Batteries store and produce energy as needed. In PV systems, they capture surplus energy generated by your PV system to allow you to store energy for use later in the day. Like technologies such as fuel cells, a battery converts chemical energy to electrical energy. Rechargeable batteries also convert electrical energy into chemical energy. Depending upon your particular PV system, batteries may help you to use more of the energy collected by your PV system. Batteries can provide power when electrical loads require more power than the PV panels are generating. This can be due to the generation of less electricity due to adverse weather conditions, greater than normal power usage, or other anomalies with the PV power collection. Batteries also help establish the DC operating voltage for the required auxiliary components in the PV system. However, they can be expensive, and each system needs to be designed extremely well so that they are worth the added expense.
During the day:
• The PV system generates solar energy
• The system will check to see if all of the energy generation can be used to power your household
• Any surplus energy will be used to charge the battery
• If the battery is already charged, excess energy will be exported to the grid
At night or when there is low power generation:
• The PV system generates little to no solar energy
• Energy is obtained from the battery system
• After the battery is discharged, electricity can be obtained from the grid
Solar PV systems that do not have a method of energy storage will transport surplus energy to the local energy grid, and when the PV panels are not generating enough energy for your needs, electricity needs to be supplied by the grid.
PV systems that have battery backup cost significantly more to install because there is a lot of other equipment required, such as inverters, batteries and charge controllers. The design and installation of these systems involve conducting a load analysis and specific wiring in specific subpanels. Battery sizing is based upon the average daily electrical load and the number of days of battery storage.
There are many types of batteries that can be used in PV systems. The lead-acid type of the most common, but lithium-ion batteries are becoming more popular. Table 1 compares these two most common battery types. A motive power (traction battery) is a lead-acid battery designed for use in deep discharge applications, such as electric vehicles. Traction batteries are used in stand-alone PV systems and differ from deep discharge batteries because they use heavier, thicker plates and strong intercell connections to withstand the mechanical stresses from deep discharges.
|Lithium-ion Batteries||Lead-acid Batteries|
• Becoming more common in domestic grid-connected solar PV storage systems
• More expensive
• Lighter and smaller
• Requires integrated controller to manage charging and discharging
• More efficient
• Can discharge more stored energy
• Longer expected lifetime
• Used for off-grid storage systems where additional storage is required
• Less expensive
• Heavier and larger
• Requires good charging and discharging process to maintain battery health
• Less efficient
• Shorter expected lifetime
Table 1: Two Most Common Types of Batteries for PV System Storage
Flooded batteries have a liquid electrolyte solution. Vented lead-acid batteries release hydrogen and oxygen gases; therefore, adequate ventilation must be provided for both vented and sealed battery systems. It is usually advisable to provide adequate ventilation requirements similar to a combustion water heater. There are no specific ventilation requirements, vented cells should incorporate a flame arrestor to help prevent explosions from external ignition sources, and sealed batteries must have pressure relief vents.
Valve-regulated lead-acid (VRLA) batteries have a gel-based electrolyte and do not have the same maintenance requirements as vented lead-acid batteries. However, they are less tolerant of overcharging and higher temperatures as well as being more expensive.
Battery storage systems often have power ratings in kiloWatts (kW) and are typically between 1 – 7 kW. The power rating is the capability of the battery to provide power. The measurement for battery storage capacity is in ampere-hours (Ah) or kilowatt-hours (kWh). A 12-volt battery rated at 480 Ah stores 2.25 kWh of energy. This is usually larger than the batteries actual capacity because:
• Batteries lose some energy during charging and discharging
• Batteries cannot be fully discharged
The battery power output is affected by the battery design and chemistry -- so the number of plates, plate dimensions, and electrolyte specific gravity affects the power output. The actual battery capacity is always less than the rated battery capacity. The battery capacity over time affect the usable battery capacity, i.e., the battery age, operating temperature, and discharge rate. The “depth of discharge” for typical lead-acid battery systems is approximately 50%, and it is 75% for lithium-ion batteries.
The charge or discharge rate is expressed as a ratio of the battery capacity (C) to the charge or discharge time period in hours. A 50 ampere-hour battery discharged at 5 amps for 10 hours is rated C/10 or a 10-hour discharge rate. Electrical capacity can alternatively be given in Amp-hours (Ah); where Ah x Voltage = kWh. The percent of available battery capacity is called the state-of-charge. The depth-of-discharge is the percent of capacity that has been removed. These two are related by the following:
state-of-charge + depth-of-discharge = 100%
The allowable depth-of-discharge is approximately 75 – 80% for deep cycle batteries. In very cold climates, batteries must be protected from freezing because the electrolyte density decreases with increasing temperature.
A battery’s lifetime depends upon the battery chemistry and the number of cycles (complete battery charge and discharge cycles) that they undergo. The battery life expectancy is usually given in years or charge-discharge cycles. An example would be a specification of 10 years or 10,000 cycles. Often the battery performance will be monitored via the PV system so that you will know when the batteries need replacing.
When looking at battery specifications, there are many factors to consider:
• Charge/discharge rates
• Flooded or VRLA
• Environmental conditions
• Storage requirements
PV systems require a charge controller to protect batteries from overcharge. A general rule of thumb is if the max charge rate from the PV modules multiplied by one hour is equal to 3% of the battery nominal amp-hour capacity or greater, a charge controller is required. A battery that is overcharged can create a hazardous condition and greatly reduces the battery life. Therefore, may charge controllers have overcharge protection built-in.
The requirements of battery installations limit the voltage of lead-acid batteries to no more than 48 volts. This implies 24 2-volt lead-acid cells connected in series or 40 1.2-volt alkali type nickel-cadmium cells in series. Battery installations must include adequate working spaces, clearances, and ventilation. If the batteries connected together produce more than 48 V, then the batteries should be separated or connected in a way to only allow a maximum of 48 volts.
Racks and trays for mounting battery systems are usually made from metal, fiberglass or other non-conductive materials. Metal racks should be painted or covered with insulation to provide protection from electrolyte leakage from the batteries. Conductive racks should not be located within 150 mm from the tops of the battery cases. Any conductive racks should be properly grounded, and current-limiting overcurrent devices must be installed.
When installing battery systems, special safety precautions are necessary. Batteries present hazards such as caustic electrolyte, high short-circuit currents, arc flash hazards and hydrogen and oxygen gas. High voltage systems may have arc flash hazards. Batteries are also very heavy and sometimes may need to be moved using the appropriate methods supported by the manufacturer.
The number of days that a fully charged battery can meet the system load without recharging is called autonomy. Autonomy can be calculated using the average daily load, nominal battery capacity, and the maximum allowable depth of discharge. The basics for selecting the appropriately sized battery is to:
• Calculate the system load
• Select the number of days that the battery needs to deliver autonomy in the system
• Use the appropriate depth of discharge to calculate the usable amp-hours
If the system load is 75 Ah per day and a 300 Ah battery is selected with an allowable depth-of-discharge of 75%, then there would be 225 Ah usable. The battery design would then deliver 225 Ah/(75 Ah/day) = 3 days. Certain loads may need more than 3 days of autonomy, and if there is another backup source such as a generator, then less autonomy may be required.
The cost of the battery system with the estimated benefits should be compared:
• The cost of the system, including the costs to run the system
• The cost of replacing the battery storage system at least once during the PV panel lifetime
• Every kWh from the battery is a kWh you do not have to use from the grid. This depends upon your PV system sizing, battery backup system, and your load profile throughout the year.
If the total cost is compared with the benefits per year, you can calculate how many years for the system to pay for itself. If possible, it would be beneficial to factor in usage change, electricity price change, and inflation.
We reviewed the basics of the battery storage system, the most popular battery types, battery specifications, autonomy, and costs. When comparing battery systems, it makes sense to get several quotes for similar systems – either stand-alone battery systems or the entire PV + battery storage system.