Solar Knowledge: Batteries
IntroductionIn stand-alone solar home system batteries are used to store the electrical energy generated by Solar panels in chemical form during the day time to be used during the time of need as the demand of energy does not always coincide with its production.
The main functions of battery bank in solar home system are:
- Energy storage capacity and autonomy: to store electrical energy whenever PV module generates it and supply energy to the load as needed.
- Voltage and current stabilization: to supply power to the loads at the stable voltages and current by suppressing the transients that may occur in solar PV system.
- Supply current surge: To supply high operating current to meet the demand.
- Low temperatures reduce capacity.
- High discharge rate reduce capacity.
- High end of discharge voltages reduce capacity.
- Limitation on depth of discharge (DoD) reduces capacity.
- Failure to properly recharge a battery limits its capacity.
- Excessive periods of high temperature and/or overcharge may result in the loss of water from the electrolyte, premature aging, and limit capacity of batteries
General classification of Lead acid batteriesLead acid batteries are most commonly used in PV system because of their low initial cost and their easy availability nearly everywhere in the world. Despite low energy to weight ratio and low energy to volume ratio, they maintain high power to weight ratio. These features along with their low cost make them attractive for PV system and motor vehicles.
Lead acid batteries are formed by two plates, positive and negative, immersed in dilute Sulphuric acid solution. The positive plate is made up of lead dioxide and negative plate is made of lead.
Lead acid battery chemistryDuring discharge cycle the battery is connected to an electrical load and current flows from the battery to the load. In this process the active materials are converted into lead sulfate (PbSO4) as given by the following chemical equation:
During charging (electrode signs as in charging circuit)
(+) electrode: PbSO4(s) + 2H2O(l) → PbO2(s) + 4H+(aq) + SO42-(aq) + 2e-
(-) electrode: PbSO4(s) + 2e- → Pb(s) + SO42-(aq)
Discharging (electrode signs as for cell)
(+) electrode: PbO2(s) + 4H+(aq) + SO42-(aq) + 2e- → PbSO4(s) + 2H2O(l)
(-) electrode: Pb(s) + SO42-(aq) → PbSO4(s) + 2e-
The overall, reversible cell reaction is therefore:
PbO2(s) + 4H+(aq) + 2SO42-(aq) + Pb(s) ⇌ 2PbSO4(s) + 2H2O(l)
As the battery is discharged the active materials PbO2 and Pb in the positive and negative plates respectively, combine with the sulfuric acid solution to form PbSO4 and water. The dilution of the electrolyte has important consequences in terms of specific gravity and freezing point of the electrolyte. Two Generic types of lead acid batteries are:
Many types of lead-acid batteries are used in PV systems, each having specific design and performance. Generally they are one of the following three categories
Staring, lighting and ignition(SLI) batteries are designed for shallow cycle and used mostly to power automotive starters. Large number of plates per cell of such battery enables it to deliver high discharge currents for short period. However they are not suitable for long life under deep cycle services. SLI batteries are sometimes used in PV System particularly in developing countries where only one type of locally manufactured battery are available or when the difference in the up-front cost of the deep cyble battery and SLI battery is very high. Although generally not recommended in most PV applications, SLI batteries may provide 2 or more years of useful services in small stand-alone PV systems, where the average daily depth of discharge(DoD) is limited to 10-20% and the maximum allowed DoD is limited to 40% to 50%.
Motive Power or Traction Batteries
There batteries are designed for deep discharge cycle services as generally required in electric vehicle or forklifts etc. These batteries have fewer number of plates per cell than SLI batteries, however the plates are made much thicker and constructed more durably. These types of battery are also very popular in PV systems due to their deep cycle capabilities and longer life cycles.
Stationary batteries are used commonly in un-interruptible power supplies (UPS) to provide back-up power to computers, telecommunication equipment etc. These batteries are designed for deep discharge and limited cycle service; and they are commonly float charged continuously.
Captive Electrolyte Type of Special Lead-Acid Battery
Valve Regulated Lead-Acid Battery
Valve regulated lead-acid(VLRA) batteries are captive electrolyte type of lead-acid batteries where the electrolyte is immobilized by means of specially designed pressure regulating mechanism on the cell vents and the battery is sealed under normal operating conditions. Electrolyte cannot be generally replenished on these types of batteries; therefore, they should not be subjected to excessive discharge.
In gelled lead-acid batteries, electrolyte is specially “gelled” by adding silicon dioxide in the electrolyte. Cracks and voids develop within the gelled electrolyte during first few cycles, which provide path for gas movement between the positive and negative plates, facilitating in recombination process.
