Chapter 6 Batteries Types and Characteristics Functions and Features Specifications and Ratings 2012 Jim Dunlop Solar
Overview Describing why batteries are used in PV systems. Identifying the basic components of battery construction. Defining battery terminology for battery specifications, ratings and operating parameters. Classifying common types of storage batteries and their performance characteristics. Understanding the NEC and OSHA requirements for battery installations and safety. 2012 Jim Dunlop Solar Batteries: 6-2
Battery Fundamentals A battery is an electrochemical cell that stores energy in chemical bonds. Chemical energy is converted to DC electrical energy when a battery is connected to a load. Batteries are used in PV systems for the following purposes: To store energy produced by the PV array and supply it to electrical loads as needed. To operate the PV array and electrical loads at stable voltages. To supply surge currents to electrical loads or appliances. 2012 Jim Dunlop Solar Batteries: 6-3
Battery Cell Design A cell is the basic electrochemical unit in a battery. + - Electrical load Positive plate Separator Negative plate Electrolyte Case 2012 Jim Dunlop Solar Batteries: 6-4
Battery Components: Definitions Plate: The positive or negative electrode in a battery cell, consisting of a grid and active material. Active Material: The reactant materials that comprise the positive and negative plates. Grid: A metal alloy framework that supports the active material on a battery plate. Separator: A porous, insulating divider between the positive and negative plates. 2012 Jim Dunlop Solar Batteries: 6-5
Battery Components: Definitions (cont.) Electrolyte: A conducting medium which allows the flow of current via ionic transfer between the battery plates. Case: A container which encloses the plates, separators and electrolyte in a battery. Vent: a cap on flooded battery cells that also serves as a means to fill the battery. For sealed batteries, the vent is also a pressure regulating valve. 2012 Jim Dunlop Solar Batteries: 6-6
Battery Construction The major components of battery construction are shown in this cut-away view. 2012 Jim Dunlop Solar Batteries: 6-7
Battery Capacity Capacity is a measure of the stored electric charge or stored energy that a battery can deliver under specified conditions. An ampere-hour (Ah) is the common unit of battery energy storage capacity, equal to the transfer of one ampere for one hour. Voltage (V) High discharge rate Cut off voltage Low discharge rate Capacity depends on the battery temperature, discharge rate and cut-off voltage. Capacity (Ah) 2012 Jim Dunlop Solar Batteries: 6-8
Battery Discharging Discharging is the process when a battery delivers current under the application of an electrical load. The discharge rate is the time in hours required to fully discharge a battery at a given current to a specified cutoff voltage. Lower temperatures and high discharge rates reduce available battery capacity. Percent of 25 o C Capacity 120 110 100 90 80 70 60 50 40 C/500 C/120 C/50 C/5 C/0.5 30-30 -20-10 0 10 20 30 40 50 Battery Operating Temperature ( o C ) 2012 Jim Dunlop Solar Batteries: 6-9
Battery State-of-Charge State-of-charge (SOC) is the percentage of energy stored in a battery compared to a fully charged condition. Depth-of-discharge (DOD) is the percentage of capacity that has been withdrawn from a battery compared to the total fully charged capacity: DOD = 100% - SOC Allowable depth-of-discharge is the maximum limit of battery discharge in system operation. Cut-off voltage is the lowest voltage that a battery is allowed to operate and defined by the charge controller or equipment low voltage disconnect set point. The cut-off voltage defines the allowable depth-of-discharge and usable battery capacity at a specific discharge rate. 2012 Jim Dunlop Solar Batteries: 6-10
Battery State-of-Charge 100 0 State of Charge (%) 80 60 Average daily DOD Max allowable DOD 20 30 Depth of Discharge (%) Summer Winter 2012 Jim Dunlop Solar Batteries: 6-11
