Introduction
Generally, there are two types of lead-acid storage batteries, based on their method of construction. These batteries are either classified as flooded (vented) or sealed. Flooded and sealed batteries also differ in their operation. All lead-acid batteries produce hydrogen and oxygen gas (gassing) at the electrodes during charging through a process called electrolysis. These gases are allowed to escape a flooded cell, however, the sealed cell is constructed so that the gases are contained and recombined. It should be noted that hydrogen gas is explosive in air at only 4% by volume. Flooded and sealed lead-acid batteries are discussed in the following paragraphs.
Flooded Lead-Acid Batteries
Flooded cells are those where the electrodes/plates are immersed in electrolyte. Since gases created during charging are vented to the atmosphere, distilled water must be added occasionally to bring the electrolyte back to its required level. The most familiar example of a flooded lead-acid cell is the 12-V automobile battery.
Flooded Lead Acid Automobile Battery
Sealed Lead-Acid Batteries
These types of batteries confine the electrolyte, but have a vent or valve to allow gases to escape if internal pressure exceeds a certain threshold. During charging, a lead-acid battery generates oxygen gas at the positive electrode.
Sealed lead-acid batteries are designed so that the oxygen generated during charging is captured and recombined in the battery. This is called an oxygen recombination cycle and works well as long as the charge rate is not too high. Too high of a rate of charge may result in case rupture, thermal runaway, or internal mechanical damage.
The valve-regulated battery is the most common type of sealed battery. It was developed for stationary and telecommunication battery applications. These types of sealed batteries have a spring-controlled valve that vents gases at a predetermined pressure. Typical pressure thresholds are from 2 to 5 psig, depending on the battery design. Although the term "valve-regulated" is often used synonymously to describe sealed lead-acid batteries, not all sealed batteries are valve-regulated. Some battery designs employ replaceable vent plugs or other mechanisms to relieve excess pressure. Sealed batteries were developed to reduce the maintenance required for batteries in active service. Since electrolyte levels are preserved by trapping and recombining off-gasses, there should not be any need to add distilled water over the life of the battery. These batteries are often misnamed "maintenance free”. In fact, all maintenance practices applicable to unsealed type batteries are applicable to sealed type batteries. The only exception is that electrolyte levels cannot, and should not need to be, maintained.
Sealed type batteries are often avoided for backup power source applications for several reasons. One reason is that the state of charge of sealed type batteries cannot be ascertained by the usual specific gravity measurement. Reliable alternative methods to measure the state of charge for sealed type batteries are under development. A second reason is their sensitivity to high temperatures.
Battery Components and Operation
Cells vs. Batteries
A battery is a device that converts the chemical energy contained in its active materials into electrical energy by means of an electrochemical reaction. While the term "battery" is often used, the basic electrochemical element being referred to is the cell. A battery consists of two or more cells electrically connected in series to form a unit. In common usage, the terms "battery" and "cell" are used interchangeably.
Primary and Secondary Cells and Batteries
Batteries are either primary or secondary. Primary batteries can be used only once because the chemical reactions that supply the electrical current are irreversible. Secondary (or storage) batteries can be used, charged, and reused. In these batteries, the chemical reactions that supply electrical current are readily reversed so that the battery is charged.
Primary batteries are common since they are cheap and easy to use. Familiar primary battery uses are in flashlights, watches, toys, and radios. The most common use for secondary (storage) batteries is for starting, lighting, and ignition (SLI) in automobiles and engine-generator sets. Other applications include uninterruptible power supplies (UPSs) for emergency and backup power, electric vehicles (traction), telecommunications, and portable tools. The remainder of this article will be concerned only with storage batteries except where general operating characteristics of batteries are discussed.
Battery Components
A cell has five major components as shown in the image below.
Major Components of a Cell
The negative electrode supplies electrons to the external circuit (or load) during discharge. In a fully charged lead-acid storage battery the negative electrode is composed of sponge lead (Pb).
The positive electrode accepts electrons from the load during discharge. In a fully charged lead-acid battery the positive electrode is composed of lead dioxide (PbO2). It should be noted that the electrodes in a battery must be of dissimilar materials, or the cell will not be able to develop an electrical potential and thus conduct electrical current.
