4.4.2 Battery parameters
Now that we have covered some background on electricity, I will return to discussing batteries.
What do you think would be the important characteristics of a battery for a portable IT device such as a camcorder or a mobile telephone?
The things that I thought of as the most important were:
Weight: if the device is to be portable it must not be too heavy
Size: for a portable device it needs to be kept small
Running time: the user does not want to have to replace or recharge the battery too frequently
Cost: it should not be too expensive.
I'd like to explore how these parameters are specified in more detail. To make the discussion more concrete I'll compare examples of some of the most widely available types of rechargeable battery. I shall look at two different sizes: AA and C. You are probably familiar with these sizes because AA is widely used in portable radios and CD players while the larger C size is used for torches, bicycle lights and portable stereos, among other things. Batteries described as LR6 and MN1500 are the same size as AAs, while R14 and MN1400 are the same size as Cs.
For each of the sizes I shall compare two battery technologies: nickel–cadmium (abbreviated to the chemical symbols NiCd, or called 'NiCad') and nickel–metal hydride (abbreviated to NiMH). At the time of writing, although both NiCd and NiMH batteries are widely available, NiCd batteries are declining rapidly in popularity. I shall have more to say about why this is later, but for the moment it is convenient for my purposes to compare the two technologies.
Batteries produce electricity by a chemical reaction, and nickel–cadmium or nickel–metal hydride refer to the chemicals used in the battery. All NiCd batteries will have some similar characteristics because they use the same chemistry, but different sizes and physical constructions will lead to some differences. Likewise for all NiMH batteries, or any other chemistry.
Some basic data on specific examples of each of these four batteries is given in Table 1.
|AA||NiCd||1.2 V||50 mm||15 mm||24 g||0.8 Ah||£1.40|
|AA||NiMH||1.2 V||50 mm||15 mm||24 g||2.1 Ah||£2.50|
|C||NiCd||1.2 V||60 mm||26 mm||75 g||1.7 Ah||£4.00|
|C||NiMH||1.2 V||60 mm||26 mm||80 g||3.5 Ah||£7.00|
FootnotesAh: amp-hours. Back to main text
All four batteries in Table 1 provide 1.2 volts. This is a consequence of the chemistry used, and the fact that each one is a single 'cell'. The cell is the basic building block of the battery, and to get higher voltages, cells can be connected together, as shown in Figure 14. This way of connecting cells or batteries, with the positive terminal of one connected to the negative terminal of the next, is know as connecting in series, and results in an output voltage that is the sum of the voltages of the individual batteries. The voltages just add together. You might be familiar with this way of connecting batteries from when you have put batteries in radios or torches, where they are nearly always connected in series.
Strictly the term 'battery' should only be used when there is a combination of cells used together, not for the single cells of an AA or C 'battery', but this is a distinction that is rarely adhered to.
Batteries using a different chemistry produce different voltages from a single cell. A single cell of an alkaline battery (the technology used for the most common non-rechargable batteries) for example, produces 1.5 volts. The chemistry of NiCd and NiMH is similar, so they produce the same voltage as each other
The battery size, AA or C, characterises the dimensions, expressed as the battery height and diameter.
You can see that the weights of the two AA batteries are the same, and there is only a small difference between the weights of the two batteries. In fact it is only when we come to the battery capacity and the prices that there is a significant difference between the NiCd and the NiMH batteries.
The NiMH batteries are more expensive but have a greater capacity. The units of capacity, Ah, are 'amp-hours', amps multiplied by hours. The idea behind this is that you can't specify a single value for the length of time a battery can be used because it depends upon the current being drawn from it. If you draw a lower current the battery will last longer. However if you multiply the value of the current being drawn by the length of time it can be used, you get a constant value: the battery capacity.
For example, a battery with a capacity of 1 Ah could supply 1 A for 1 hour, or else it could supply 2 A for half an hour or 0.5 A for 2 hours. More generally, if a battery can run at a current i for t hours, then its capacity is:
If you know the capacity of a battery and want to know how long it can be used to supply a given current, then you divide the capacity by the current.
