| Batteries are used to provide electrical power to portable electronic systems. |
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How does it operate?
In most electronic systems, nearly all the power from the battery is used by the output device(s). The first task in selecting a battery is therefore to investigate the output devices needed and to find:
the voltage they need (the battery will provide this voltage);
the current consumed (this will directly influence the battery’s useful life, depending on the battery ‘capacity’ – see below).
An ‘ideal’ battery provides a steady voltage, and as much current as is needed. In practice, the voltage available from a battery falls as it is discharged and as the current drawn increases.
Environmental Issues
The average UK household uses 21 batteries a year. On average, only one of these is recycled. The others go into landfill sites. Much of this waste could be eliminated by using rechargeable batteries.
Some battery types contain cadmium or mercury. These chemicals cause environmental damage and batteries that contain them should not be disposed of in a rubbish bin. Consult your local waste disposal service.
Key features of a battery are:
Type
The battery ‘type’ refers to the materials that are used to make the battery. Common battery types include: zinc chloride, alkaline, nickel cadmium, nickel hydride, lithium polymer, lithium iron sulphide, lead acid, silver oxide and lithium manganese.
Case
Common cases include AAA, AA, C, D, PP3, coin cell and button cell. The type of case provides details of the shape and dimensions of the battery.
Voltage
Manufacturers and suppliers usually provide information about the voltage that a battery provides when it is fully charged, and not supplying current.
In practice, the voltage a battery provides falls as it is discharged by providing current to the electronic system. Some manufacturers provide graphs of typical discharge characteristics in data sheets.
In addition, the voltage a battery actually provides when a current is drawn is less than its voltage when it is not supplying a current. This is due to the battery’s ‘internal resistance’ (see below).
Capacity
The capacity of a battery indicates how much current it can provide over a period of time. Capacity is usually specified in milli Amp hours (mAh) – the current provided multiplied by the time to discharge. For example, if a battery has a capacity of 2000 mAh, it could provide a current of 2000mA for one hour before it was exhausted, or 1000mA for two hours, or 100mA for 20 hours.
The capacity of a battery depends on the battery type (some types last longer than others) and on its case (generally speaking, the bigger the battery, the higher its capacity).
The tables below show battery capacities for a range of battery types and cases.
Internal Resistance
A practical battery behaves as if it were made up of an ‘ideal’ battery (providing a constant ‘ideal’ voltage VI) in series with a resistor (called the battery’s internal resistance – RI) inside the practical battery.
In consequence, when a battery provides a current (IB) the voltage across the battery terminals (VB) is VI minus the voltage dropped across the internal resistance (IB RI). So:
The most important point to bear in mind about the internal resistance of a battery is that the battery type should be chosen so that its internal resistance is well below the resistance of the load. Otherwise most of the battery’s energy will be wasted as heat in the battery’s internal resistance – not in providing energy to the load.
The internal resistance depends on the battery type and case and generally increases as the battery discharges. The table below shows typical values for a range of battery types and cases.
Details of Battery Types
Probably the battery case most widely used in schools is the ‘AA’. The table below gives details of common AA battery types.
The data given for the ‘Initial voltage’ (the voltage of a ‘fresh’ battery) is taken from manufacturers’ data sheets. Some manufacturers’ data sheets give the battery Capacity directly. In some cases the Capacity has been estimated from graphs of the battery discharge characteristics given in data sheets.
Data on internal resistance is not always quoted by manufacturers. In those cases the estimated figure quoted is based on measurements made on one battery sample. The figures given for internal resistance are for a fresh battery. The internal resistance increases as the battery discharges.
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Non-rechargeable batteries |
| Zinc Chloride |
| This is the cheapest type of commonly available battery. If the battery type is not quoted on the packaging it is usually of this type. It has a low capacity. |
| Initial voltage: 1.5V |
Capacity: 680mAhrs |
Initial internal resistance: 0.3W (estimated) | |
| Alkaline |
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This battery type is widely available. The data given in the table below is for the Duracell Plus alkaline and the (electrically identical) Procell alkaline. Procell alkaline batteries cost about three times as much as the cheapest zinc chloride batteries but have four times their capacity, so are better value for money than zinc chloride.
