|All batteries can be described as either primary or secondary. Primary batteries are those batteries that are used only once and then discarded; they cannot be recharged. They have the dual advantages of having both a higher initial voltage and longer life than secondary batteries of the same size. Secondary batteries are the rechargeable batteries. While the initial voltage and battery life is less, they have the significant advantage of being reusable.
Primary battery construction ranges from the basic construction used in carbon zinc and zinc chloride batteries to the more complex construction of more powerful batteries such as alkaline and lithium manganese. Changes in the components and the construction allow for the improved battery life of alkaline and lithium batteries.
Carbon zinc, zinc chloride and alkaline batteries are similar in that they each have a zinc anode, a manganese dioxide cathode, and a paste electrolyte. In carbon zinc and zinc chloride batteries, the zinc anode is the battery cylinder (or can). The electrolyte is a zinc chloride paste, and the manganese dioxide cathode mix is in the center of the can, surrounding a carbon rod electrode.
In alkaline batteries, the zinc anode is a zinc powder in the center of the can, surrounding a brass current collector. The electrolyte is potassium hydroxide, and the zinc and potassium hydroxide are combined in a gel. The manganese-dioxide cathode is contained between the can wall and the separator, which keeps the cathode and anode from direct contact. The can wall in alkalines is steel, rather than zinc.
Lithium batteries have a lithium foil anode, a manganese dioxide cathode, and a lithium-based electrolyte. Lithium manganese batteries use a variety of shapes and constructions, with the most common being button cells, solid-core cylindrical batteries and wound-core cylindrical batteries.
Regardless of the battery type, all batteries operate in a similar manner. In broad terms, a three-way chemical reaction takes place between the electrolyte, cathode and anode. This reaction produces negative ions at the negative terminal (the base), and positive ions gathered at the positive post (the small post on top). The generation and gathering of the positive and negative elements creates the battery energy.
As energy is used, the effectiveness of the basic battery ingredients diminish. The cathode and anode materials become depleted, and the reaction between the anode and the electrolyte produces byproducts. As these byproducts build up, the battery's voltage diminishes and performance drops. For this reason, the life of the battery increases as the amounts of the anodic, cathodic and electrolytic chemicals increase.
In most primary batteries, the electrochemical reactions produce a gradual decline in voltage, as opposed to the steady decline in voltage produced by lithium manganese and most secondary batteries. The figure to the left shows a typical discharge curve of voltage over time for a primary battery. The figure to the right shows a typical flat discharge curve of voltage over time for (a lithium manganese battery (starting at 1.5 volts) or) a secondary battery (starting 1.25 volts). The exact slopes will depend on the type of battery, its size, rate of current discharge, operating temperature, and other conditions.