Self-Discharge Rate Self-discharge is due to internal losses within a battery that reduce state-of-charge over time. Higher temperatures result in higher self-discharge rates. Lead-antimony types and older batteries have higher selfdischarge rates. Self Discharge Rate (%/month) 20 15 10 5 0-50 -25 0 25 50 Lead-Antimony Grid (end of life) Lead-Antimony Grid (new) Lead-Calcium Grid (typical) Operating Temperature ( o C) 2012 Jim Dunlop Solar Batteries: 6-12
Battery Charging Charging is the process when a battery receives or accepts current from a charging source, such as a PV array, and quantified by the charge current or rate. Bulk Stage Absorption Stage Float Stage Battery Voltage Increasing Voltage Bulk Charge - Constant Voltage Float Voltage Battery Current Maximum Charge Current Reducing Absorption Current Float Current Time 2012 Jim Dunlop Solar Batteries: 6-13
Effects of Charge and Discharge Rates on Battery Voltage 16 Battery Voltage (V) 15 14 13 12 11 10 C/5 Charging C/20 Charging C/20 Discharging C/5 Discharging 0 20 40 60 80 100 Battery State of Charge (%) 2012 Jim Dunlop Solar Batteries: 6-14
Lead-Antimony Charging Voltage vs. Battery SOC Cell Voltage (volts) 3.0 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0 Lead-Antimony Grids Charge Rate C/2.5 C/5 Gassing Voltage at 0 o C C/20 Gassing Voltage at 27 o C Gassing Voltage at 50 o C 0 20 40 60 80 100 Battery State of Charge (%) 2012 Jim Dunlop Solar Batteries: 6-15
Chemical Reactions for the Lead-Acid Cell At the positive plate or electrode: + 2+ PbO + 4H + 2e Pb + 2H O 2 2+ 2 Pb + SO PbSO 4 At the negative plate or electrode: Pb Pb + 2 e 2+ 2+ 2 Pb + SO PbSO 4 Overall lead-acid cell reaction: PbO + Pb + 2H SO 2PbSO + 2H O 2 2 4 4 2 4 4 2 2012 Jim Dunlop Solar Batteries: 6-16
Sulfation and Stratification Sulfation is the process where lead-sulfate crystallizes on battery plates when left at partial state-of-charge, and reduces capacity. Stratification is a condition that can occur in taller batteries when electrolyte concentration varies vertically in the battery cell. Resulting higher concentrations at the bottom of the cell accelerate battery degradation and loss of capacity. Proper charging can minimize the effects of sulfation and stratification. 2012 Jim Dunlop Solar Batteries: 6-17
Battery Types Primary batteries are not rechargeable. Secondary batteries are rechargeable. Lead-acid batteries are classified based in their design and intended service: Starting, Lighting and Ignition (SLI) not typically used in PV systems Motive Power or Traction Standby or Stationary Batteries are also classified as either flooded or sealed valveregulated types. 2012 Jim Dunlop Solar Batteries: 6-18
Flooded and Sealed Batteries Flooded batteries have a liquid (fluid) electrolyte. Open-vent types have removable caps that permit electrolyte additions. Sealed-vent types have non-removable caps and do not permit electrolyte additions. Valve-regulated lead-acid batteries have an immobilized electrolyte and sealed pressure-relief vents. Gelled types immobilize the electrolyte by the incorporation of additives. Absorbed glass mat (AGM) types immobilize the electrolyte in glass separator mats. 2012 Jim Dunlop Solar Batteries: 6-19
Types of Lead-Acid Batteries Flooded Lead-Acid Batteries Valve-Regulated Lead- Acid Batteries Gelled Absorbed Glass Mat 2012 Jim Dunlop Solar Batteries: 6-20
Flooded Nickel-Cadmium Batteries Flooded pocket-plate nickel-cadmium batteries are used in some critical and low temperature PV applications. Advantages include long life and low maintenance, and excellent deep discharge and low temperature performance. Disadvantages include high cost and limited availability. Electrolyte is a flooded potassium-hydroxide solution. 2012 Jim Dunlop Solar Batteries: 6-21