The data sheet for GP alkaline batteries (GP15A) indicates that they have a lower capacity (about 2,300mAhrs) than Procell batteries but they cost only about 50% more than zinc chloride types, and still have more than three times as much capacity. They therefore represent much better value for money.
Power One alkaline batteries have a capacity of 2,600mAhrs and cost about 10% more than GP alkaline, so they represent vary similar value for money. They can be purchased in bulk packs of 500, which gives a further saving of about 10%. |
| Initial voltage: 1.5V |
Capacity: 2,700mAhrs |
Initial internal resistance: 0.12W | |
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Ultra Alkaline
The Duracell Ultra M3 batteries have the highest capacity of any of the batteries investigated. The data below is for the Duracell Ultra M3. However, they are more than three times as expensive as the GP15 alkaline batteries, and their capacity is only about 40% greater. They are therefore only recommended in situations where maximum battery life is essential.
The GP Ultra alkaline has a capacity of about 2,400mAhrs, less than Power One alkaline, and they cost about 20% more. |
| Initial voltage: 1.5V |
Capacity: 3,200mAhrs |
Initial internal resistance: 0.08W | |
| Nickel Zinc |
This battery type does not appear to have any clear advantages over other battery types. |
| Initial voltage: 1.5V |
Capacity: 920mAhrs |
Initial internal resistance: 0.2W (estimated) | |
| Lithium iron sulphide |
This battery type does not have as high a capacity as the Duracell Ultra M3 but costs about three times as much. |
| Initial voltage: 1.5V |
Capacity: 2,900mAhrs |
Initial internal resistance: 0.1W (estimated) | |
Rechargeable batteries
In principle, rechargeable batteries can help to reduce waste and pollution. However, there are potential problems in using some types of rechargeable batteries in school electronics. In particular, because the internal resistance of nickel hydride and nickel cadmium batteries is much lower than non-rechargeable types, they deliver a much larger current if a short circuit is accidentally applied across them. If this happens, the battery can overheat and possibly explode. This can present a health hazard and can also damage the battery.
If the battery can be completely enclosed (to prevent short circuits) and a voltage regulator (hotlink to data sheet) can be included inside the enclosure, then the voltage regulator will prevent hazards and damage to the battery because it includes current limiting circuitry.
| Nickel Hydride (NiMH) |
| These batteries are sometimes referred to as nickel metal hydride. It is important to note that the initial voltage of these batteries is 1.2V, rather than the 1.5V of most non-rechargeable batteries. NiMH batteries are about five times as expensive as alkaline manganese batteries, and their capacity is about 20% less. However, they can be recharged more than 500 times, so they can give large cost and environmental savings. The values quoted below are for GP high capacity NiMH batteries – type GP210AAH. |
| Initial voltage: 1.2V |
Capacity: 2,100mAhrs |
Initial internal resistance: 0.024W | |
| Nickel Cadmium (NiCd) |
| Nickel cadmium batteries have many of the same features as nickel hydride batteries. However, they contain cadmium, which is a serious environmental pollutant (http://www.liv.ac.uk/~preston/metals.htm). The capacity of NiCd batteries is not as high as NiMH. For these reasons NiMH is superior to NiCd for use in schools. The values quoted below are for GP NiCd batteries – type GP100AAS. |
| Initial voltage: 1.2V |
Capacity: 1,000mAhrs |
Initial internal resistance: 0.018W | |
| Rechargeable Alkaline |
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Rechargeable alkaline batteries have several important advantages, for use in schools, over NiMH and NiCd. The most important is that their internal resistance is higher, and so they do not present a safety hazard if a short circuit is applied across them.
The initial voltage is close to the value of typical non-rechargeable batteries, rather than the 1.2V of NiMH and NiCd.
One limitation of rechargeable alkaline batteries is that they have a limited cycle life. The manufacturer’s data sheet states that this can vary between 25 and 500+, depending on the rate of discharge, end point voltage and depth of discharge.
The batteries cost about five times as much as GP alkaline batteries, so even if their life is limited to 25 cycles, they represent a cost saving by a factor of five, as well as greatly reducing waste. |
| Initial voltage: 1.57V |
Capacity: 2,000mAhrs |
Initial internal resistance: 0.15W | |
| Rechargeable Alkaline XL |
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Rechargeable alkaline XL batteries are almost identical electrically with standard rechargeable alkaline batteries. They key difference is that their cycle life is increased to 50 to 500+ cycles.
They are only slightly more expensive than standard rechargeable alkaline batteries, and so they represent much better value for money in the longer term. |
| Initial voltage: 1.57V |
Capacity: 2,000mAhrs |
Initial internal resistance: 0.15W | |
| Lithium Polymer |
| Lithium polymer batteries are widely used in portable computers. Recently lithium polymer batteries have become available in a PP3 case (but not AA). They have the advantages of relatively high capacity – almost as high as alkaline, fast charging – one hour (a special charger is needed), a lifetime of at least 100 cycles (so they are at least 10 times more cost effective than disposable alkaline batteries), and the supplier has conducted tests that indicate that they are protected by internal circuitry against damage or danger when short circuited. |
| Initial voltage: 8.4V |
Capacity: 500mAhrs |
Initial internal resistance: internally protected | |
Safe battery recharging
There are important safety limits to the current and voltage that can be used to recharge batteries. Commercial battery rechargers work within these limits. However, if you are planning to build your own recharger, these limits need to be observed.
Most modern batteries can be charged at quite a high current. For example you could charge a 2,000mAhrs battery with a 500mA current for just over 4hrs and it would be fully charged. However, if you keep on charging it beyond that 4hrs you could seriously damage the battery (or even cause an explosion). NiMH batteries have a protective mechanism when they get overcharged and attempt to dissipate the excess current as heat. However they can usually only managed to discharge one tenth of their total current capacity as heat. What this means in practical terms is that if you charge a 1,300mAhrs battery with 130mA then it will survive, but get warm if you overcharge it for a while. However if you overcharge it with a 500mA current the risk of explosion occurs.
Cases
The information above applies to AA case batteries – which are widely used in schools. There are situations where other battery sizes are useful, and many of the battery types described above are available in a range of cases.
To estimate the capacity of a particular type of battery in the relevant case size from the table below, multiply the capacity of the AA battery of the same type (listed above) by the ‘Case Capacity Factor’. For example, if we wanted to find the capacity of a Power One alkaline C battery:
- the capacity of a Power One alkaline AA battery is 2,600mAhrs;
- the Case Capacity Factor for a C battery is 2.9;
- so the capacity of a Power One alkaline C battery should be 2,600 ´ 2.9, which is about 7,550mAhrs.
| Case |
Dimensions |
Case Capacity Factor |
Notes |
| D |
L 61.5 mm
D 34.2 mm |
6.7 |
High capacity. Large and heavy. |
| C |
L 50 mm
D 26.2 mm |
2.9 |
Quite high capacity. |
| AA |
L 51 mm
D 15 mm |
1.0 |
Widely used. |
| AAA |
L 44.5 mm
D 10.5 mm |
0.44 |
Useful if small size or weight is important. |
| PP3 |
H 48.5 mm
L 26.5 mm
W 17.5 mm |
0.20 |
Initial voltage is about 9V. |
Battery Holders
Once the battery type and case has been selected, a suitable battery holder can be chosen. There are a number of points to consider when selecting a suitable holder:
Holders are available for one, two, three or more batteries. The batteries are automatically connected in series, and so their voltages are added together. For example, a battery holder for three AA (nominally 1.5V) batteries would provide a voltage of 4.5V.
Electrical connection can be made between the battery holder and the circuit board with: flying leads (these are cheapest, but not very flexible); solder tags (wires are soldered to the tags); press studs (connection is made with a detachable battery clip); or PCB mounting pins (these make the circuit and battery holder a single integrated unit).
Most battery holders have screw holes for mounting the holder on the PCB or the product case. It is important to take these into account when designing the PCB or the product case.
Coin and Button Cells
Coin and button cells are used in situations where small size and/or weight are very important. They have considerably less capacity than ‘standard’ batteries.
Safety – coin and button cells are small, and could be swallowed by young children. They should not be used in products for children unless they are securely contained.
There are a very wide range of coin and button cells available. However, to be of practical value in school electronics, they need to be fitted into specialised battery holders and only a limited range of these are available. The coin and button cells described below are limited to those where suitable holders are readily available.
Button Cells
Only one button cell holder appears to be readily available (Rapid Order code 18-0077). This holder is PCB-mounted on the track side of the PCB, and a pad needs to be placed in the centre of the holder to make electrical contact with the button cell. |
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Two suitable batteries are available. Both have a diameter of 7.9mm and a height of 3.6mm.
The Silver Oxide SR51 button cell has as and initial voltage of 1.55V, a capacity of 40mAhrs and an estimated initial internal resistance of 7W. |
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The Alkaline Manganese L736 button cell has an initial voltage of 1.5V, a capacity of 35mAhrs.
The L736 has a slightly lower capacity but is about half the cost of the SR51.
Zinc air button cells are not recommended for school use because they contain mercury which is a serious environmental pollutant, and so they cannot be disposed of in landfill sites. |
Coin Cells
Two widely available PCB-mounting holders for coin cells are the 2032 style and 2430 style – suitable for coin cells with corresponding codes. |
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Lithium coin cells that fit into these two holders are the CR2032 and the CR2430.
The CR2032 has an initial voltage of 3.0V, a capacity of 170mAhrs and an estimated initial internal resistance of 15W. However, this rises steadily as the battery discharges (see below). The cell has a diameter of 20mm and a height of 3.2mm. |
The CR2430 has an initial voltage of 3.0V, a capacity of 360mAhrs and an estimated initial internal resistance of 15W which also rises steadily as the battery discharges. The cell has a diameter of 24.5mm and a height of 3.0mm. |
Detailed measurements on the CR2032 and the CR2430 show that, as well as having a relatively high initial internal resistance, their internal resistance increases significantly as they discharge. In consequence, the current they can deliver is limited to a few mA.
The graph shows how the voltage and internal resistance of the CR2032 vary as it is discharged. The blue curve (labelled VI) shows how the ideal voltage of the battery varies with time as it is discharged. The red curve (labelled VB) shows the battery voltage when it is supplying current to a 150W resistor. The black curve (labelled RI) shows how the internal resistance varies. The time shown on the graph is the time that the battery has been supplying current to the 150W resistor.
As can be seen, the internal resistance steadily increases from its initial value of 15W to about 70W after five hours. That is why VB is only about two thirds of the value of VI when delivering current to the 150W resistor after five hours.
Because of their high internal resistance (particularly when they are partially discharged) lithium coin cells cannot be used to deliver currents of more than a few mA.
Possible applications
The key application of batteries is providing power to electronic systems.
Making
The main points to consider are:
- Making sure that the battery is inserted into the battery holder the correct way round (otherwise the voltage supply will have the negative and positive connections interchanged, which can damage the electronic system)
- If the battery is connected to the PCB with flying leads, connecting them the right way round (for the same reason).
Testing
- Check the battery voltage with a multimeter before inserting it in the battery holder. Then check that the correct supply voltage (of the correct polarity) is present on the power supply tracks of the PCB.
Fault finding
If the battery voltage is correct, but the voltage on the PCB track is not, check:
- the connections to the PCB;
- that there is no short circuit between the power supply tracks on the PCB;
- for any gaps or cracks in the tracks on the PCB.
Alternatives
Web links
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