In order to charge a battery (increase its voltage), the PV module must apply a voltage that is higher than that of the battery. If the PV module's Vpp is just slightly below the battery voltage, then the current drops nearly to zero (like an engine turning slower than the wheels). So, to play it safe, typical PV modules are made with a Vpp of around 17V when measured at a cell temperature of 25°C. They do that because it will drop to around 15V on a very hot day. However, on a very cold day, it can rise to 18V!
The function of a MPPT is analogous to the transmission in a car. When the transmission is in the wrong gear, the wheels do not receive maximum power. That's because the engine is running either slower or faster than its ideal speed range. The purpose of the transmission is to couple the engine to the wheels, in a way that lets the engine run in a favorable speed range in spite of varying acceleration and terrain.
Let's compare a PV module to a car engine. Its voltage is analogous to engine speed. Its ideal voltage is that at which it can put out maximum power. This is called its maximum power point. (It's also called peak power voltage, abbreviated Vpp). Vpp varies with sunlight intensity and with solar cell temperature. The voltage of the battery is analogous to the speed of the car's wheels. It varies with battery state of charge, and with the loads on the system (any appliances and lights that may be on). For a 12V system, it varies from about 11 to 14.5V.
In order to charge a battery (increase its voltage), the PV module must apply a voltage that is higher than that of the battery. If the PV module's Vpp is just slightly below the battery voltage, then the current drops nearly to zero (like an engine turning slower than the wheels). So, to play it safe, typical PV modules are made with a Vpp of around 17V when measured at a cell temperature of 25°C. They do that because it will drop to around 15V on a very hot day. However, on a very cold day, it can rise to 18V!
Choosing the Right MPPT Solar Charge Controller
in an Off-Grid or Hybrid (Grid-tied, with battery back-up) PV system, the Inverter is generally the heart of the system. Inverter size is determined by the maximum loads that the site will experience and how long those loads will last.
Inverter and Charge Controller - a delicate balance
In an Off-Grid or Hybrid (Grid-tied, with battery back-up) PV system, the Inverter is generally the heart of the system. Inverter size is determined by the maximum loads that the site will experience and how long those loads will last. The most basic inverter function is to efficiently drain the battery bank and supply clean AC power until the batteries are exhausted (or a preset point is reached where the inverter shuts off to avoid ruining the batteries). Most inverters can also charge the batteries in a Hybrid system by using power from a generator or the utility grid. The on-board battery charger of the inverter usually does not use power from a solar array directly (unless it’s an SMA Sunny Island) - that is the job of the DC to DC Solar Charge Controller and the subject of this article.
The solar charge controller has the important function of getting PV energy into the batteries faster than the inverter can drain them - and in a well designed system, the charge controller takes advantage of any available energy that the solar array can provide.
There are three basic decisions in choosing the right charge controller for your project:
Determine Amperage and Power
Once the demand load schedule is calculated, the battery is sized and the solar array is sized to service the battery, then the capacity in amperage of the charge controller can be determined.
Amperage options for MPPT charge controllers range from 20Adc to 80Adc and they can be run in parallel for systems requiring more than 80A. Another article focuses on sizing the charge controller. (How to size a charge controller).
Determine System (battery) Voltage
System voltage, in this article, is different from array voltage. The system voltage is really all about the battery and inverter interaction. System voltage options will be in the 12 - 60Vdc range. The default system voltage is 48V for modern MPPT charge controllers used in a residential application, where an inverter will be powering appliances with 120/240 Vac. Other voltages, like 12Vdc or 24Vdc, might be used in mobile or marine applications where 12 or 24Vdc appliance are being run directly from batteries.
Determine Array Voltage
Array Voltage is open circuit voltage of the PV array at the lowest recorded temperature. Most array voltages are limited to 150Vdc - the maximum dielectric protection of the charge controller components and the the maximum difference between the array and the battery: A 150Vdc to 12Vdc conversion.