What is a solar charge controller?

A solar charge controller is a device that acts as the interface between solar PV modules and batteries. It does the following:

  • Limits the rate at which current is added to batteries. If batteries are charged at rates higher than their recommended rates, it reduces battery life.
  • Prevents the battery from over-charging, which reduces battery life drastically. Over-charging or charging abusively can also lead to emission of gases, both in the case of FLA batteries as well as VRLA batteries.
  • Limits the rate at which current is drawn from the batteries. If batteries are discharged at rates higher than their recommended rates, it reduces battery life.
  • Prevents the battery from over-discharging, which reduces battery life drastically, especially if the batteries are not designed to handle deep discharge conditions. Overcharging or charging abusively can also lead to emission of gases, both in the case of FLA batteries as well as VRLA batteries.

Essentially, a solar charge controller does the job of increasing the life of batteries to the extent possible.

What does “rated voltage” mean in the context of a solar charge controller?

Rated voltage is the battery voltage that a solar charge controller is designed to work with. It is usually 12V, 24V, or 48V.

For a 12V solar charge controller, you can use one 12V battery, or two 6V batteries in series, or six 2V batteries in series.

For a 24V solar charge controller, you can use two 12V batteries, or four 6V batteries in series, or twelve 2V batteries in series.

For a 48V solar charge controller, you can use four 12V battery, or eight 6V batteries in series, or twenty four 2V batteries in series.

What does “full charge cut-off” mean in the context of a solar charge controller?

Full charge cut-off is the voltage to which the battery has to be charged. It is also the voltage at which battery charging stops. It varies from battery to battery. Read the battery specifications properly and set the full charge cut-off accordingly.

What does “low voltage cut-off” mean in the context of a solar charge controller?

Low voltage cut-off is the voltage at which the battery discharging stops. It varies from battery to battery. Read the battery specifications properly and set the low voltage cut-off accordingly.

What does “reconnect voltage” mean in the context of a solar charge controller?

When the low voltage cut-off is reached, battery stops discharging and starts charging again, which leads to increase in its terminal voltage. Reconnect voltage is the voltage at which the solar charge controller allows the battery to be discharged again.

What are the two types of solar charge controllers?

The two types of solar charge controllers are: PWM solar charge controller and MPPT solar charge controller.

What does PWM stand for?

PWM stands for Pulse Width Modulation.

What is a PWM solar charge controller?

A PWM solar charge controller uses pulse width modulation to slowly lower the amount of power added to the batteries as they get closer to getting fully charged. This technique allows the battery to be more fully charged, while putting minimal stress on the battery. Therefore, the battery life is extended quite a bit.

A PWM solar charge controller can also keep the battery in a fully charged state (called “float”) indefinitely.

What does MPPT stand for?

MPPT stands for Maximum Power Point Tracking.

What is an MPPT solar charge controller?

An MPPT solar charge controller is more advanced compared to a PWM solar charge controller. So it does everything that a PWM solar charge controller does. Additionally, the most important thing that it does is extract maximum power from the solar PV modules, which is why it gets its name.

mppt-chart

The figure above shows the V-I characteristics of a solar PV module. The curves are different for different solar radiation (also called “insolation”) levels.

On any given curve, the point on the X-axis represents the maximum voltage (or VOC) point, whereas the point on the Y-axis represents the maximum current (or short circuit current or ISC) point. The power at both these points is zero. The maximum power point lies in between these two points, where the inflection point of the curve is.

An MPPT solar charge controller adjusts the output voltage of the charger such that it tries to stay as close to the maximum power point as possible. And this is something that you can’t do once and forget about it; it has to be done continuously. Why? Because, in reality, the insolation levels keep changing. Therefore, the solar PV module keeps shifting from one curve to another, which is why the maximum power point also keeps shifting.

Is an MPPT solar charge controller more efficient than a PWM solar charge controller?

Yes, an MPPT solar charge controller is more efficient than a PWM solar charge controller.

mppt-efficiency

The above figure illustrates why an MPPT solar charge controller is more efficient than a PWM solar charge controller, and by how much.

A PWM solar charge controller will work at point A, whose voltage is slightly higher than the battery voltage of 12V.

VPWM = 13.2V

IPWM = 9.8A

PPWM = 13.2V x 9.8A = 129.36W

On the other hand, an MPPT solar charge controller will work at point B, which represents the maximum power point.

VMPPT = 18.4V

IMPPT = 9.3A

PMPPT = 18.4V x 9.3A = 171.12W

In the above example, the MPPT solar charge controller extracts 32% more power from the solar PV module compared to the PWM solar charge controller. In reality, the figure can be anywhere between 15% to 35% since it depends on many factors: V-I curves of the solar PV module used, insolation levels, ambient temperature, charge controller efficiency, etc.