Batteries

All you need to know about batteries.

Summary

There are various types of battery based on different chemistries. To get the best out of them and to avoid problems you need a basic understanding of their advantages and disadvantages, as well as a little about purchasing, maintaining and disposing of them.

Safety

• Old batteries often leak, causing corrosions of the battery contacts, so preventing new batteries from working.
• Lithium batteries can catch fire and burn violently if over-charged, shorted, punctured, or physically damaged in any way.
• Button cells must be kept away from small children. If swallowed, this is a medical emergency as death can result in just a few hours, as a result of electrochemical action in the stomach.

How batteries work

(You can skip this section if you like, though a little more knowledge than you actually need is always helpful.)

You can make a very simple battery by pushing a copper coin and a galvanised nail into a potato (but not touching each other). Touch one probe of a voltmeter on the coin and the other on the nail. It should show around 1 volt.

Both the copper of the coin and the zinc on the galvanised nail want to dissolve in the potato juice, each atom leaving behind a couple of negatively charged electrons in the process. However, the zinc is more eager to dissolve than the copper, so it does so, shedding electrons which flow through the voltmeter to the copper coin. There, they flow back into the potato juice and neutralise positively charged atoms, forming bubbles of oxygen, and at the same time holding the copper back from disolving.

Copper, zinc and potato juice don’t make the best battery, but all batteries consist of two electrodes of different metals (or a metal and carbon) immersed in an electrically conducting electrolyte Different formulations give different voltages and other characteristics such as cost, weight and capacity.

Strictly, the potato isn’t a battery but a galvanic cell, usually just called a cell. Properly speaking, a battery consists of a number of cells connected together, each adding to the “push” of the one behind it. In this way, 6 cells each of 1.5V can be combined into a battery giving a total output of 9V.

In practice, everybody talks about a battery, even if it only consists of one cell, as in the case of the familiar AA battery (or rather, cell). A 9V alkaline battery consists of 6 cells, each giving 1.5V.

The potato is an example of a primary cell. In operation, it consumes some of its constituents, in this case the zinc in the galvanised nail dissolves. The process is not reversible. A rechargeable battery is known as a secondary battery, and uses constituents which can be restored to their original state by driving an electric current back through it in the reverse direction. Even if the chemistry is completely reversed, the physical properties of the electrodes will generally degrade to some extent, limiting the number of discharge/recharge cycles that are possible.

Non-rechargeable (primary) batteries

Type	Advantages	Disadvantages	Comments
Zinc carbon and zinc chloride	Cheap. Available in standard shapes and sizes.	Short life and slow death.	Zinc chloride is a heavier duty version of zinc carbon. Alkaline batteries are preferred in almost all applications.
Alkaline	Good life at a reasonable cost. Available in standard shapes and sizes, and also as button cells as cheap alternatives to silver.		This is the most economic general purpose type.
Silver oxide	High capacity	Expensive	Normally only available in small sizes as button cells for watches and calculators, on account of the cost.
Zinc-air	Very high capacity	Short life once the tab has been remove to activate it by letting the air in	Used in hearing aids in the form of button cells.
Lithium	Very long shelf and service lives.	Relatively expensive.	Various different lithium-based chemistries have somewhat different characteristics. Mainly used in smoke alarms and cameras.

Rechargeable (secondary) batteries

Type - Nominal voltage	Advantages	Disadvantages	Comments
Lead Acid - 2V	Rugged and reasonably cheap.	Lead is toxic and cannot be disposed of in landfill.	At its best when mainly kept fully charged, hence widely used for (petrol/diesel) car batteries, uninterruptable power supplies, emergency lighting, security alarms, but also used in milk floats, golf buggies etc. Smaller sizes generally come as non-spillable sealed units.
Nickel Cadmium (NiCd) - 1.2V	Very rugged Can deliver a high current and accept a very fast charge	Contains cadmium, which is toxic Repeated partial discharge causes a "memory effect"	Mainly used nowadays in power tools and radio controlled model boats and cars.
Nickel Metal Hydride (NiMH) - 1.2V	Greater capacity than NiCd No toxic cadmium No memory effect Available as direct replacements for common sizes of non-rechargeable batteries	Relatively high self-discharge rate Low self-discharge variants have reduced capacity May not work in all equipment designed to accept alkaline batteries on account of a lower voltage (1,2V as opposed to 1,5V for alkaline)	NiMH has replaced NiCd in all but specialist applications.
Lithium - 3.7V	Very high energy density	Very dangerous if abused. Protection circuitry is essential	As with primary lithium batteries, there are various chemistries and formulations with somewhat different characteristics.

External links

• Battery University