Solar Regulator Information

What is a solar regulator?
A solar regulator (also known as a charge controller) is used in a stand-alone solar power generation system. It is connected between a solar panel and storage battery to regulate the charging level to the battery.
A typical 12 volt solar power setup consists of a 12 volt solar panel, solar regulator, 12 volt battery and the load or appliances being charged. The solar power is passed through the regulator to a battery bank to be stored for later use. The main purpose of regulation is to protect the battery from being overcharged by the solar panel. Some also control the power supply from the battery to the load (ie. appliances) to prevent flattening the battery (deep discharge).

Types of solar regulators
Modern solar regulators use PWM (pulse width modulation) to regulate the solar panel's current flow.
PWM is a rapidly acting electronic switch used to produce a certain overall level of current.  The PWM stays switched on when the battery demands as much current as the solar panel can produce but when the battery approaches full charge it requires less and less current. The PWM controller then rapidly switches on and off many times a second, creating pulses of current flow, the pulses switch on for shorter periods as the battery nears its fully charged state. To reduce the overall current the 'on' periods become shorter and the 'off' periods longer until the battery is fully charged and the current is stopped completely.

It is usually recommended for larger solar installations because it will deliver solar power to the storage batteries more effectively than a standard PWM solar regulator.
MPPT solar regulators also use pulse width modulation to perform the charging but have added circuitry to charge the battery faster. MPPT, which means maximum power point tracking, is special electronics to get the maximum power out of the solar panels. It does this by tracking the maximum power point (MPP) of the solar array. The MPP is the voltage level where the solar panel is producing the most power,. It can change depending on light levels and temperature. For instance, depending on conditions, a 12V solar panel MPP might be 18 volts but the solar regulator only needs about 15V to charge the battery, the extra 3 volts is not used. The MPPT tracks the relevant voltage levels then uses a DC/DC voltage converter to convert the extra volts to amps to increase the charge current.

What size solar regulator is required?
They are sized according to maximum solar panel amps they can safety accept without overheating and damaging the sensitive electronic circuitry.
For instance a 12V solar regulator rated for 10A maximum can be connected to any 12V solar panel that generates a maximum current of less than 10 amps, and its wise to include a 20% safety factor when estimating the maximum amps. A 150W solar panel generates about 8 amps maximum, so it would be the largest solar panel recommended for a 10A solar regulator.

Is a solar regulator always necessary?
A small solar panel can be connected directly to a battery if the charging current is just enough to make up loss of charge for a battery kept in storage. Larger solar panels used to bulk charge a battery should be regulated to prevent overcharging.
Generally speaking, a 5 watt solar solar panel can be connected directly without a solar regulator but larger solar panels should use a solar regulator to prevent any chance of overcharging the battery. To fully understand the reasons we need to discuss the battery charging process and charging rates versus the battery capacity.

  • The amp hour capacity of the battery being charged
    The maximum charging current (amps) is often specified by battery manufacturers as C/5 where C refers to the Amp hour (Ah) capacity of the battery, so a 10Ah battery being charged at a C/5 rate corresponds to a 2.0A charging current. An easy rule to remember is that the charging current should not be more than 5% of the rated capacity of the battery, so an 80Ah battery can be directly charged by a solar panel with a current of less than 4 amps (60 watt solar panel)
  • The type of battery
    Ask your battery supplier about the recommended charging regime. Although most Lead acid batteries are fairly robust, some batteries using different compounds and chemicals need very specific charging sequences.
    The table below shows the charging recommendations for a 12Ah VRLA battery, which is a sealed lead acid battery so the charging voltage must be set slightly lower than normal flooded types to prevent over gassing.
Charging Method
(Constant voltage)
Cycle Use Initial current Maximum 4.8A The table shows the charging specifications for a 12 volt 12 Amp-hour Panasonic VRLA battery.
It specifies a maximum charging current of 4.8 amps so the maximum size solar panel used for recharge should be about 80 watts.
The maximum trickle charge is 1.8 amps so the maximum size solar panel used for providing a trickle charge to a standby battery should be about 20 watts.
Control voltage 14.5V to 14.9V
(@25 degC)
Trickle use Initial current Maximum 1.8A
Control voltage 13.6V to 13.8V
(@25 degC)