Solar Panels And Charging
Solar panels are not a new thing as they have been around for many years, but investment in solar technology has seen them developed and improve at a phenomenal rate over the last decade, and with most of the latest technology coming within the last five years. Technological breakthroughs have meant that for off roading and expedition work they have become a viable option for those with the capability to invest in them, and it’s opened up the market for cheap copies from certain countries.
Many people invest in solar panels, only to be disappointed by their performance and failure to live up to their often fantastic claims, most of this comes because of over enthusiastic claims by sales people, and considerable ignorance by potential purchasers. Overcoming either of these problems ensures potential purchasers are knowledgeable enough to see through the carefully worded, often outlandish claims by sellers and gives them the knowledge to correctly evaluate their potential and viability for their purpose.
If we assume we look at a fairly normal 60w solar panel many people would logically assume it gives a maximum of 5 amps at 12 volts, nothing is further from the truth as a 60w solar panel is tested under laboratory conditions which are totally removed from their actual working environment. If we correctly assume that as a solar panels voltage rises as its current or output rises, then apply a little simple maths, then look at the panels specifications we can calculate the maximum output for ourselves. Specifications for a typical 60w solar panel show its maximum voltage is 17.2 volts we can calculate its amperage output for ourselves.
Volts Multiplied by Amps = Watts
Watts Divided by Volts = Amps
Watts Divided by Amps = Volts
From our calculations we can use our 60 watts and divide it by the maximum voltage the panel supplies, so 60W divided by 17.2v = 3.48 amps; so now we know our assumption of a 60W solar panel providing a maximum of 5 amps is incorrect as our calculation shows it only provides a maximum 3.48 amps. This is a massive discrepancy over the claims made by sales people, and anyone using the claimed maximum output to formulate their own calculations will have considerable problems when they find the unit they have purchased isn’t living up to expectations. Many feel disappointed that they have spent a large sum of money because they have listened to sales people, often large amounts of money they could have spent on a small generator which would cost less than a decent solar panel, and actually delivered what it claimed in actual output.
Many sales people will talk about crystalline or amorphous solar panels; again I would dismiss this as such technicalities are not what off roaders want to know, leave this to the scientists as all we really need to know is how much a solar panel will cost, and how much power it will actually deliver.
I will now make a claim:
Why is your 60w panel only a 28.5w solar panel? For a solar panel to work efficiently it has to be pointing at the sun when the sun is at its highest, and we assume we have a bright summers day, this gives us its maximum output the panel can deliver.
If the panel is mounted on a vehicle roof it’s not pointing directly at the sun, also the sun is not bright enough to generate anywhere near its maximum output, and remember that only 2 degrees away from directly pointing at the sun will reduce a panels efficiency dramatically.
The first and last two hours of the day means it only generates 15w per hour or 1.25 amps per hour due to the solar panels oblique angle and not working efficiently, therefore we can say over the four hour period it has generated 60w or 5 amps of power.
If we assume that with a brighter midday sun, and the fact its overhead and at a better angle to our roof mounted solar panel it generates 3.5 amps of power per hour or 14 amps of power over the other four hours it’s working.
We can add the 5 amps from its first and last two working hours to the 14 amps it generates from the four hours when the sun is brighter and at its highest; we now have a total of 19 amps of power generated over an eight hour period. If we multiply the 19 amps by 12 volts as seen in our calculations formulae we can conclude that V x A = W; or 19A X 12V = 228 Watts.
If we take our 228 watts of generated power and divide this by the time it’s taken to produce it we can conclude 228 Watts divided by 8 hours = 28.5 Watts per hour. Therefore we have shown that our claimed 60w panel is in reality only a 28.5w panel as its actual average output over an 8 hour working day is 28.5w per hour, or 2.375 amps/hour. If we then base this our sample panel having a maximum voltage of 17.2 volts instead of 12 volts the panel only provides13.25 amps over an eight hour working period.
I can prove my claims; ask a manufacturer to do the same and you find they can only quote theoretical maximum output; and my figures are taken from actual testing out in the field in real world conditions, not hypothetical laboratory figures.