Absorbed Glass Mat Batteries
In absorbed glass mat (AGM) lead-acid batteries, the electrolyte is absorbed in glass mats which are sandwiched in layers between the plates. Under controlled charging condition the pressure relief vents in AGM batteries are designed to remain closed, preventing the release of any gas and water loss.
Modern Rechargeable Batteries
Nickel-Cadmium Rechargeable Battery
In nickel-cadmium rechargeable batteries, nickel oxyhydroxide (NiOOH) is generally the active material in the charged positive plate and cadmium metal is the active material in the charged negative plate of the battery. The primary cell voltage is commonly 1.2volts. These batteries have found diverse applications in consumer and electronics market. For the same capacity range they are quite costlier than the similar lead-acid batteries; although they have a longer life cycle and higher allowable DoD. The main disadvantage of nickel-cadmium battery are
- It has got memory effect.
- It is environmentally hazardous as it contains toxic material viz. Cadmium.
Nickel Metal Hydride Rechargeable Battery
Lithium Ion Battery
Battery SpecificationsThe photovoltaic system designer must consider following variables when specifying and installing stand-alone PV system
- Days of autonomy
- Battery capacity
- Rate and depth of discharge
- Life expectancy
- Environmental conditions
Days of autonomy:It refers to the number of days battery bank can provide power to the load without being charged by photovoltaic array. To determine days of autonomy designer must consider the weather conditions, types of load and total load.
Weather conditions determine the number of “no sun” days which plays significant role in determining autonomy. Local weather patterns and micro-climate like three days foggy winter storm, cloudy periods in rainy season must be taken into consideration.
The most important factors in determining the autonomy for the system are the size and types of load that system services. It is important to answer several questions about each load.
- Is it critical that the load operates at all times?
- Is the load simply a convenience?
- 2 to 3 days for non essential uses
- 5 to 7 days for critical load with no other power source.
Battery capacity:Batteries are rated in amp-hour (AH) capacity. Using water analogy, one can think of battery as a bucket, stored energy as water and AH capacity as bucket size. The AH rating tells how large the bucket is. In theory, 100AH battery will deliver one amp load for 100 hours and 2 amp load for 50 hours before it is considered fully discharged. If more storage capacity is required for PV system, these batteries are combined in series and parallel. Many factors can affect battery capacity including rate of discharge, depth of discharge, temperature, age and recharging characteristics.
There is misconception about PV system being highly modular. New batteries should not be added to the old battery bank as older batteries will degrade the performance of new batteries and could result in reduced system voltage when connected in series. In addition if battery capacity is to be increased, one might add in parallel to the existing system to increase ampere hour capacity and maintain system voltage. It is advisable to minimize excessive “paralleling” because this increases total number of cell thereby increasing potential for failure from bad cell. Maximum recommended parallel strings in a battery bank are limited to 4. Slightly higher battery capacity is specified than required capacity because battery loses their capacity as they age.
Rate and depth of discharge:The rate at which battery is discharged directly affects its capacity. A common battery specification is the battery's capacity in relation to the number of hours that is discharged. Most batteries are rated at C/20 meaning that a battery would provide 180AH if discharged over 20 hours. If the battery is discharged over 5 hours then same battery would not provide rated 180 AH.
Figure: Battery Depth of Discharge Vs number of cycleSimilar consideration should be taken while charging batteries. Most flooded lead acid batteries should not be charged more than C/5 rate.
Depth of discharge refers to how much capacity will be withdrawn from the battery. PV systems are designed for discharge of 40 percent to 80 percent. Battery life is directly related to how deep battery is cycled. For example, if battery is discharged to 50 percent every day, it will last about twice as long as if it is cycled to 80 percent.
Life expectancy:Battery life is expressed in terms of quantity of cycle, but most people think of life expectancy in terms of year. Batteries lose their capacity over time and are considered at the end of their life when 20 percent of their original capacity is lost, although they can still be used. While sizing the system, this fact should be taken into consideration.
A battery with shallow-cycling of only 25 percent of DoD would be expected to last 4000 cycle while battery cycle to 80% of DoD would last 1500 cycle. If one cycle equaled to one day, shallow-cycled battery would last for 10 years, while deep-cycled battery would last for 4.12 years.
Environmental conditions:Batteries capacity is highly sensitive to temperature of the environment. Batteries are rated at 25 C. Battery capacity decreases with decrease in temperature and increase at higher temperature. Even though battery capacity decreases at lower temperature its life increases. Similarly battery life decreases with increase in temperature [Ref. 11].
Figure: Effect of temperature on batteryColder temperature affects more than battery capacity, it might freeze the electrolyte. Temperature at which battery freeze depend upon the state of charge.