12v Solar setup part 1: components

As we’re planning to install solar panels for Freyja, we thought it would be useful to share with you what we learn about installing these setups for a campervan. The first post in this three-part series will look at the components needed to put together a solar setup.
A 12v solar setup that is usually used in a campervan has three or four basic components: a solar panel, a battery, a charge controller, and possibly an inverter.

  • The solar panel supplies the energy.
  • The 12v battery stores it.
  • The charge controller is like a switch that turns on to charge the battery when needed, and off to prevent overcharging when the battery is “full”.
  • The inverter changes direct current (DC) to alternating current (AC). You may need an inverter if you intend to use 240V AC loads (mains appliances) rather than 12V DC loads.

The options can become confusing if you don’t know what the different types, names, or abbreviations are. So this post tackles describing some of the differences in component options.


Solar Panels

What are the different kinds of solar panels?
There are various types of solar panel, from an increasing range of manufacturers. But, what’s most worth knowing is that the “panel efficiency” quoted by manufacturers has very little bearing on the energy generation, it just affects how much roof space is needed for the same powered system. A 3kW system will generate a very similar amount of electricity whether it is 3kW of poly, mono, thin film or hybrid panels.

Having said that, let’s take a look at the most common types of panel available:

Monocrystalline Panels: The solar cells in monocrystalline panels are slices cut from pure crystalline silicon bars. The entire cell is aligned in one direction, which means that when the sun is shining brightly on them at the correct angle, they are extremely efficient. They have a uniform blacker color because they are absorbing most of the light.
Polycrystalline Panels: Also known as muticrystalline, these are made up from silicon offcuts molded into blocks. Because the individual crystals are not necessarily perfectly aligned and there are losses at the joints between them, they are not quite as efficient in direct sunlight. The appearance is also different – you can see the random crystal arrangement and the panels look a little bluer as they reflect some of the light.
NOTE: Because of the UK’s duller conditions it is  arguable that polycrystalline can outperform monocrystalline because the crystal misalignment means cells can work better with light from all angles, or in low light, etc. but the difference is marginal in both performance and space used.
Thin-film solar cells: Manufactured by depositing one or several thin layers of photovoltaic material onto a substrate. The advantages are flexibility and cost, but they take up more space and don’t last as long as crystalline.
Hybrid Panels: Currently comprised of a thin layer of solar film behind the monocrystalline cells. The extra layer extracts even more energy from the available sunlight, particularly in low light conditions. These are the most efficient panels available, so they take up the least space. However, unless space is a crucial factor, choose crystalline, or you will pay a lot more to produce the same power.

What is a blocking diode?
This is a component connected within the cable that prevents the solar panel discharging the battery when there is no sunlight. This is normally used in smaller panels that are used for trickle charging or charging things like mobile phones directly, they are not necessary when panels are connected to a battery through a controller as the controller will provide this function.

Why do 12 Volt Panels actually put out 16-18 Volts?
Although many panels may have a nominal output of 12 volts, in reality they are capable of significantly higher voltages (i.e. 16-18V). This higher voltage is referred to as “maximum power point”. The reason for this is that if they simply output 12V, the panels would only provide power in optimal conditions (i.e. in ideal temperatures and direct sun) which is not something you can count on in most places. Therefore, the panels need to provide extra voltage so that when the sun is low in the sky, you have heavy haze, cloud cover, or high temperatures, you still get some output from the panel. A fully charged “12 volt” battery is around 12.7 volts at rest (around 13.6 to 14.4 under charge), so the panel has to put out at least that much in worst case conditions.


Batteries

Why should I use a battery rather than run things directly from a solar panel?
The obvious answer is that you can’t run appliances if the sun isn’t out, but there is another reason to use a battery. The solar panel will produce a higher or lower current depending on how bright the sun is shining. By storing that energy in a battery, the battery acts as a current regulator so that the current you draw is stable.

