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

• 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, from caustic chemicals generated by electrochemical action in the stomach.
• Lithium batteries can catch fire and burn violently if over-charged, shorted, punctured, or physically damaged in any way.
• Old batteries often leak, causing corrosions of the battery contacts, so preventing new batteries from working. Always remove spent batteries from equipment.
• Improperly disposed of batteries can damage the environment. Some types comntain toxic materials. Always dispose of used batteries in a responsible way.

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 of the galvanised nail would like to dissolve in the potato juice, each atom leaving behind a couple of negatively charged electrons as it does so. However, the zinc is more determined to dissolve than the copper, so it does so, shedding electrons and creating a positive charge in the potato juice. This positive charge discourages any copper atoms from dissolving. Meanwhile, the electrons abandoned by the zinc atoms flow through the voltmeter to the copper coin, where they find positively charged atoms in the potato juice less determined to dissolve than the zinc. The electrons neutralise these positively charged atoms, creating bubbles of oxygen.

N.B. Don't eat the potato, even cooked, when you finish the experiment!

Copper, zinc and potato juice don’t make the best battery but all batteries, like the potato battery, 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, as in the case of the familiar AA battery (or rather, cell) it only consists of one 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, in part at least, because the oxygen bubbles escape. 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.

Purchasing batteries

Domestic batteries

Zinc-carbon, zinc-chloride, alkaline and rechargeable NiMH batteries all come in the common C, D, AA, AAA and PP3 sizes and are widely available on the high street. Generally, you can expect to get what you pay for though a good quality battery may be just as good as a more expensive premium brand one.

Most can be found more cheaply online, especially if you buy them in multi-packs.

Very cheap batteries sold in discount stores are probably best avoided as they may be old stock or poor quality. In any case, check the best-before date.

In the case of rechargeable NiMH batteries, check the capacity. It may well be worth paying extra for higher capacities ones.

NiCd batteries have been almost completely superseded by NiMH. Their only advantages are that they can supply a heavier current (useful for powerful toy cars or boats and for power tools) and that they have a lower self-discharge rate. On the other hand, they contain toxic cadmium and suffer from the memory effect.

Lithium batteries

Lithium versions of domestic batteries are available in a few sizes, and are mainly useful for smoke alarms where their very long life is an asset.

Buying replacement laptop, mobile phone and tablet batteries is something of a minefield. Check online and find out the range of prices for the battery you need. At the cheapest end, these are likely to be poor quality or used batteries pulled from equipment, and are best avoided.

Manufacturer's branded batteries are usually sold at a high price, which is not necessarily a guarantee that they are fresh. (An unused lithium battery can deteriorate markedly in 2 - 3 years just sitting on a shelf!)

An upper-middle price may well be best value, but before buying check the seller's guarantee and returns policy. A personal recommendation for a seller may well be worth following.

External links

• The Battery University contains a wealth of information about all types of battery.