Solar (PV) Charge Controllers and Regulators:
*** A very Basic Primer
There are 4 main components that make up a complete solar
- charge controller(s)
The solar modules produce DC power, which is wired through a charge controller, before it goes on to the battery
bank, where it is stored and/or utilized.
The main function of a controller or regulator
is to fully charge a battery
without permitting overcharge, while preventing reverse current
flow at night. If a (non-self-regulating) solar array is connected to lead acid batteries, with no overcharge protection, battery life will be compromised. Simple controllers contain a transistor that shunts the PV charging
circuit, terminating the charge at a pre-set high voltage and, once a pre-set reconnect is reached, opens the shunt, allowing charging
The circuitry in a controller reads the voltage of the batteries to determine the state of charge. Designs and circuits vary, but most controllers read voltage to control the amount of current
flowing into the battery as the battery nears full charge.
There are many different types of charge regulators available on the market, the simplest switch on/off regulators, PWM charge regulators which charge the battery with constant voltage or constant current (they are the most often used regulators in PV systems), to the most complex MPPT
charge regulators. The first 70% to 80% of battery capacity is easily replaced, but the last 20% to 30% requires more attention and therefore more complexity.
Pulse Width Modulation (PWM)
is an efficient charging method that maintains a battery at its maximum state of charge and minimizes sulfation build-up by pulsing the battery voltage at a high frequency. When the battery voltage reaches the regulation setpoint, the PWM algorithm slowly reduces the charging current to avoid heating
and gassing of the battery, yet the charging continues to return the maximum amount of energy to the battery in the shortest time possible. The result is a higher charging efficiency, rapid recharging, and a gentle mixing of (stratified) electrolyte.
Maximum Power Point Tracking (MPPT)
is a newer charging method designed to extract the most power possible out of a solar module by altering its operating voltage to maximize the power output.
Maximum Power Point Trackers (MPPT) allow a PV charging source to operate at Vmp (maximum power voltage*). When operating at Vmp, the array produces the maximum power possible for a particular light and temperature condition. When operating at other voltages, such as a battery float voltage or the voltage of a discharging battery, array power is reduced. Using solid state dc-dc converter circuits and a power comparing algorithm, MPPT
charge controllers allow the array to operate at Vmp on the input side while maintaining the proper battery voltage on the output side.
* The Maximum Power Point is the point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage is maximum. For a typical silicon cell, this is at about 0.45 volts.
ie: Shell Solar’s SP75 is rated at 4.4 amps @ 17 volts (4.4 times 17 = 74.8 watts)
But this 75 watts does NOT equal 75 watts of charging capacity, since your battery is only charging near 13.5 V. The output of a solar module is characterized by a performance curve of voltage versus current known as its I-V curve. For crystalline modules, the current remains fairly constant as the voltage changes relative to the voltage of battery it is charging. A battery charging at 13 V is only using 57.2 watts of power not the full 75 watts - a loss of about 24%. In an extreme case, such as a fully discharged battery at 10.5 volts, you would get nearly 7 amps at 10.5 volts from the MPPT into the battery!
MPPT's are most effective under these conditions:
1. Cloudy or hazy days - when the extra power is needed the most.
2. Cold weather
- solar module output increases in cold temperatures during the winter when sun hours are low and you need the most power.
3. Low battery charge - the lower the state of charge in your battery, the more current a MPPT puts into them - another time when the extra power is needed the most.
4. With higher voltage solar arrays of 300 Wp or more.
Don’t confuse “Smart” Controllers or Regulators with Blocking & Bypass diodes or Shunt Devices:
- A semiconductor connected in series with a solar cell or cells and a storage
battery to keep the battery from discharging through the cell when there is no output, or low output, from the solar cell. It can be thought of as a one-way valve that allows electrons to flow forwards, but not backwards.
- A diode connected across one or more solar cells in a photovoltaic module such that the diode will conduct if the cell(s) become reverse biased. It protects these solar cells from thermal destruction in case of total or partial shading of individual solar cells while other cells are exposed to full light.
- A charge controller that redirects or shunts the charging current away from the battery. The charging current is controlled by a switch, connected in parallel with the photovoltaic (PV) generator
. The controller requires a large heat sink to dissipate the current from the short-circuited photovoltaic array.