Batteries 

A battery consists of  one or more cells, each of which use stored chemical energy to produce electrical energy, There are many types of cells and these are combined in a multitude of ways to produce the many battery types in use.

Batteries are divided in to primary and secondary:

• Primary batteries – have an irreversible chemical action and can only be used once
• Secondary batteries – have a reversible chemical action and can be recharged

The modern battery came into being due mainly to work carried out by Alessandro Volta at the end of the 18th century. He was able to make a cell by using silver and zinc discs sandwiched between  cardboard soaked in salt water.  By combining many cells in series he created what has become known as the Voltaic Pile.

Theory

When two dissimilar metals [electrodes] are placed in an electrolyte, ions will flow from the more reactive electrode to the other, building up charge.  This build up of charge is neutralised by the flow of electrons [current] in the circuit.

Splug.com (physics) have a good article which looks at the chemistry of how a cell works in more detail.  A link to the page is included below.

Approximate EMFs of Cells

Capacity (Ampere-hour)

Capacity measures the amount of energy stored in a battery.  In a battery, capacity is measured in ampere-hours (Ah).  This is the amount of amperes which can be drawn from the battery for one hour.  If a lower current is drawn the battery will last longer than an hour (conversely if more current is drawn it will last less).

In practice, varying terminal voltages, temperature and other factors make the assessment of capacity complex.

Internal resistance

Batteries are not perfect devices.  To understand their operation they can be modelled as a perfect voltage source (one with zero internal resistance) in series with an internal resistance. The voltage V_t at the terminals of a battery is given by:

• V_oc is the open-circuit voltage of the battery
• R_i is the battery's internal resistance
• I is the current flowing through the battery

This can be rearranged to calculate the internal resistance given the other quantities:

As the battery runs down and its internal resistance increases.  Consequently the voltage drop across the internal resistance increases and the terminal voltage decreases.

Battery Types

Alkaline Battery - invented by Canadian engineer Lewis Urry in the 1950s, alkaline batteries are a type of primary battery or rechargeable battery which utilizes alkaline as the electrolyte . Alkaline batteries have a higher output than many other battery types and account for the largest share to the battery market worldwide.

Common Consumer Battery Sizes

There are a vast number of battery types in use.  The table below lists the most common, which are used in everyday appliances.  For a more complete list, Wikipedia has a good page – see the external links section below.

Diagram	Size	Capacity (mAh)	V	ANSI NEDA	IEC	Diam. (mm)	Height (mm)	Mass (g)
	AAAA	625	1.5	25A	LR8D425	8.3	42.5	6.5
	N	1000	1.5	910A	LR1	12	30.2	9
	AAA	1250	1.5	24A	LR03	10.5	44.5	11.5
	AA	2850	1.5	15A	LR6	14.5	50.5	23
	J	625	6	1412A	4LR61	prismatic	48.5	30
	9V	625	9	1604A	6LR61	prismatic	48.5	45.6
	C	8350	1.5	14A	LR14	26.2	50	66.2
	D	20500	1.5	13A	LR20	34.2	61.5	148

Notes: capacities are typical only.

Battery Life

Primary Batteries – naturally lose 8% to 20% per year at a temperature of about 20°-30°C due to self discharge.  Lowering the battery temperature (keeping in the fridge for example, can aid in reducing the self discharge.

Secondary Batteries – generally self-discharge quicker than primary batteries.  As batteries are recharged, they lose capacity – typical recharge cycles are in the range of 500 to 1000 charges.

Battery life can be seriously affected by temperature.  The link [effect of temperature on lead batteries], at the end of the article, explains this phenomena in detail.  

External Links

• Splug.com - Electrical Cells and Batteries
  http://www.splung.com/content/sid/3/page/batteries, accessed April 2012
• Wikipedia - List of battery sizes
  http://en.wikipedia.org/wiki/List_of_battery_sizes, accessed April 2012