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    BIG AL DRYBURGHParticipant

    Hi All,

    Can someone please advise me on the solar power requirements and equipment required to run my Exclusive.Fridge,lights TV

    Has anyone got any bright ideas on how to safely contain a small pet dog on your site.

    Thank you kindly.

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    Fyko on

    Since the solar panels will be recharging your caravan battery all of the loads you mention, fridge, lights, tv will need to be driven from that battery, and the daylight charge capacity of the solar panel must equal or exceed the day and nighttime load on the battery.

    Solar panel charge capacity is generally given in Watts because this gives a falsely optimistic impression of the actual useful capacity of the panel array.

    How does that happen? The best-case power capacity of  panel is calculated from the maximum output voltage (usually about 19V) multiplied by the maximum current produced at that voltage. For example an average panel will drive a 4.5A current at 19,5V. Multiply that out and you get 87,75W. Very impressive, but totally false.

    Why? because when that panel is connected to you battery (through a regulator please) that current won’t increase above 4,5A just because the opposing voltage in the battery is only 12,6 to 13,8V depending on the state of charge of the battery. Read the short-circuit current specified on the sticker and you’ll see some thing like 4,7A. So you will never get more than 4,7A charging your battery, think 4,6A at absolute max, and when you multiply that out you get 4,6A x 13,8V = 63,5W. Not so impressive anymore.

    Where your calculation should in reality start is with how much power are you going to suck out of your battery in a 24 hour period so you can figure out how to put that back during the following daylight period.

    First the fridge. Many caravans are equipped with absorption fridges, the kind with a heater and no compressor. Absorption fridges work by heating a water solution of ammonia which drives off the gas which is cooled and later re-absorbed into the water. No moving parts but quite energy inefficient. If you do the arithmetic you find that the energy contained in LP gas (46MJ per kg @ R20/kg) work out much the same as municipal electricity (1 unit = 3,6MJ @ R1,80 per unit = R23 for 46MJ), and a lot less than solar power (over low-frequency use), so you’re better off running your absorption fridge on gas. If you have a compressor fridge disregard the above.

    Most tv sets require mains power. Dstv decoders can be driven from 12V or mains power. Lights are always driven from 12V, or should be. Mains power must be supplied by an inverter which is powered from 12V.

    To know how all of these loads add up over a 24 hour period is not difficult, but must be known in Amperes and not in Watts, since the Amperes x hours going out must be at least matched by Amperes x hours going back in. Of course the ‘back in’ time is shorter than the ‘going out’ time.

    The lights are the easiest – add all the wattages together and divide the total by 12. If you run an average of 20W of lights for 4 hours every night that works out to 1.6A x 4 = 6,6 Ampere/hours. Round that up to 8 for safety.

    The fridge is running on gas and we won’t count the lamp inside the fridge if that is 12V, and it won’t be working if it’s not. If you have a compressor fridge you need to multiply its current requirement (usually about 3A) by its duty cycle (think 50%), so an average of 1,5A x 24hours or 36Ampere/hours

    The tv and decoder take about 25W each – 50W total. They are going to be driven from an inverter which is about 85% efficient, so the total draw is 59W, call it 60. Divide that by 12 and you get 5A. Running them for 5 hours will eat 25Amp/hours.

    So now (excluding the compressor fridge) you have to replace 33Ampere/hours during the daylight hours of what? 6 hours at max charging current? To generate 33A/hours in just 6 hours requires 33/4,6 = 7,1 hours, so that single panel will not quite cut the mustard. However if you accurately track the sun to get maximum efficiency you might just make it.

    If you have a compressor fridge then you’re getting ready to write a cheque for a second panel just about now.

    You have here everything you need to know to make you own calculations. Always add a little to be on the generous side since there’s parasitic losses all over the place.

    And you see how useless the Wattage specification of the panel is, it’s all about the current.



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    Simon Tasman
    Simon Tasman on

    You say I should use a regulator. What kind? And what for?

