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Solar Charging Guide

A solar charger is essentially a solar panel connected to a battery.
Sunlight falls on the solar panel and some of it is converted to an electric charge. The size of the charge depends the exposed surface area of the solar cell and the amount of sunlight absorbed into the cells. The solar panel is connected to a suitable battery to accumulate and store the charge.

To fully understand solar chargers you must first understand the functions and limitations of these two components

Solar panels
Solar panels are more accurately known as solar modules, they are composed of multiple solar cells, each solar cell produces a small charge but not enough to be of much use, so they are connected (strung) together to produce a total charge that is useful for general applications.
There are three important facts you should know about solar panels

  1. Shade is the enemy of solar power. There are some manufacturers who claim their solar panels are not affected by shade. That is not true, they all need bright sunlight for maximum power.
  2. Solar output decreases as heat increases. Maximum solar power is produced on cool sunny days. The rated maximum power is determined when the module temperature is at 25˚C. An increase of 20˚C will cause power output to drop by 10%. But when the ambient temperature is 25˚C the actual temperature of the solar panel surface is probably 20˚C higher so the power output will already have dropped by 10%.
  3. Maximum power is only produced when the panel is facing squarely at the sun. Most of the time it won't be optimally aligned for maximum exposure so over the length of a day it will probably only produce about 50% of the maximum possible solar power. This is taken into account by referring to meteorological data for your location, look up the equivalent Peak sun hours and optimum installation angle for solar collectors. See chart below

Batteries
A battery is simply a container that holds electrical charge. Once all the charge has been drained by connecting it to anything that uses electricity (an electrical load) it is empty (flat) and needs to be recharged or replaced.

There are six important facts you should know about batteries

  1. They can be damaged by overcharging and also by being completely discharged.
  2. The amount of power available from a storage battery is specified as amp-hours (Ah)
  3. The specified Ah capacity is available over a period of 20 hours. For example a 50Ah battery will supply 2.5 amps for 20 hours before it is completely dead.
  4. It can supply less or more than the 20 hr amp rating, according to the load, but that will affect the available capacity, thus large currents that drain the battery in less than 20 hours will not provide the specified Ah capacity and small currents that drain the battery slowly so it lasts longer than 20 hours will provide more than the specified Ah capacity. In other words connecting a 100Ah battery to a load that draws a very high current will decrease the available capacity to less than 100Ah and a load that draws a very low amperage will provide more than 100Ah capacity.
  5. Large batteries will be able to provide larger currents than small batteries.
  6. Batteries are consumables and only have a certain lifespan. This lifespan depends on how many charge cycles it is designed to accomplish before the internal plates are worn out. Each charge cycle consists of one full charge from zero capacity to full capacity, so a partial discharge, say to 50% then recharge to 100% counts as half a charge, or a discharge to 25% then full recharge counts as one quarter of a charge cycle..etc. Partial discharge before recharging adds even more to the life of a battery beyond the specified number of cycles because partial discharge incurs less damage to the plates.

Solar charging process

A small solar panel can be connected directly to a large battery when providing a small current to trickle charge it. But a solar panel large enough to fully recharge a depleted battery in one day will overcharge the battery and possibly damage the battery plates if connected to it for long periods. The solar charge needs to regulated to protect the battery. For this we need a solar regular, also known as a charge controller. Many solar regulators also allow the load to be connected through it to monitor the battery voltage and disconnect the load from the battery to prevent complete discharge and possible damage of the battery

Voltage Regulation
In order to ensure proper charging a solar regulator takes the output from the solar panel and maintains the voltage at about 14.5 volts to ensure the battery has has the right voltage for optimum charging. The regulator will monitor the battery charge level and recommence charging once the battery charge level has dropped. More sophisticated solar regulators continue to provide a small charge to maintain a full charge once the battery is fully charged .

Components for 12 volt solar charger

Components for 12 volt solar charger. The solar regulator's main role is to control the solar charge going to the battery. This controller can also disconnect the battery supply to the load if the battery voltage drops below 11.5 V. Once the solar panel has recharged the battery the regulator will reconnect the load.