Choosing the Right Batteries for Your Solar System

Alternate energy is the future of energy. Certainly, solar energy is the most important source of alternate energy. It is free and green. Use it directly and there is no charge. Convert it to electricity and use the electricity. Even then there is no charge except that there is a conversion cost. But a major snag is, availability is not guaranteed at all times. As a result, you must store it somehow for use when you need it. Battery is the currently the best form of energy storage. That makes the battery solar systems very important. Since the battery is such a critical part of the energy system, it must be chosen carefully. We discuss some important parameters to keep in mind while choosing the right batteries for your system.

Energy Power and Capacity

The unit of energy is Joule (J). For practical reasons we rarely use this unit in working systems. Power is the rate at which energy is consumed. The standard unit is the Watt (W). By definition, a steady rate of one Joule in one second means the power is flowing at the rate of one watt. In dealing with practical systems, Kilowatt (KW) is a more convenient unit because even the smallest household will draw a few KW. In electrical systems, power is the product of voltage and current. If a battery system delivers 100 A at 24 V, it is delivering 2400 Watt (W), or 2.4Kilowatt (KW). Capacity of a battery system is the total energy it can hold for use later. It is the product of power used with the time duration of use. A 1 KW appliance running for one second will consume 1000 Joule of energy. Since energy is the product of time and power, it is much more meaningful to use the units of Watt-hour or kilowatt-hour. A 1KW device running for one hour will consume 1KW-hour(KWh). Batteries manufacturers normally specify the ampere hour (AH) rating of their battery. Multiply the AH with the battery voltage and you get the power rating of the battery in Watt-hour.

Determining the Capacity

The total capacity of your battery system will be determined by the total energy it may be required to store and supply. Plan carefully, how much power the system will be required to supply for how long. Then put a margin of 50% on it to allow for capacity degradation and unforeseen situations. We can connect Smaller batteries  in parallel to achieve higher currents and higher capacity.

Battery Life-Charge-Discharge Cycles

Battery life is specified in terms of number of discharge cycles. This guaranty is available when used under certain restrictions specified by the manufacturer. Charging rate allowed is a function of temperature with a maximum and lower temperature limits. Similarly, there are restrictions on discharge temperature. Higher, charge rates will reduce the capacity and life. The dependence is different for different technologies. Study the manufacturer’s guidelines properly. An important parameter is the allowed depth of discharge (DoD).

Depth of discharge (DoD)

This is one reason for keeping a large margin in the capacity of your battery system. Practical batteries do not like being completely discharged. They get damaged. If the batteries in your system are specified for 80% depth of discharge (DoD), this means you can drain 80% of their total charge and expect them to give you the specified number of charge-discharge cycles. Draining them deeper than this will affect their life. Different battery technologies allow different levels of DoD. Thus, DoD also matters to your calculation of system capacity.

Round Trip Efficiency

Like everything else in this world, batteries also do not return to you the total of what you feed into them. Round-trip efficiency is the energy recovery efficiency of the battery. If your system can safely return 4.5 Kwh safely after being charged with 5 KWh, then the round-trip efficiency is 4.5/5=0.9 or 90%.

Manufacturer

Experience and reputation matters. A longtime manufacturer is likely to provide a more reliable service and be better at meeting the warranty conditions.

Cost of Operation

This is about the ultimate test for choosing the right batteries. For every battery type being considered, calculate the cost of one KWh over the specified life of the battery if cycled once per day keeping in mind the DoD and round-trip efficiency. The one with the lowest cost of one KWh is your choice. If there is a tie, compare the manufacturer’s reputation.