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    Fyko on

    There’s been a few other questions sent privately so I’m going to back up a few steps before I address yours.

    There’s lots of convenient analogies between electric current and water flow. A water pipe does the same as a length of wire, water behaves similarly to electricity, water pressure can represent Voltage, and a water tank can look like a battery.

    If you have a water tank that waters your garden catches runoff from the roof  when it rains,  or a city has a storage dam that supplies water year round but gets filled only in the rainy season, you have a similar situation as a solar panel array charging a battery.

    Water is measured in litres. The unit of measure for the flow rate of water is a cusec, which count the cubic metres  (1000 Litres to a cubic metre) per second. The electrical equivalent to a cusec is an Ampere, where one coulomb, which is a large number of electrons, flows past a point in one second. So 1L/second is a thousandth of a cusec, and 1 coulomb (look it up if you want to see how many electrons it contains) flowing past a point is 1 Ampere.

    So waterflows out of the tank or the dam continuously but only gets replenished when it rains. The battery supplies electric current continuously but gets recharged when the sun shines on the solar panel.

    If the battery has a capacity of 100Ah then it will store 360,000 coulombs when full. (3600 seconds in an hour x 100 coulombs per second). Not all of those electrons are available on discharge. Think of it as a tank who’s outlet is not quite at the bottom – some water will always remain unavailable. Also discharging a battery completely is not advisable because it causes some loss of capacity.

    Of course the tank stores the physical water, whereas the battery actually stores the energy that makes the current flow. When you charge a battery the same number of electrons flow out of the negative pole as flow into the positive pole. The battery stores the energy required to make the same amount of current flow in the opposite direction to the charging current.

    If you overfill the water tank or the dam they overflow without hurting the tank or the dam, but if you keep pushing a charging current into a battery when there is no place to store the excess energy that this current brings with it. This energy must be dissipated in a way that doesn’t harm the battery – and that’s not easy.

    In the past the battery would just ‘gas off’ the excess energy by splitting the water in the electrolyte into its hydrogen and oxygen components. This led to a loss of electrolyte volume as well a interfered with the concentration of the sulfuric acid electrolyte (already at max in a charged battery).

    Nowadays they put in other stuff like silver and cadmium which are intended to recombine the hydrogen and oxygen peacefully, but the energy must still be dissipated, which means the battery is going to get hot. Better to prevent the overcharge before it happens.

    To get back to the water analogy, I think of those toilet cisterns of yesteryear where there was a float on the end of a transverse rod. When the tank was empty the ball would drop and open the entry valve to allow the water to fill the tank. As the water approached the maximum level the float would progressively close off the entry valve and prevent the tank overflowing. It mostly worked well but it took the last litre a long time to fill because the valve was near closed and made the water flow very slow.

    The modern cistern valve allows a full flow for virtually the entire filling phase until water overflows into a little cup right at the top of the tank, which causes a float to jump up inside the cup and direct the incoming water pressure to actuate a valve which slams to water supply closed abruptly.

    Battery chargers and charge regulators are similar to this. You get the simple float chargers that progressively pinch off the charging current as the battery cells approach 2,3V or 13,8V cumulatively. The battery is protected from overcharging but the last charging phase goes slower and slower. And charging time is precious when the sun is about to go down.

    Intelligent chargers and regulators work the way of the second cistern valve. They apply a higher voltage than the traditional maximum of 13,8V (as high a 16V)  for such a time as they estimate the battery is not yet full, and then they cut this back abruptly to a float charge of 13,8V when they consider the time to be right.

    Intelligent chargers and regulators also have a de-sulphating capability whereby they batter the perceived lead sulphate coating the plates and reducing charge capacity with rapid short pulses (200KHz) at a high voltage. This is supposed to  soften the lead sulphate layer and make it amenable to be re-integrated into the battery’s chemistry.

    So – the short answer. You definitely need a regulator to protect your battery. And the more you spend the more you’ll get. A good regulator from the solar panel supplier will cost only a little less than the battery it is intended to protect, but you need it.

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