Charge controllers, also known as charge regulators are an integral part of the solar power systems and are often termed as battery life enhancers. These devices limit the amount of current drawn and fed to the batteries bank, which stores DC energy from the solar panels. Life span of a battery is curtailed if overcharged and same is the case when current is drawn in an under charge condition. The charge controller circuit is intelligent enough to tackle both these situations by allowing or suppressing amount of charge to the batteries. In case of solar systems, Most 12 volts panels put out about 16 to 20 volts, so if there is no regulation the batteries will be damaged because of overcharging. Most batteries need around 14 to 14.5 volts to get fully charged.
Under what circumstances are solar charge controllers required? A general rule of thumb is that if the panel outputs 2 to 3 watts or less for each 50 ampere-hours of battery, a charge controller is not required. For instance, a standard car battery is around 210 amp-hours. In order to keep up a series pair of them (12 volts) probably for storage purpose, a panel is required that is approximately 5 watts. A generally available 5 watt panel meets the requirement and doesn’t need a charge controller.
The next logical question that comes up is that why panels aren’t exactly 12 volts, the reason is that the panels will yield output power only under perfect environmental conditions and full sun which is impossible in real scenario and these panels have to supply some extra voltage so that when the sun is low in the sky, in heavy haze, cloudy weather or elevated temperatures, you will still have some output from the panel. A fully charged 12 volts battery is around 12.7 volts under steady state (which is approximately 13.6 to 14.4 under charged condition), so the panel has to put out at least that much under worst case situations. Hence the primary duty of charge controller turns out to regulate panel’s 12-16 volts to the battery required level (which may vary from 10 volts to 14.5 volts depending upon battery charge conditions).
There are three majorly classified types of solar charge controllers. First kind comprises of those maintaining the voltage and current in one or two stages and rely primarily on relays and shunt transistors employed for switching purposes. The methodology used is simple in principle as these relays and transistors cut off panel from battery in case of over charge condition and disconnect load in under charged situation. These controllers have become obsolete but are still found on a few old systems. The only advantage is the simple design and low cost which is overshadowed by their low performance.
The second type is known as PWM (pulse width modulation) or three stage controllers and are pretty much the industry standard now, but the older shunt/relay types are still found in some low-tech industrial or domestic applications. These controllers adjust the current rate which depends mainly on the battery’s current charge level to allow input charge in proximity of the battery’s maximum capacity as well as monitor battery temperature to prevent it from overheating. Another startling feature of PWM controller is that it harvests constant output efficiency isolated from the effect of solar panel array size.
The third kind is commonly known as MPPT (maximum power point tracking) controller. Each PV array has an ideal voltage at which it can output the maximum power, and this voltage varies by the intensity of sunlight and temperature level. As mentioned above, the battery voltage varies depending upon state of charge and load factor, for a 12V battery voltage may oscillate from 11 to 14 volts approximately. In order to get battery charged, the voltage of PV array must be higher than that of the battery. MPPT manages to keep this balance between two voltages and by intelligently finding the point at which panel can provide its maximum output. This type of controller is found in abundance these days in both stand alone and grid tied solar panels.
Most controllers come with some kind of indicator, either LED arrays or digital meters. Many newer ones such as MPPT and PWM controllers now have dedicated computer interfaces to monitor and adjust the voltage and current intelligently. The simplest usually have only a couple of small LED lamps which indicate presence of output power and input charge just like those available in laptops, a side blinking led lamp notifies of input charging. Alternatively those control panels having display meters have the reading of both voltage and the current coming from the panels and the battery which provides visual indication of current state of input and output for manual switching.
When it comes to chose between PWM or MPPT, the later one has a leverage of higher efficiency over the former one because it has proven to output 30% more power as compared to PWM controller. The MPPT controllers are compatible with multiple panels connected in series for greater voltage levels, keeping the current flow to minimal thus reducing the size of wire used for transmission, especially for long distances. Another perspective of choosing a charge controller is the amount of load to be served, wattage of the solar array and the maximum rating of the power inverter used for DC-AC conversion. The solar charge controller technology is changing its shape day by day and it won’t be incorrect to say that the future charge controllers will be monitoring and controlling the circuit parameters at array level, thus producing output far better than we have now.