Now we have the basics to understand how solar panels work, dispel the outlandish claims made by sales people and calculate the accurate output using the specifications, and hopefully choose whether or not to buy a solar panel, and which type or size to buy.
Mounting Solar Panels
Mounting solar panels is not difficult but many people get it wrong, we can see a fixed solar panel on a vehicle roof, or a panel strapped to a roof rack is not working efficiently, so how do we mount them, and where do we mount them. For maximum efficiency they can be leaned up against a tree or camp chair as that can be continually moved to follow the angle of the sun throughout the day, remember they have not only be moved around to follow the sun, but their lean angle has to be continually altered as well. Moving a solar panel around every 30 minutes to ensure it points directly at the sun will increase its output considerably, thus maximising what you already have.
Solar panels come as two predominant types, these are the solid or flexible types, flexible types can be rolled up as they are on a rubber backing, and solid types are solid in that they are mounted in a solid frame. Solid types are very fragile and should never be mounted to a vehicle roof as they can’t stand the continual vibrations and vehicle flex without damaging the panel. Solid types produce considerably more power than flexible types, usually twice as much power, often more; the decision is do I permanently mount a flexible type to the vehicle and suffer the lower power output, or do I carry a solid type and lean it up at the camp site. This is an individual decision based on everyone’s individual circumstances, if you want the power without running the vehicle then a fixed type is the way to go, if you are using purely as a supplementary source of power which may be seen as a bonus, then a flexible type. Flexible types are more robust and much more suitable for off roading type vehicles.
When a solar panel is mounted it needs to be pointing towards the sun at all times, anything which casts a shadow over it such as tree branches cut its output significantly, dirty panels and even an insect walking over it will do the same. Many solar panels come with a wiring kit, often these are cheap and solar panels are sensitive to wiring as well as dirty connectors, always ensure the connectors are clean and use electrical cleaner to keep them clean at all times. Work out the maximum amperage your chosen panel will produce, treble this, and use only copper cables which are designed to work with current at this trebled amperage; keep all cabling as short as possible to reduce voltage drop, and you should have an efficient solar panel delivering its maximum output at all times.
Power Output V Batteries
Many people assume because that because a solar panel produces 12 volts that it will charge a battery, another incorrect assumption as batteries only have a nominal rating of 12 volts, they also have a maintenance voltage rating and a charging voltage rating. To maintain a battery at full charge you need to provide more voltage than the battery contains, in simple terms a 12 volt battery which is fully charged is actually reading around 12.6 volts, test this with your own battery by using a multimeter. To provide enough voltage to charge it you need the maintenance voltage of 13.8 volts being supplied to the battery to charge it, if your solar panel is 12 volts output your battery will flatten as you need at least 13.8 volts going to it, therefore your solar panel must produce at least 13.8 volts to charge a battery.
Charging voltages are anything between 13.8 and 14.6 volts, this is what your vehicle alternator will provide to keep it charged, so anything above 13.8 volts will charge your battery and anything below, but close to 14.6 volts is better.
Many solar panels produce peak voltages well above 14.6 volts and this means they need a regulator to control the voltage to keep it within the batteries charging voltage range, and to regulate the current also to prevent overcharging. Regulators are also something which need careful consideration as cheap ones only control voltage, and not that efficiently so could still overcharge a battery. Once again stick to a quality regulator as this is money well spent as they will control both the voltage and amperage and last for many years, ensure it is matched to your solar panels maximum or peak output for both voltage and amperage, and it will ensure your battery is kept at peak charge without damaging it.
Buying Solar Panels
Before you buy a solar panel ensure its suitable for your use, ignore its stated wattage rating and use the specifications to calculate its actual output.
Always ensure the solar panel is rated above 13.8 volts so it will provide enough voltage to charge your battery.
Always buy a reputable make and avoid the cheap and nasty Chinese and Indian made copies as these are total rubbish and only last weeks.
Compare its low light performance with other brands, good quality solar panels offer significantly better low light performance than cheap solar panels, often its double the low light output, in many cases its three to four times the low light output than cheap solar panels.
Mon Mar 05, 2012 7:15 am
