convert
them to watts to find out how many panels are needed. Use this formula: Amps * 12 (volts) =
watts.
2) SIZING PANELS:
Below is an extreme example, but it illustrates the relationship of power usage, system sizing, and
charging times.
Our goal is to get a solar set up that can be used year round to recharge our trailer battery so we
have unlimited dry camping capacity.
We estimate our winter usage at 51 amps per 24 hours. We have a pet and want to keep them warm
during winter months. Most of the usage is the furnace fan. Our daily total in
watts is: 612 watts. In spring and fall months the usage will be less. In summer months it may be close to 51 amps
if we run our Fantastic Fan all the time. So we will stay with 51 amps per day (612 watts).
How many panels do we need and what do we do with them?
Solar panels are not 100% efficient. Some numbers say as low as 80% efficient in full sun (different
brands, age, engineering, etc.). This puts a 100 watt panel at 80 watts in full sun. In winter you
may only get about 40% of that 80 watts, due to clouds, the angle of the sun, etc.
Lets work with compensating for the loss of power in the panel - it's 80% efficient. That means of our
612 watts, we will loose 122.4 watts right off the top. So we need 735 watts. 612 + 122.4 = 735
watts. This will over-ride the inefficiency of the panel and give us our full power usage per hour in
full sun.
That seems like a lot! Remember, this is sized for winter and it will be cloudy and the sun will be
low. We know we won't see the full power output. We will be lucky to see 40% of that 735 watts per
hour, and we only have a short window of sun time to recharge those batteries!!! This is why we have
such a large system. It can be made smaller for
periods of longer daylight.
Where we live, we can plan on about 2.5 hours of useable sunlight for generating power in the winter,
based upon sun charts that list hours of useable light for each season. Because it is also cloudy in
winter, that will eat into the panel's ability to generate power - and we don't know by how much
because that is variable. By going with 735 watts
we have oversized our set up for winter use. We hope that if the panels can generate 40% of their
capacity in that 2.5 hours of useable daylight, then
we MIGHT be able to get our battery recharged - or close to it. The success of that depends on what
type of charge controller we have.
That off-the-shelf RV system? It comes with 100 watt panel with a 7 amp charge controller (explained
below). We will have 80 watts after panel inefficiencies. During the winter we *might* get 35
watts. This would give us 2.9 amps per hour. 2.9
amps would not run the furnace fan. We would have nothing to recharge the batteries with. The system
is under powered.
Summer may be a different story. In the summer that same 100 watt off-the-shelf setup could generate 6.6
amps per hour. If we had 7 hours of power generating sunlight and no power was being run in
the trailer - it might work for us. But if we have an animal and leave a fantastic fan running on high
(3 amps), we might break even. If it is cloudy or
shady, we won't break even with a fan running.
In the summer our 735 watt set up would generate up to 61.25 amps for up to 7-10 hours depending on
where we were! We could probably leave some panels at home for summer usage since the days are longer.
3) CHARGING BATTERIES:
It's winter and we've got 2.5 hours to get 51 amps back into our batteries... hmmm.
You can not just hook a solar panel up to a battery because there would be nothing to regulate the
amount of power going to the battery. You could ruin the battery by boiling off the electrolyte,
overheating it, or worse. That is the purpose of a charge controller. It regulates the flow of power
to the battery.
The optimum charge rate for lead acid batteries is 20-25% of their capacity (gel or AGM batteries have
rates specified by the manufacturers that may be higher). So a 100 Amp Hour (AH) lead acid battery
can handle being charged at between 20 and 25 amps.
Most off the shelf auto store battery chargers can't do 20-25 amps for a deep cycle battery. Most can
only do 12 amps and then a slow trickle charge. A good quality "3 stage" charger or charge controller
will be able to do 20-25 amps or more.
What is a three stage charger/charge controller? A three stage charger/charge controller monitors the
battery voltage (and temperature) and adjusts the amount of amps flowing to the battery to what the
battery can handle. Batteries do not absorb the same amount of amps during the whole recharge
process. The amount they absorb decreases as the battery gets full. 3 stage chargers will have a
period of: bulk charging, absorption charging, and a final period of float or trickle charging.
During the bulk stage, a large number of amps can be absorbed by the battery. This period will stop when
the battery reaches between 60-75% of it's capacity. High amperage bulk charging is very important if you
have a large amount of amps that need recharging in
short periods (such as winter using solar panels). So if you have a 100 AH battery drained to 50 amps.
The bulk charge would put 25 amps into the battery
until the battery reached between 60-75%. With a good charger working at 20-25% of the batteries
capacity, the bulk recharge portion could take as
little as one hour. When the charger sensed that the battery has reached 60-75% of it's capacity, the
battery charger would then switch to the "absorption
stage". Our battery is now at between 60-75% capacity, and we have 1.5 hours of sun remaining...
During the Absorptions stage, the battery absorbs a decreasing amount of charge as the battery moves
closer to full, until it reaches about 90%. The charger switches to a "float" or "trickle" charge.
I don't have any time estimates or references
regarding times for this, but one would hope that in the remaining 1.5 hours of winter sun that you could
get up to 90% charged.
With a float charge, a battery will absorb about 1 amp per hour.
The 7 amp battery charger in the off-the-shelf kit would need over 7 hours to recharge our battery. We
don't have that much time so we would need a better charger able to handle a higher bulk charge rate and
able to monitor the battery while charging.
4) CONNECTING SOLAR TO THE TRAILER:
With trailers, it is usually recommended to use free standing solar panels. This lets you park the
trailer and move the panels to where the sun is
best. It is best to use thick wire to minimize power loss. Many solar web sites have charts for
figuring the proper gauge for length of runs.
How do we get the power to the trailer battery?
One way is a charge controller as discussed above. This is what most off-the-shelf RV solar
set ups consist of. Two leads off the controller are attached to the battery posts. The other end is
connected the solar panels. These are made so they can be connected and disconnected. They pretty much
slowly recharge the battery through out a long sunny
day. There are a variety of charge controllers out there that for the solar market and they range in
their amp ratings. Shop around.
Another option is an inverter that has a built in 3 stage charge controller. This provides the greatest
flexibility because it recharges the batteries with
high bulk charges AND gives AC power to the trailer from either the solar panels or battery.
Inverters are measured in watts because inverters also have the ability to take a 12 volt supply, and
convert it over to 120 volts. When using an
inverter always get an inverter which matches the "INPUT" wattage of your appliance. The inverter has
to be able to sustain the required "Input" wattage to the appliance - not the output wattage - so that
the appliance will run properly.
Certain inverters by Trace Engineering are built for RVs and have a three stage charge controller built
into them, accept a DC voltage source such as solar panels, have bulk charge outputs from 25 amps to 50
amps, and have a maximum rated wattage from 2,500
watts (at 18 amps) to 3,400 watts (at 63 amps).
With such an inverter you can scale your solar needs to the time of year and conditions - just add or
remove panels to meet your needs. During the summer months, you could be recharging your batteries and
using appliances without ever being hooked up to
electrical or running a generator. During the winter you could build a small array and recapture
your daily power needs.
Enjoy,
Chris |
