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 - 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 10A to 80A and they can be run in parallel for systems requiring more than 80A. Another article focuses on sizing the 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.
For any complete energy-harvesting system designed to provide power to anything but small, short-duration loads, storage batteries represent a necessary but significant portion of the initial expense. The cost of batteries over the lifetime of the system can have an even larger impact if care is not taken to maximize the useful life of the battery component. What’s more, if unit growth continues for photovoltaic and other energy-harvesting systems relying on large-capacity storage batteries, designs that fail to maximize battery life could have a negative environmental impact due to the extra material and energy consumption needed to manufacture replacement systems as well as dispose of exhausted units.
Storage battery specifications such as robustness and projected lifetime depend, to a great extent, upon the chemistry of the cells; Li-ion, LiFePO4 and sealed lead acid are widely used battery chemistries. Most commercial Li-ion cells can now be charged to 4.2 V/cell. A LiFePO4 battery allows a much higher charge and discharge rate, but the energy density is lower. The typical cell voltage is 3.6 V. The charge profile of both Li-ion and LiFePO4 is preconditioning, constant current and constant voltage. For maximum cycle life, the end-of-charge voltage threshold could be lowered to 4.1 V/cell. Battery chemistry notwithstanding, there are several design criteria engineers should know that affect the life expectancy of any type of storage cell.
One of the most common battery killers is overcharging. Battery life is reduced when charging is continued for an excessive length of time or at too high a voltage. Simple charging circuits either exceed optimum charge voltages or make a trade-off for excessive charging cycle time. The particular chemistry determines the voltage level to use, and manufacturers provide detailed specs for charging and discharging voltages and rates.
A basic system with a photovoltaic panel for energy input provides either a blocking diode or switch in series with the battery to prevent draining the battery during periods of low light. To avoid overcharging, excess current can be shunted from the solar cell once the battery is fully charged. In practice, even this simplified design requires monitoring and control electronics.
Although those of us living in frigid climates tend to associate battery death with the coldest part of the year when your car won’t start in a blizzard, most battery damage actually can occur in hot weather. Temperature plays an important role in ensuring long storage battery life. All batteries depend on chemical reactions that are thermally activated. With strong temperature dependence, these reactions can get out of control, leading to sudden and serious damage. At low temperatures, battery electrolytes can also freeze when the charge is depleted. This is especially important in photovoltaic energy harvesting since the lowest temperatures are often coincident with the lowest solar energy levels (because the energy source is lost at night) compounding the detrimental temperature effect.