Charge controllers: Who needs it and why?

19th Feb 18  

India at the end of 2017 had installed a total of (appx.) 17.6 GW of solar power out of which around 600 MW of power came from off-grid systems. Additionally, the government in this budget (for FY 2018-19) has allocated fair funds to Saubhagya scheme, Deendayal Upadhyaya Gram Jyoti Yojna (DUGJY) scheme, etc. which has been instrumental in rural electrification. With government’s vision to electrify all the un-electrified homes by 2020, it is expected that off-grid solar systemmay expect a meteoric rise in the coming months. While we briefly explained off-grid solar system in our previous article "Business model: How to go the solar way?” it is however necessary to explain the readers what is at the heart of off-grid plant i.e. charge controller.

As the name suggest, an off-grid system (Figure 1)only has battery and not the grid to endure load variations. Hence, such system has to adjust these variationseither from the power output available from the solar panel (which is intermittent in nature) or from the charge stored in battery. The battery howeverneeds continuous charging to maintain its State of Charge (SOC) (toensure its healthy life).All such functions needs charge controller, a smart electronic device which is capable of supplying (from solar panel to battery) and/or extracting (the battery to load) the right amount of charge for the right amount of time. While we explained the importance of charge controller, this article also aims to explain its readers what the different types of charge controllers are and where can they be used.

Figure 1:Flow of energy in off-grid solar PV power plant(Source: Google images)

Pulse Width Modulation (PWM) charge controller

The Pulse Width Modulation (PWM) charge controller as the name suggests sends pulses (of varying characteristics) to the battery for charging it. At the end of each pulse, the charge controller measures the charging characteristics to adjust the controller’s output voltage accordingly. The charge controller essentially acts as a switch between the solar panel and battery controlling both the voltage and current flowing into the battery. The nominal voltage of a typical lead acid battery is 12V. However, it may vary from 11V during discharged condition to around 14V when almost fully charged. The charge controller hence takes higher voltage (around 16.5V to 19V) from the solar moduleto charge the battery. In order to better understand charging from PWM, its charging is shown infigure 2 below.
The battery is charged in 3 different stages i.e. bulk charging, absorption charging and float charging. During bulk charging, the charge controller charges the battery at constant current (around 10A) while gradually raising the charging voltage. Here the solar panels are connected directly to the battery meaning that the voltage of the solar panel is drawn down to meet the charging voltage. The excess generated energy from solar panel (if any) is wasted (generally as heat). This stage however contributes to 75-80% of battery charging. Further charging at such high current may damage the battery (and its life) and hence an alteration in current is required. This leads to second charging stage known as absorption charging. In this stage, the battery is charged at a constant voltage (around 14.4V) while the current keeps reducing to a minimum level (to 2A). This ensures that the battery is safely charged and overheating in the battery is prevented. Once the battery reaches to almost 98~99% SOC, the charge controller switches to final stage of charging known as Float or Trickle charging. In this stage the battery are charged by trickle of current sent by the charge controller at constant voltage (around 13.9V). This ensures that the battery stays safely at 100% SOC for longer durations.

Figure 2: Input characteristics for typical battery charging by PWM controller (Data source: Waaree Energies)

The advantages of the above charging mechanism can be described as follow:

  • Increase the charge acceptance of the battery
  • Avoid desulfation and gassing of battery because of quick and continuous recharging
  • Maintaining the battery at high SOC (at around 90%)
  • Regulate for voltage drop and temperature effects of solar plant
  • Self-regulate for voltage drops and temperature effects in solar systems

While there are advantages, PWM charge controller does not fully utilize the solar power output of the panel.Additionally this charge controller are not suggestible for strings having higher power (and voltage) output as all such power would be wasted in form of heat. This limits its use for a bigger systems.

Maximum Power Point Tracking (MPPT) charge controller

The Maximum Power Point Tracking (MPPT) charge controller utilizes the maximum available power from solar panel and transfers it to the battery. The battery as we had mentioned earlier, is at different voltage level while fully discharged (11V) and fully charged (14V). Thus in order to efficiently charge the batteries, it is important to regulate the charging voltage and the charging current. Additionally the output from solar panel is continuously varying throughout the day. The controller optimizes both these conditions by continuously sensing the output from panel & the voltage of battery. It then feeds the correct amount of voltage to the battery by extracting the maximum amount of power required from solar panel, thanks to its DC to DC converter. The DC-DC converter (as the name suggests) converts the DC power output from solar panel to AC power which is converted to the required DC power output for battery.A representative example of working of MPPT charge controller is shown in Figure 3 below.
The IV curve of solar module (shown in purple) and P-I curve (shown in red) are shown below. As evident, the solar module is operated such that it produced maximum power output (i.e. 100W). Further the controller senses the exact voltage requirement of the battery (which is around 12.7V here) given the poweroutput from solar panel. The controller uses its DC to DC converter and delivers the power to battery at the determined voltage and current. Such charge controller delivers higher than rated charge capacity to the battery for efficient charging. Thus in this system all the power from the solar panel can be put to use.

Figure 3: Concept of MPPT tracking in solar module (Source: Google Images)

The main advantages of using this charge controller are as follow:

  • It can be used for practical applications where power requirement is high and there is voltage mismatch between panel and battery
  • It forces the module to work on maximum power point at low/high temperatures and low irradiance ensuring the battery is charged even in the worst conditions
  • When the battery is at lower SOC (say at 11.2V) it requires higher current to charge swiftly. The MPPT controller (say extracting power of 100W from panel) would exactly do that (charging current fed to battery = 100W/11.2V = 8.90A) thus making it a perfect solution
  • Most M PPT's are around 93-97% efficient making it an ideal choice

Thus after developing understanding of both PWM and MPPT charge controller one can conclude that PWM charge controller can be used when there is a perfect match of solar panel output (of small size) characteristics and battery charging characteristics. The MPPT charge controller can be preferably used when we use multiple modules and/or when there is mismatch between the electrical characteristics of module and battery.However to ensure optimum charging efficiency and maximum power utilization, it is suggested to use MPPT charge controller.
Waaree Energies have expertise in both on-grid and off-grid solar PV projects. We are capable of offering complete solutions to all of our customer requirements. Such installations boost our morale and inspire us to deliver the best to our customers.

Let us all pledge to make solar energy the primary source of energy in the near future.