Battery Characteristics BATTERY TYPE ADVANTAGES DISADVANTAGES FLOODED LEAD-ACID Lead-Antimony Lead-Calcium Open-Vent Lead-Calcium Sealed-Vent Lead-Antimony/Calcium Hybrid low cost, wide availability, good deep cycle and high temperature performance, can replenish electrolyte low cost, wide availability, low water loss, can replenish electrolyte low cost, wide availability, low water loss medium cost, low water loss high water loss and maintenance poor deep cycle performance, intolerant to high temperatures and overcharge poor deep cycle performance, intolerant to high temperatures and overcharge, can not replenish electrolyte limited availability, potential for stratification VALVE-REGULATED LEAD-ACID Gelled Absorbed Glass Mat medium cost, little or no maintenance, less susceptible to freezing, install in any orientation medium cost, little or no maintenance, less susceptible to freezing, install in any orientation fair deep cycle performance, intolerant to overcharge and high temperatures, limited availability fair deep cycle performance, intolerant to overcharge and high temperatures, limited availability NICKEL-CADMIUM Sealed Sintered-Plate Flooded Pocket-Plate wide availability, excellent low and high temperature performance, maintenance free excellent deep cycle and low and high temperature performance, tolerance to overcharge only available in low capacities, high cost, suffer from memory effect limited availability, high cost, water additions required 2012 Jim Dunlop Solar Batteries: 6-22
Effects of Temperature on Battery Life Lower operating temperatures reduce battery capacity but increase cycle life. For vented batteries, a 10 C increase in average operating temperature above 25 C reduces battery life by 50%. This is worse for VRLA batteries. Higher temperatures accelerate corrosion of the grids and result in greater gassing and electrolyte loss. Battery Life (% life at 25 o C) 1000 100 10 Effects of Temperature on Battery Life Lead-Antimony Grids Lead-Calcium Grids Nickel-Cadmium 5 10 15 20 25 30 35 40 45 Battery Operating Temperature ( o C) 2012 Jim Dunlop Solar Batteries: 6-23
Electrolyte Properties Electrolyte concentration is measured by its specific gravity, and related to battery state of charge. Batteries must be protected from 1.05 freezing at low state-of-charge. 1 Electrolyte Specific Gravity 1.35 1.3 1.25 1.2 1.15 1.1 0-3.3-7.8-15 -27-52 -71 Electrolyte Freezing Temperature ( o C) Specific Gravity H 2 SO 4 (Wt%) H 2 SO 4 (Vol%) Freezing Point ( o C) 1.000 0.0 0.0 0 1.050 7.3 4.2-3.3 1.100 14.3 8.5-7.8 1.150 20.9 13.0-15 1.200 27.2 17.1-27 1.250 33.4 22.6-52 1.300 39.1 27.6-71 2012 Jim Dunlop Solar Batteries: 6-24
Battery Design and Selection Criteria Electrical properties Voltage, capacity, charge/discharge rates Performance Cycle life vs. DOD, system autonomy Physical properties Size and weight Maintenance requirements Flooded or VRLA Installation Location, structural requirements, environmental conditions Safety and auxiliary systems Racks, trays, fire protection, electrical BOS Costs, warranty and availability 2012 Jim Dunlop Solar Batteries: 6-25
Battery Connections Batteries are first connected in series to achieve the desired DC system voltage for utilization equipment. Series connections of batteries are connected in parallel to increase energy storage capacity (amp-hours). 2012 Jim Dunlop Solar Batteries: 6-26
Series Battery Connections Battery 1 + 12 volts - 100 amp-hours + Battery 2 12 volts - 100 amp-hours Total: + 24 volts - 100 amp-hours 2012 Jim Dunlop Solar Batteries: 6-27
Parallel Battery Connections + Battery 1 + 12 volts - 100 amp-hours Battery 2 + 12 volts - 100 amp-hours - Total: 12 volts 200 amp-hours 2012 Jim Dunlop Solar Batteries: 6-28
