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What are the principles of operation of electrical cells?

In our last article, we looked at the basic electrical parameters in D.C. circuits. In this article, we’re going to move on to the principles of operation of electrical cells. We’ll look at the different types of cells and batteries, their mechanisms, and their applications.

What are cells and batteries?

A cell is a single unit device that converts chemical energy into electric energy. They’re normally very light and compact, as they’re a single unit. A battery is normally a group of cells and can either be a primary or secondary battery. This means it can be rechargeable or non-chargeable (single use).

Batteries are classified into 4 categories

Primary cell

A primary cell or battery is one that can’t easily be recharged after one use. They are discarded after discharge. Most primary cells use electrolytes that are contained within absorbent material or a separator. They are known as dry cells.

, What are the principles of operation of electrical cells?

Secondary cell

A secondary cell is one that can be electrically recharged after use by passing a current through the circuit in the opposite direction to the current during discharge. Secondary batteries fall into two subcategories depending on their application:

Energy storage devices – delivering energy on demand. Typically connected to primary power sources so they remain fully charged. An example would be something like an emergency no-fail and standby power sources.
Recharged after use – they are essentially used as primary cells but are recharged after use instead of being discarded. An example is a portable consumer electronic device or electric vehicle.

Reserve cell

These are also known as deferred-action batteries. Reserve batteries are special purpose primary batteries, normally designed for emergency use. The electrolyte is stored separately from the electrodes, which remain in a dry inactive state.

The battery is only activated when it is actually needed by introducing the electrolyte into the active cell area. This has the double benefit of avoiding deterioration of the active materials during storage, and at the same time it eliminates the loss of capacity due to self discharge until the battery is needed. They can be stored for 10 years or more and provide full power in an instant when it is required.

Most reserve batteries are used once and then discarded. They are used in timing, temperature and pressure sensitive detonation devices in missiles, torpedoes, and other weapon systems.

The fuel cell

These were originally developed for space vehicles, however recent development has led to them being used in a variety of systems, such as electric vehicles, utility power and on-site generators.

Fuel cells can deliver the zero-pollution, high-efficiency answer to much of our air pollution and global warming dilemmas. A fuel cell is an electrochemical power generation device that combines hydrogen fuel, with oxygen from air, to produce electricity, with water and heat as the only by-products.

Unlike traditional power generators, Fuel cells are more fuel-efficient, operate with very little noise and produce no harmful emissions at point of use. Fuel cells and batteries are similar as they both generate electricity. However, a battery stores energy, while a fuel cell uses an external fuel such as hydrogen allowing it to continue operating as long as fuel is available.

Unlike conventional batteries however, fuel cells do not contain harmful materials, and very little maintenance since there are no moving parts

Cell capacity

The capacity of a cell is measured in ampere-hours (Ah). A fully charged 50 Ah battery rated for 10 h discharge can be discharged at a steady current of 5 A for 10 h, but if the load current is increased to 10 A then the battery is discharged in 3–4 h, since the higher the discharge current, the lower is the effective capacity of the battery. Typical discharge characteristics for a lead-acid cell can be seen in the image below: