Part IA Linear Circuits and Devices

Contents

Batteries as sources of electrical power

This section is about batteries - those little power-sources in portable electrical devices. 

Types of battery

There are a number of different types of household batteries, used for a variety of purposes. 

The three main types are:

General purpose disposable household batteries include:

Dry-cell rechargeable batteries for household use include: 

Used for cordless power tools, personal stereos, portable telephones, lap-top computers, shavers, motorised toys etc, with a life of 4-5 years.  Using rechargeable batteries reduces the number of batteries requiring disposal, but 80% of them contain nickel cadmium, a known human carcinogen, and therefore need to be disposed of safely.

The Environmental issues

The average household uses 21 batteries a year. The UK generates 20 - 30,000 tons of waste general purpose batteries every year, but less than 1,000 tons are recycled.

In 2001 we bought 680 million batteries in the UK.  Most of these (89%) were general-purpose batteries.  This amounted to almost 19,000 tonnes of waste general purpose batteries requiring disposal in the UK.  

Currently, only a very small percentage of consumer disposable batteries are recycled (less than 2%) and most waste batteries are disposed of in landfill sites. The rate for recycling of consumer rechargeable batteries is estimated to be 5%. 

Whilst the exact chemical make-up varies from type to type (see below), most batteries contain heavy metals, which are the main cause for environmental concern. When disposed of incorrectly, these heavy metals may leak into the ground when the battery casing corrodes.  This can contribute to soil and water pollution and endanger wildlife. Cadmium, for example, can be toxic to aquatic invertebrates and can bio-accumulate in fish, which damages ecosystems and makes them unfit for human consumption. Some batteries, such as button cell batteries, also contain mercury, which has similarly hazardous properties.  Mercury is no longer being used in the manufacture of non-rechargeable batteries, except button cells where it is a functional component.  The major European battery suppliers have been offering mercury-free disposable batteries since 1994.

Recycling batteries

An increasing number of householders recognise the residual value of spent batteries and separate them from their general household waste for recycling.  A number of local authorities now collect waste household batteries in kerbside collections.  Rechargeable batteries can also be recycled once they have reached the end of their useful lives. 

Batteries contain a range of metals which can be reused as a secondary raw material. There are well-established methods for the recycling of most batteries containing lead, nickel-cadmium, nickel hydride and mercury. For some, such as newer nickel-hydride and lithium systems, recycling is still in the early stages.

UK's first battery dedicated recycling plant for household batteries was recently opened in West Bromwich. It is estimated it will be able to recycle up to 1800 tonnes per year; the opening of this plant is expected to stimulate a significant increase in domestic battery recycling rates in the UK.

What can we do to help?

A note on the cost of energy supplied by batteries

If we look at the choices we have for household batteries, and attempt to compare their cost, we should look at cost per kilowatt-hour.  the AC mains supply is billed in the same units, and the cost of 1 kWh - the quantity of energy required to run a typical one-bar electric fire for one hour) - is currently about 10 pence.  Disposable batteries are a far more expensive way to use energy. Depending on the type, capacity, and cost of the battery, disposables carry a price tag ranging from 300 to over 10,000 per kilowatt-hour. In contrast, the cost for using rechargeable batteries is of the order of 1 per kilowatt-hour!

Are rechargeable batteries cost-effective? 

On the basis of the argument above, in pure cost-of-energy terms, yes.  But, it depends on the application. For a portable CD-player, YES! Definitely worth it! For a calculator, where the battery life may be considerable, it's less obvious. The way to decide is to work out how much a set of rechargeables (plus the necessary charger) will cost, compared with the cost of disposable  batteries. Divide the results and you have a number representing how many sets of dry batteries you could purchase for the same outlay!

In some cases equipment manufacturers advise against the use of rechargeable batteries.  While there are few circumstances where the use of rechargeables would impact normal operation, it is important to be aware that rechargeables can go 'flat' quite suddenly - that is, their terminal voltage can fall to the point where the equipment stops working, without warning; and they will discharge quite noticeably even if no current is drawn.  Disposable batteries can be obtained with a shelf-life of a year or more; when installed they gradually decline in performance, with a prolonged period while the terminal voltage progressively falls.  They go flat like a long-distance runner getting tired, whereas rechargeable batteries go flat like a car running out of fuel.  Rechargeables should never be used in emergency equipment - e.g. smoke alarms, emergency lighting etc, since the rapid fall in terminal voltage may go unnoticed, and the device may fail to operate when it is needed.

How rechargeable batteries work 

A battery is made up of one or more separate cells.  However, the term battery is widely used for both batteries and single cells.  All batteries convert stored chemical energy into electrical energy. This is achieved through causing electrons to flow whenever there is an external conductive path between the cell's electrodes. Electrons flow as a result of an electro-chemical reaction between the cell's two electrodes that are separated by an electrolyte. The cell becomes discharged when the active materials inside the cell are depleted and the chemical reactions slow. The voltage provided by a cell depends on the electrode material, their surface area and material between the electrodes (electrolyte). Current flow stops when the connection between the electrodes is removed. Rechargeable cells operate on the same principle, except that the chemical reaction that occurs the discharge can be reversed if the cell is charged.  This involves causing a current to flow through the battery in the reverse direction, by applying an external voltage between the terminals.   When connected to an appropriate charger, cells convert electrical energy back into potential chemical energy. The process is repeated every time the cell is discharged and recharged. 

Different cells use different electrode materials and have different voltage outputs (1.2, 1.5, 2 and 3.6 volts for the types discussed here). Higher voltages are possible by connecting cells in series.  

The capacity of cells is determined by the materials used in its construction, and is expressed in amp-hours (Ah) or milliamp-hours (mAh). The approximate time that a battery will last per charge can be found by dividing the battery capacity (often printed on the battery itself) by the average current consumption of the device. 

Thus a 600 mAh battery can be expected to power a receiver that takes 60mA for 10 hours. 

Batteries can be visualised as consisting of one or more ideal cells with a resistor in series - the internal resistance. You won't find an actual resistor should you split open a battery pack, but the effect is the same. Certain battery types have higher values of internal resistance than others. High internal resistance doesn't matter if powering items that draw fairly low currents (eg a clock or small receiver).  However, if you are running something like a powerful torch or audio amplifier, owing to Ohm's Law a battery with a high internal resistance may not deliver the current asked of it.   

Nickel-cadmium (NiCad) 

Nickel-cadmium cells are the most commonly used rechargeable batteries in consumer applications.  They use nickel and cadmium as electrodes and aqueous potassium hydroxide as electrolyte. They come in similar sizes to non-rechargeable cells, and can often directly replace non-rechargeable alkaline or zinc-carbon cells. NiCads have a somewhat lower voltage output than non-rechargeable cells (1.2 vs 1.5 volts). This difference is not important in most cases. NiCad battery packs have voltages of 2.4, 3.6, 4.8, 6, 7.2, 9, 10.8 volts, etc. This corresponds to 2, 3, 4, 5, 6, 7, 8 and 9 cells respectively. NiCads perform best between 16 and 26 degrees Celsius. Their capacity is reduced at higher temperatures. Below 0 degrees, hydrogen gas is created and there is a risk of explosion when cells are used. NiCad batteries have a low internal resistance. This makes them good for equipment that draws large currents (eg portable transmitting gear). However, the low internal resistance means that extremely high currents (as much as 30 amps for a C-sized cell!) will flow if cells are short-circuited.  Short-circuiting should be avoided as it can cause heat build-up and cell damage. 

The normal charging rate is 10 per cent of a battery's capacity for 14 hours. For example, if a battery pack has a 600 mAh rating, its correct charging current is 60 mA. Because the charging process is not 100% efficient, the charger needs to be left running for about 14 hours instead of 10 hours. Higher charging currents are possible, but the charging time needs to be proportionally reduced. NiCads can be left on a trickle charger indefinitely if the charging current is reduced to 2% of the battery's amp-hour rating. Avoid the build up of heat during charging for long battery life. NiCad batteries require a constant current charger; ie one where the current provided to the battery is fixed over the entire charging period. Such a charger can be something as simple as an unregulated DC power supply with a series resistor to limit the charging current into the cells. If the charger's voltage and the battery's desired charging current is known, Ohm's Law can be used to calculate the correct series resistor value. Because NiCads have a low internal resistance, proper charging can occur with several cells in series. For best life, NiCads should not be discharged to less than 1.0 volt per cell. When charging, NiCads should read 1.45 volts per cell. If the cell voltage is higher during charging (eg 1.6 or 1.7 volts), the cell is faulty and should be discarded. 

The so-called 'memory effect' exhibited by NiCad cells is often discussed. This refers to the claimed tendency of cells not to deliver their rated voltage when placed in a charger before being fully discharged. Evidence suggests that true 'memory effect' is rare, and these observations are in fact due to continuous overcharging, which may cause electrolyte to crystallise inside the cell. Fortunately this effect can be overcome by subjecting the battery to one or more deep charge/discharge cycles. Another term often heard is cell reversal. This can occur when a battery of cells is discharged below its safe 1.0 volt per cell. During this discharge, differences between individual cells can lead to one cell becoming depleted before the rest. When this happens, the current generated from the remaining active cells will 'charge' the weakest cell, but in reverse polarity. This can lead to the release of gas and permanent damage to the battery pack. 

NiCads occasionally develop an internal short-circuit owing to the build up of crystals inside the cell, and this usually spells an end to its useful life.  A lifespan of 200 to 800 charges and discharges is typical for NiCad batteries. 

Nickel metal hydride (NiMH) 

Similar to NiCads, nickel-metal hydride cells provide 1.25 volts per cell.  The cadmium of the NiCad is replaced with metal hydrides, which are less of an environmental threat.  Battery manufacturers claim that NiMH cells do not suffer from the 'memory effect' and can be recharged up to 1000 times. NiMH cells are not as suitable as NiCads for extreme current loads, but do offer a greater capacity in the same cell size. A typical AA NiCad, (often used in cycle lamps and tape or CD players) may have a 750 mAh capacity, but a NiMH may provide 1100 mAh - 45 percent more. This makes NiMH cells a good choice for applications where long life is desired but current demands are not high.  The charger required for NiMH batteries is similar to that needed for NiCads; it must provide a constant-current, but typically the charging time needs to be lengthened in view of the larger cell capacity. 

The main enemy of rechargeable cells is heat. If cells get hot during charging, the charging current must be reduced to prevent damage. 

Lithium ion (Li-Ion)

Lithium ion cells are the most recent of the battery types discussed here to come onto the market. They offer higher cell voltage (3.6 volts) and greater capacity for a given volume. This makes them especially suitable for handheld equipment where long operating times are important, such as mobile phones. As an example, a typical lithium ion battery pack is 55x45x20mm but provides 7.2 volts with a 1100 mAh capacity. Lithium ion batteries are still quite expensive, but are coming into domestic use use through their inclusion in cameras, camcorders, palmtop computers and mobile phones. A special charger is required; one designed for NiCad or NiMH must not be used.

Some Links

Welcome to Battery University

Batteries as components - from: The educational encyclopedia

http://www.energizer.com/learning/howbatterieswork.asp

Battery Backup Application Handbook

http://www.wasteonline.org.uk/resources/InformationSheets/Batteries.htm

http://www.cycom.co.uk/howto/electricity_consumption_measurement.html

A Case Study on some sustainability/environmental issues relating to choice of single-use or rechargeable batteries to power a Walkman can be found here (PDF).  It is organised as a problem with a suggested solution, and you might like to discuss it with your supervisor.

Charging a battery


David Holburn  October 2005