The electrolyte completes the internal circuit in the battery by supplying ions to the positive and negative electrodes. Dilute sulfuric acid (H2SO4) is the electrolyte in lead-acid batteries. In a fully charged lead-acid battery, the electrolyte is approximately 25% sulfuric acid and 75% water.
The separator is used to electrically isolate the positive and negative electrodes. If the electrodes are allowed to come in contact, the cell will short-circuit and become useless because both electrodes would be at the same potential. The type of separator used varies by cell type. Materials used as separators must allow ion transfer between the electrolyte and electrodes. Many separators are made of a porous plastic or glass fiber material.
The above components are housed in a container commonly called a jar or container.
Cell and Battery Voltage
In order for a cell or battery to be able to deliver electrical current to an external circuit, a potential difference must exist between the positive and negative electrodes. The potential difference (usually measured in volts) is commonly referred to as the voltage of the cell or battery. A single lead-acid cell can develop a maximum potential difference of about 2 V under load. A completely discharged lead-acid cell has a potential difference of about 1.75 V, depending on the rate of discharge.
Capacity and Battery Ratings
In general terms, the capacity of a cell/battery is the amount of charge available expressed in ampere-hours (Ah). An ampere is the unit of measurement used for electrical current and is defined as a coulomb of charge passing through an electrical conductor in one second. The capacity of a cell or battery is related to the quantity of active materials in it, and the amount of electrolyte and the surface area of the plates. The capacity of a battery/cell is measured by discharging at a constant current until it reaches its terminal voltage (usually about 1.75 volts). This is usually done at a constant temperature, under standard conditions of 25ºC (77ºF). The capacity is calculated by multiplying the discharge current value by the time required to reach terminal voltage.
The most common term used to describe a battery's ability to deliver current is its rated capacity. Manufacturers frequently specify the rated capacity of their batteries in ampere-hours at a specific discharge rate. For example, this means that a lead-acid battery rated for 200 Ah (for a 10-hour rate) will deliver 20 amperes of current for 10 hours under standard temperature conditions (25ºC or 77ºF). Alternatively, a discharge rate may be specified by its charge rate or C -rate, which is expressed as a multiple of the rated capacity of the cell or battery. For example, a battery may have a rating of 200 Ah at a C/10 discharge rate. The discharge rate is determined by the equation below:
C/10 rate (amperes)= 200 Ah/10 h = 20 amperes
Battery capacity varies with the discharge rate. The higher the discharge rate, the lower the cell capacity. Lower discharge rates result in higher capacity. Manufacturer's literature on batteries will normally specify several discharge rates (in amperes) along with the associated discharge time (in hours). The capacity of the battery for each of these various discharge rates can be calculated as discussed above.
The rated capacity for lead-acid batteries is usually specified at the 8-, 10-, or 20-hour rates (C/8, C /10, C /20). UPS batteries are rated at 8-hour capacities and telecommunications batteries are rated at 10-hour capacities.
Series and Parallel Connections
Cells and batteries may be connected in series, parallel, or combinations of both. Cells or batteries connected in series have the positive terminal of one cell or battery connected to the negative terminal of another cell or battery. This has the effect of increasing the overall voltage but the overall capacity remains the same. For example, the 12-V lead-acid automobile battery contains 6 cells connected in series with each cell having a potential difference of about 2 V. Another example of cells or batteries connected in series is shown in the image below.
Cells or batteries connected in parallel have their like terminals connected together. The overall voltage remains the same but the capacity is increased. For example, if two 12-V automotive batteries were connected in parallel, the overall voltage for the batteries would still be 12 V. However, the connected batteries would have twice the capacity of a single 12-V battery. Another example of cells or batteries connected in parallel is shown in the image below.
Cells Connected In Series
Cells Connected in Parallel
Batteries may also be connected in a series/parallel combination. Batteries are added in series until the desired voltage is obtained, and in parallel until the battery bank meets capacity requirements. Only like cells or batteries should be connected together. Connecting cells or batteries of different rating or manufacturer may produce undesirable or even dangerous results.
How Lead Acid Batteries Work
Additional Resources
https://electrical4u.com/zinc-carbon-battery
https://britannica.com/technology/battery-electronics/Lithium-batteries
https://batteryuniversity.com/learn/article/lead_based_batteries