Eye on electronics

Motor, Nov 2003 by Dale, Mike

In our cordless, wireless, portable world, it often seems that everything needs a battery. Mike explains why some are better than others, depending on their intended use.

Besides the big one under the hood, there are all sorts of batteries in vehicles and around your service bay-inside hand-held electronic test equipment, portable phones in the shop, laptop computers and, of course, the ubiquitous flashlight. Batteries can cost you plenty in at least two ways: When your DVM doesn't work, the value of the lost time will greatly exceed the value of a replacement battery. And if you ever need to replace the battery in your laptop because you left it in direct sunlight, you're going to be in for a nasty-and expensive-surprise.

Every year more than 15 billion batteries are made and disposed of worldwide. Choosing the right one and using it the way it was intended to be used can save you money and ensure that your battery-operated equipment will work when you need it.

When it comes to batteries, what you want to know is how many milliamps (mA) for how long at a given voltage you're going to get for your money. According to the Real Goods Solar Living Sourcebook, the cost per kilowatt-hour from disposable batteries varies from $400 to $20,000, depending on the technology used. In contrast, rechargeable batteries can supply that kilowatt-hour for less than a dollar.

Dr. Carl Gassner in Germany invented the first dry-cell battery in 1886. He used a zinc shell to hold all the chemicals and to form the negative electrode. Later, he added zinc chloride to the electrolyte. This slowed the corrosion of the zinc shell, adding greatly to the shelf life of the battery. When you buy an ordinaiy flashlight battery today you're buying basically what Dr. Gassner invented.

Todays dry-cell battery construction uses an outer shell of steel with a zinc lining on the inside. This makes the cell leak-resistant as the chemical reaction eats away the zinc. On the inside of the cell is an electrolyte separator soaked with an electrolyte paste made of water and ammonium chloride. As Dr. Gassner found out, adding zinc chloride to the electrolyte adds shelf life to the battery. Inside the electrolyte layer is a quantity of manganese dioxide that makes the connection between the carbon rod at the center of the battery and the electrolyte paste that reacts with the zinc.

The performance of the carbon-zinc cell varies tremendously depending on how it's used. As the battery is used up, its output voltage falls. As the battery gets cold, its performance decreases. As the rate of current draw increases, battery life decreases at a faster rate. At 0°F, a carbon-zinc battery loaded initially to 667mA will start with an output of only 1.2 volts (1.5 volts is nominal), put out less than 1.0 volt in ten minutes and basically be dead in 30 minutes. The same battery stored at 125°F will have a very short shelf life due to internal corrosion. However, if the same battery were to be used at 150mA (at 70°F), it would last 30 to 40 hours. These types of cells do okay at room temperature with light loads that are not sensitive to decreases in output voltage.

The major improvement in dry-cell batteries came with the introduction of alkaline cells. For one thing, the electrolyte is an acid, not a base. The use of a potassium-based electrolyte mixed with water lowers the internal cell resistance, allowing higher discharge rates and higher power densities. In an alkaline cell, the cathode is made of a high-purity manganese dioxide mixed with carbon and in direct contact with the steel shell. The anode is a gelled mixture of amalgamated zinc powder and potassium hydroxide. A special nonwoven separator is used to keep particles of the anode or the cathode from moving around inside the cell. A piece of brass in the electrolyte collects the electrons.

The main claim to fame for alkaline cells is that they output up to twice as much current as carbon-zinc cells. But like carbon-zinc cells, alkaline cells have the same problem in that the output voltage falls as the battery's chemicals are used up. An advantage for alkaline cells is that they can be used at -20°F. Alkalines also lose relatively little to self-discharge and can be stored without losing their charge.

Rayovac, among others, is now selling what it calls a rechargeable alkaline battery system. The combination of improved venting to release gas, some adjustments to the chemistry and a special charger that controls current into the battery allows them to be recharged up to a hundred times. The batteries themselves are only slightly more expensive than disposable alkalines. Roughly $20 will buy four AA cells plus the charger. If you compare this to the $150 needed to buy a hundred new disposable alkaline batteries, the savings are obvious.

The workhorse rechargeable battery has long been the nickel-cadmium cell. Here, the positive electrode is made of the heavy metal cadmium, while the positive electrode is made of nickelic hydroxide. The electrolyte is the same potassium hydroxide mixed with water used in alkaline cells. Current Ni-Cd cells are designed so the gases formed inside the battery recombine and are not vented to the outside. The result is that Ni-Cds can be recharged as many as a thousand times if properly maintained. Even though the purchase price is high, Ni-Cds are actually the least expensive form of portable power.