What is a leisure battery?
There are two main types of lead acid batteries:
Starter (SLI) batteries: Used for starting engines. A vehicle battery has to provide a high current surge to start an engine, but once that has been achieved, the vehicle’s alternator immediately starts to replenish the power it provided. If starter batteries are repeatedly deeply discharged this will irreversibly damage the battery.
Axillary Power (Leisure/Deep cycling) batteries : In contrast to a starter battery, a leisure battery has to provide a steady flow of current over a prolonged period, often to the point when it’s virtually ‘flat’, and then has to be recharged. A ‘cycle’ in terms of leisure batteries is a discharge/recharge cycle. A deep cycle leisure battery is designed to perform a high number of discharge/recharge cycles without loss of battery health. To cope with a life of deep cycling, leisure batteries are made differently to starter batteries.

What types of leisure battery are there?
Conventional (flooded) wet lead acid batteries: must always be kept upright so that the electrolyte doesn’t spill, in a vented area so that the gas produced can escape, and insulated against extreme heat or cold. A lot of leisure batteries are wet lead acid as they are cheaper.
Valve-regulated lead-acid (VRLA/Sealed/maintenance free) batteries: are a newer type of lead-acid battery. There are two primary types of VRLA batteries:
Gel batteries: Have a jellified electrolyte, so can be stored in any orientation and are safe to handle. They are also extremely low gassing so can be safely installed inside the cabin of a motorhome. The downsides are that they can be expensive and you have to be very careful not to overcharge them.
AGM batteries: Instead of using gel, AGM batteries use a fiberglass type material to hold the electrolyte in place, making them spill-proof and extremely vibration resistant. Again, they are expensive and you must be careful not to overcharge them.

What is ‘state of charge’?
State of charge refers to how much charge is in your battery, you can think of it like a fuel gauge. When measuring your battery remember to turn off any loads connected. This table shows you what state of charge your battery is in at different voltages:

What about ‘capacity’?
Capacity is the measure of how much charge can be stored by the battery. A battery’s capacity is measured in Ampere-hours (Ah). 1Ah is the amount of charge in a battery that will allow one ampere of current to flow for one hour. Put simply, the higher the Ah the longer it will last between charges.
One characteristic associated with lead acid batteries is that the capacity will decrease as the rate of discharge increases. On the battery case you should find capacity figures. If you find just one Ah figure such as 100ah, it is normally rated for discharge over 20 hours. Multiple figures such as “K5 75Ah – K20 100Ah – K100 110Ah” show the Ah at different discharge rates, (so in this case if discharged over 5 hours you get 75Ah, but if discharged over 100 hours you get 110Ah). Hence, it is important when comparing battery capacities to compare the same discharge times.

So which battery should I choose?
Your battery should be chosen depending on your usage of it, and it’s environment, not just the capacity. Here is a quick list of things to remember to take into consideration:

  • The Ah size of the battery you need will depend on what you’re running from it (see sizing in the next post).
  • As well as how much power your appliances use, when they use it is another consideration:
    If you use all your energy during the day while your battery is charging (ie your battery is never really deep cycling), you probably don’t need the best/priciest battery, as your battery is acting more like a current regulator than a storage device.
    If you use run all your appliances at once, then you need to pay extra attention to discharge rates.
  • The higher the Ah figure, the physically larger the battery, so make sure the physical dimensions will fit where you want to house your battery.
  • The ventilation of the area you will house your battery needs to be considered to avoid igniting gasses that batteries give off (most important with conventional flooded lead acid batteries).
  • As a rule of thumb, heavier batteries will last longer. Heavier = more lead.

How should I look after my leisure battery?
The more care you take with your battery the longer its life will be. 

  1. Regularly inspect the battery for cleanliness, for example clean any white deposits from the terminals with warm water, coat the terminals with petroleum jelly, and ensure connectors aren’t rusting.
  2. Check the electrolyte levels of conventional batteries monthly, and top up the liquid with distilled water if needed, the level should be just above the plates. This won’t be necessary for maintenance free battery types such as gel and AGM.
  3. Keep an eye on the state of charge. Batteries don’t do well if left to discharge for long periods of time (plates corrode), or if overcharged (gassing occurs). Ensure your charge controller is right for the job and if your solar panel won’t be seeing light for a long period of time (for example if storing your camper in a garage over winter), use a mains trickle charger to maintain your battery.

Charge Controllers

There are two types of charge controllers (also called regulators) commonly used in today’s solar setups. Both adjust charging rates depending on the panels output and the battery’s charge level, as well as monitor battery temperature to prevent overheating.

PWM controllers: With “pulse width modulation” the current from the solar panel is reduced by the charge controller as the battery reaches its fully charged state. This stops the battery becoming overcharged which reduces gassing of electrolyte so lengthens the battery life. When a battery is fully charged a PWM solar charge controller will maintain the battery at its fully charged state by reducing the charge voltage, monitoring any slight changes to the battery voltage and changing the charging voltage accordingly.

MTTP controllers: A “maximum power point tracker” is suitable for a solar power system that needs to extract maximal power from solar panel; it forces the solar panel to operate at a voltage close to it’s “maximum power point” in order to draw the most available power. MPPT allows users to use solar panels with a higher voltage output than the operating voltage of battery system. The benefits of MPPT solar charge controllers are greatest during cold weather, on cloudy or hazy days or when the battery is deeply discharged.

If maximizing charging capacity were the only factor considered when specifying a solar controller, everyone would use a MPPT controller. But the two technologies are different, each with its own advantages:

PWM Controller MPPT Controller
Array Voltage Solar panel & battery voltages should match Solar panel voltage can be higher than battery voltage
Battery State Operates at battery voltage so it performs well in warm temperatures and when the battery is almost full Operates above battery voltage so it is can provide “boost” in cold temperatures and when the battery is low.
System Size Recommended for use in smaller systems where MPPT benefits are minimal 150-200W or higher to take advantage of MPPT benefits
Cost Cheaper More expensive

Choose your controller according to your specific needs.


Inverters

What is an Inverter?
A 12V inverter takes the 12V DC (direct current) from a battery and produces a 240vAC (alternating current) for appliance use. Inverters have 2 ratings: a maximum constant rating and a peak rating, so a 500-1000watt inverter can constantly run at a maximum of 500 watt and has a maximum peak of 1000 watts. The peak rating is usually used when an appliance starts.

What is AC/DC?
Both AC and DC describe types of current flow in a circuit. “Current” is the movement of electrons through a conductor (i.e. a wire). The main difference between AC and DC lies in the direction in which the electrons flow. Here are some comparisons between AC and DC:

AC DC
Distance energy can be carried Safe to transfer over longer city distances and can provide more power. Voltage of DC cannot travel very far until it begins to lose energy.
Cause of direction of flow Rotating magnet along the wire. Steady magnetism along the wire.
Flow of Electrons Electrons keep switching directions – forward and backward. Electrons move steadily in one direction or ‘forward’.
Frequency The frequency of alternating current is 50Hz or 60Hz depending upon the country. The frequency of direct current is zero.
Obtained from A.C Generator and mains. Cell or Battery.
Types Sinusoidal, Trapezoidal, Triangular, Square. Pure and pulsating.

If you need a more in-depth explanation, check out this page.

What types of inverter are there?
There are two main types of inverter used in solar setups:

Modified or Quasi Sine Wave inverters: produce a modified AC supply in which the voltage rises and falls abruptly, the phase angle changes sharply and sits at 0Volts for some time before changing its polarity.
These inverters are more affordable and are sufficient for some applications, but will not run devices that use certain types of control circuitry. For example: when using these with devices that have transformers in their power supplies they will run hotter and be less efficient, interference may be transmitted through audio/visual systems, and digital clocks will not keep accurate time.

Pure or True Sine Wave Inverters: produce a smoothly varying mains quality AC supply. This will run any appliance without interference and are therefore are most suited for use with sensitive electronic equipment. In short a Pure Sine Wave Inverter will provide a reliable power source for all electronic appliances & devices.


We hope this information has helped you to understand the components you need for your solar setup. In the next post we’ll look at how to decide what sizes you need.

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