The STEMpunk Project: Batteries

In The STEMpunk Project: Basic Electrical Components I wrote about resistors, capacitors, inductors, and diodes, but I had originally wanted to include batteries and transistors as well. As I did research for that post however it occurred to me that these latter two devices were very complex and would require their own discussion. In today’s post I cover a remarkable little invention familiar to everyone: batteries.

Battery Basics

The two fundamental components of a battery are electrodes and an electrolyte, which together make up one cell. The electrodes are made of different metals whose respective properties give rise to a difference in electrical potential energy which can be used to induce current flow. These electrodes are then immersed in an electrolyte, which can be made from a sulfuric acid chemical bath, a gel-like paste, or many other materials. When an external conductor is hooked up to each electrode current will flow from one of them (the ‘negative terminal’) to the other (the ‘positive terminal’).

Battery cells can be primary or secondary, and are distinguished by whether or not the chemical reactions happening in the cell cause one of the terminals to erode. The simplest primary cell consists of a zinc electrode as the negative terminal, a carbon electrode as the positive terminal, and sulfuric acid diluted with water as the electrolyte. As current flows zinc molecules combine with sulfuric acid to produce zinc sulfate and hydrogen gas, thus consuming the zinc electrode.

But even when not connected to a circuit impurities in the zinc electrode can cause small amounts of current to flow in the electrode and correspondingly slow rates of erosion to occur. This is called local action and is the reason why batteries can die even when not used for long periods of time. Of course there exist techniques for combating this, like coating the zinc electrode in mercury to pull out impurities and render them less reactive. None of these work flawlessly, but advances in battery manufacturing have allowed for the creation of long-storage batteries with a sealed electrolyte, released only when the battery is actually used, and of primary cell batteries that can be recharged.

A secondary cell works along the same chemical principles as a primary cell, but the electrodes and electrolyte are composed of materials that don’t dissolve when they react. In order to be classifiable as ‘rechargeable’ it must be possible to safely reverse the chemical reactions inside the cell by means of running a current through it in the reverse direction of how current normally flows out of it. Unlike the zinc-carbon voltaic cell discussed above, for example, in a nickel-cadmium battery the molecules formed during battery discharge are easily reverted to their original state during recharging.

Naturally it is difficult to design and build such a sophisticated electrochemical mechanism, which is why rechargeable batteries are more expensive.

Much more information on the chemistry of primary and secondary cells can be found in this Scientific American article. I also found this article on how batteries work from Save On Energy to be helpful.

Combining Batteries in Series or in Parallel

Like most other electrical components batteries can be hooked up in series, in parallel, or in series-parallel. To illustrate, imagine four batteries lined up in a row, with their positive terminals on the left and their negative terminals on the right. If wired in series, the negative terminal on the rightmost battery would be the negative terminal for the whole apparatus and the positive terminal on the leftmost battery would be the positive terminal for the whole apparatus. In between, the positive terminals of one battery are connected to the negative terminals of the next battery, causing the voltage of the individual batteries to be cumulative. This four-battery setup would generate six volts total (1.5V per battery multiplied by the number of batteries), and the total current of the circuit load (a light bulb, a radio, etc.) is non-cumulative and would flow through each battery.

If wired in parallel, the positive and negative terminals of the rightmost battery would connect to the same terminal on the next battery, and the terminals for the leftmost battery would connect to the external circuit. In this setup it is voltage which is non-cumulative and current which is cumulative.  By manipulating and combining these properties of batteries it is possible to supply power to a wide variety of circuit configurations.

Different Battery Types [1]

Nickel Cadmium: NiCd batteries are a mature technology and thus well-understood. They have a long life but relatively low energy density and are thus suited for applications like biomedical equipment, radios, and power tools. They do contain toxic materials and aren’t eco-friendly.

Nickel-Metal Hydride: NiMH batteries have a shorter life span and correspondingly higher energy density. Unlike their NiCd cousins NiMH batteries contain nothing toxic.

Lead Acid: Lead Acid batteries tend to be very heavy and so are most suitable for use in places where weight isn’t a factor, like hospital equipment, emergency lighting, and automobiles.

Absorbent Glass Mat: The AGM is a special kind of lead acid battery in which the sulfuric acid electrolyte is absorbed into a fine fiberglass mesh. This makes the battery spill proof and capable of being stored for very long periods of time. They are also vibration resistance and have a high power density, all of which combine to make them ideal for high-end motorcycles, NASCAR, and military vehicles.

Lithium Ion: Li-on is the fastest growing battery technology. Being high-energy and very lightweight makes them ideal for laptops and smartphones.

Lithium Ion Polymer: Li-on polymer batteries are very similar to plain Li-on batteries but ever smaller.

The Future of Batteries

Batteries have come a very long way since Ewald Von Kleist first stored static charge in a Leyden jar in 1744. Lithium Ion seems to be the hot topic of discussion, but there are efforts being made at building aluminum batteries, solid state batteries, and microbatteries, and some experts maintain that the exciting thing to watch out for is advances in battery manufacturing.

Hopefully before long we’ll have batteries which power smart clothing and extend the range of electric vehicles to thousands of miles.


[1] Most of this section is just a summary of the information found here.

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