OSHA Requirements for Battery Installations Unsealed batteries must be installed in ventilated enclosures to prevent fumes, gases, or electrolyte spray entering other areas, and to prevent the accumulation of an explosive mixture. Battery racks, trays and floors must be of sufficient strength and resistant to electrolyte. Face shields, aprons, and rubber gloves must be provided for workers handling acids or batteries, and facilities for quick drenching of the eyes and body must be provided within 25 feet of battery handling areas. Facilities must be provided for flushing and neutralizing spilled electrolyte and for fire protection. Battery charging installations are to be located in designated areas and protected from damage by trucks. Vent caps must be in place during battery charging and maintained in a functioning condition. 2012 Jim Dunlop Solar Batteries: 6-29
NEC Requirements for Battery Installations Battery installations in dwellings are limited to less than 50 volts, nominal unless live parts are not accessible during maintenance [690.71(B)]. Live parts must be guarded for all battery systems in dwellings regardless of voltage [690.71(B)(2)]. Live parts on any battery installations 50 volts and greater must be guarded [490.9, 110.27]. Sufficient working spaces and clearances must be provided for any battery installations [110.26]. 2012 Jim Dunlop Solar Batteries: 6-30
Battery Overcurrent Protection Battery circuit conductors must be protected from overcurrent in accordance with Art. 240 [690.9]. Current-limiting overcurrent devices may be required for large battery banks with high fault currents [690.71(C)]. Fuses energized from both directions must be able to be disconnected from all sources [690.16]. 2012 Jim Dunlop Solar Batteries: 6-31
Battery Disconnects A disconnecting means must be provided for all ungrounded battery circuit conductors [690.15, 480.5]. Disconnecting means must be provided for battery systems greater than 48 volts to isolate the battery system to sections no more than 48 volts for service and maintenance [690.71(E)]. 2012 Jim Dunlop Solar Batteries: 6-32
Grounding Battery Systems Battery systems are considered to be grounded when a currentcarrying conductor of the connected PV source is grounded [690.71(A), 690.41]. Battery systems over 48 volts are permitted without a grounded circuit conductor where all of the following apply: The PV and load circuits are grounded, Both ungrounded battery circuit conductors have overcurrent protection and disconnecting means, and A ground-fault indicator is required for the battery system [690.71(G), 690.35]. 2012 Jim Dunlop Solar Batteries: 6-33
Battery Wiring Methods Flexible cables are permitted to facilitate battery connections [690.74, 400]. Cables must be rated for hard service and moisture resistance. Welding cables are not allowed [690.74]. Fine stranded cable must use lugs and terminals approved for such cables [690.74, 110.3]. 2012 Jim Dunlop Solar Batteries: 6-34
Battery Racks and Trays Metal racks must be painted or otherwise treated to resist degradation from electrolyte and provide insulation between conducting members and the battery cells [480.9]. Conductive racks are not permitted to be located within 150 mm (6 in.) of the tops of the nonconductive battery cases [690.71(D)]. Does not apply to sealed batteries that are manufactured with conductive cases. Conductive battery racks, cases or trays must also have proper equipment grounding. 2012 Jim Dunlop Solar Batteries: 6-35
Battery System Ventilation Ventilation of explosive battery gasses is required [480.9]. Vented battery cells must incorporate a flame arrestor, and sealed batteries must have pressure relief vents [480.10]. 2012 Jim Dunlop Solar Batteries: 6-36
Summary Batteries are used in stand-alone PV systems to store energy produced by the PV array for use by electrical loads as required. Batteries also establish the operating voltage for PV arrays and DC utilization equipment, such as charge controllers, inverters or DC loads. The types of batteries and their performance characteristics vary widely. Battery energy storage capacity is a function of temperature, discharge rate, cutoff voltage and age of the battery. Battery installation and safety requirements are covered in the National Electrical Code and OSHA safety standards. 2012 Jim Dunlop Solar Batteries: 6-37
Questions and Discussion 2012 Jim Dunlop Solar Batteries: 6-38