The Argosy Solar Project

Dave & Ann
Lowrey

WBCCI: 5074

Batteries

The easier of the two to size are the batteries. Batteries are measured in Amp-Hours, so is should be simple. If I draw 50 amp-hours a day, and I have a 55 amp-hour battery, I should be OK, right?

Well, as the Hertz TV commercial says, "Not Exactly".

There are several types of batteries. The battery in your car is called a "Starting" battery. It is designed to accommodate a very large current draw over a short period of time (like when you start your car). "Starting" batteries do not like to be totally discharged, and doing so often will quickly damage the battery. The batteries you use in a RV are called "Deep Cycle" or "Marine" batteries. These batteries are designed to support smaller current draws over a longer period of time. They also support being heavily discharged and recharged many times over their lifetime.

There are also several different types of Deep Cycle batteries. Standard Lead-Acid batteries are less expensive but can not be used inside a living space unless they are vented outside. This is due to the fact that they outgas hydrogen, which is explosive. Lead-Acid batteries can come in capped or sealed versions. AGM batteries do not outgas, and can be placed inside living areas. They are a lot more expensive then lead-acid batteries.

There are several factors that effect a Deep Cycle battery's performance:

• You never actually get the batteries rating from a battery
• The slower the current is drawn, the more capacity the battery will have. So, a 100 Amp-Hour battery bank may support a 5 Amp-Hour load for 18 hours (90 amp-hours total), while it might support a 20 amp-hour load for only 3 hours (60 amp-hours total).
• Battery temperature effects it's output. Colder batteries produce less power than warmer ones
• A good rule of thumb is that you never want to discharge your batteries more than 50%
• Older batteries do not perform as well as newer batteries
• Dumb battery chargers, such as the "Uni-Volt" found in old Airstreams and Argosys, tend to "cook" the battery at a constant voltage. An "intelligent" battery charger (otherwise known as a converter) is better for your batteries (I use a Inteli Charge from Progressive Dynamics).

Solar Panels

The number of Solar Panels you have determine how fast your batteries recharge.

Solar panels are rated in Watts. The amount of watts you get at what voltage is determined by the amount of sun that is falling on the panel. So the panel(s) send 'watts / volts' Amps to the battery. Most solar panels will have a label that describes these values:

So, under "ideal" conditions, this panel will output 4.54 amps at 16.6 volts. This gives us a total power rating for the panel of 75 Watts (4.54 * 16.6). If I hook them up in parallel (which is typically done in 12 volt installations), you add up the amps. So two of these panels produce 9.08 amps under "ideal" conditions.

The number of panels you have determine how fast your batteries are recharged. Under "ideal" conditions, my 50 amp hours would be recharged in under six hours (9 amps * 6 hours), assuming I had two of these panels (which I do :-) ). If I had four panels, they would be recharged in under 3 hours, etc.

Charge Controller

The charge controller controls how much voltage and current go from the solar panels to the batteries. It also keeps the solar panels from draining your batteries at night.

There are three types of charge controllers:

• Cheap - These feed a constant voltage, if possible, to your batteries. These usually have no meters.

• Standard - These vary the voltage and current based on how "full" the battery is. If the battery is below 90% charged, the controller will send 14 volts or so and all the current your panels are generating to the batteries. As the battery gets charged, the current is lowered till the battery is full and charging stops. These usually have meters showing battery voltage, solar panel output in amps and the charging amps going to the battery bank.

• MPPT Controllers - Standard and cheap controllers "throw away" excess voltage that is above what they want to send to the battery. Even if a panel is producing 16 volts, a controller may only be sending 13 of those volts to the battery. A MPPT (Maximum Power Point Tracking) controller converts any excess voltage into current, so you can  actually send more current to the batteries than the panels are producing. These generally have the same metering as a standard controller. Don't ask me how this is done. It's magic! :-)

Inverter

An Inverter converts 12V DC to 110V AC so you can run 110V AC appliances, such as a microwave off of your 12 Volt batteries.

Inverters can be a MAJOR drain on your batteries. A 1500 watt microwave will draw 125 amps at 12 volts! They range from simple "cigarette lighter" low power models, such as the 300 Watt unit I have, to multiple thousand watt hard wired models.

If you decide to use an inverter, you may also want to invest in a converter (charger) that will automatically switch the 110V AC outlets in your trailer between shore power and your inverter.

Constraints

Note the use of the word "ideal" in the above paragraphs. What that really means is "Forget it Dave, you will never see numbers like that".

• Due to mounting constraints, your panels will most likely never be pointing directly at the sun.

• Clouds, shadows, dust, pollution, bird poop, etc. all conspire to reduce your power output.

• During the winter months, you have significantly less hours of sun and that sun is coming in at a low angle (See bullet #1).

• Solar panels become less efficient the warmer they are. The numbers on the label were obtained in a lab at 25 degrees centigrade (about 77 degrees Fahrenheit). Solar panels sitting in close proximity to a metal RV roof can get very warm in the summer and will not produce optimal results.

On top of the above mentioned general constraints, I had a few personal ones, mainly due to the fact that I have a 21 foot long trailer (read: short).

• I only have room for two solar panels on top of my trailer. I could possibly squeeze in a third, maybe, but it wouldn't be in a very good location due to the fact that the air conditioner shroud would cast shadows on it.

• I don't have a lot of room for batteries. I have two 55 Amp-Hour Optima AGM batteries. I could possibly squeeze in a third (I could also squeeze in a  fourth, but then my wife would loose room for her clothing and, while "I" don't have a problem with her doing it, she doesn't like to go camping naked).

OK - given ALL the above information, it's now time to see what Hardware I bought...