The time for which the battery can be used, t , is given by:
The form of these equations should be starting to be familiar by now. Again, the relationship between capacity, running time (t) and current(i) can be represented by a formula triangle. Draw one now.
The relationship between capacity, and t was presented in two forms:
From either of these, and from what you were told previously, you can see that capacity is the quantity that should go at the top of the triangle, so that the triangle is in either of the forms shown in Figure 15.
In a test, it is found that a battery can be used for 10 hours supplying a current of 0.4 A.
What is the capacity of the battery in Ah?
If a current of 0.3 A is flowing from the battery, how long can it be used for?
capacity=i × t = 0.4 × 10 Ah = 4 Ah
time battery can be used at 0.3 A:
This is 13 hours and 20 minutes.
It is important to appreciate that the figures quoted for capacity and the length of time a battery can be used depend very strongly on the way it is being used and the temperature. Also, a battery does not just suddenly run out of electricity – it is not like a car running out of petrol where suddenly there is no more and it stops. Rather, while a battery is being used the voltage falls and as the battery runs out (goes 'flat') its voltage drops more quickly. When specifying a battery's capacity a lower limit to the acceptable voltage is specified, and the battery is defined as flat when that lower limit is reached.
Bearing all this in mind, the values for battery capacity are nevertheless useful for comparisons and estimates of battery performance.
How long could a device which uses 0.1 A be run from each of the four batteries in Table 1?
AA, NiCd. Capacity of 0.8 Ah, so it can run for 0.8/0.1 h=8 hours.
AA NiMH. Capacity of 2.1 Ah, so it can run for 2.1/0.1 h=21 hours.
C NiCd. Capacity of 1.7 Ah, so it can run for 1.7/0.1 h=17 hours.
C NiMH. Capacity of 3.5 Ah, so it can run for 3.5/0.1 h=35 hours.
As you can see from Table 1, for the batteries considered here, an NiMH battery has a greater capacity than an NiCd battery of the same size and weight, but costs more. This is generally true for NiMH compared with NiCd batteries, although as NiMH become more widely used their prices are getting lower. These are not the only considerations when choosing batteries. For example, with rechargeable batteries such as these there are also issues as to how easy they are to recharge and how many times they can be recharged. On these considerations, broadly speaking NiMH batteries come out better than NiCd batteries. Another significant consideration is the fact that cadmium is highly toxic (poisonous) and so NiCd batteries should be handled carefully and should not be disposed of with other waste, but should be recycled so that the cadmium is extracted safely. For all these reasons, NiCd batteries are falling out of favour.
Have a look for any batteries that you have, especially rechargeable batteries, and see if they say what voltage they are and what capacity they have. Compare them with those in Table 1. Alternatively, if you don't have any rechargable batteries, you can look for adverts to see what information you can find. (Non-rechargeable batteries don't generally quote their capacity.)
I had an AA NiCd battery. It was labelled as '1.2 V, 0.65 Ah'. This is lower capacity than the AA NiCd in Table 1. I weighed it on the kitchen scales (on a piece of paper as a precaution, remembering the toxicity of cadmium) and found it was about 55 g. Clearly mine is inferior to some NiCds – heavier and lower capacity – but I don't recall how much it cost (I bought it several years ago).
I also found both NiCds and NiMH batteries advertised in catalogues that I had at home. The catalogue listed NiCd and NiMH batteries with similar capacity to those in Table 1. Some of them quoted the capacity in mAh, which is milliamp-hours. The value in mAh needs to be divided by 1000 to get Ah so, for example, the capacity of a battery advertised as a 'super high capacity' NiMH AA battery was given as 2300 mAh. This is equal to 2.3 Ah.
Another important type of battery is based on chemical reactions involving lithium. 'Lithium Ion' (Li-ion) batteries are commonly used in laptop computers and other portable IT equipment.
A complication when comparing Li-ion batteries with NiCd and NiMH batteries is that the voltage delivered by an Li-ion cell is around 3.6 volts, compared with the 1.2 volts of NiCd and NiMH cells. To make fair comparisons of capacity you need to be looking at supplies at the same voltage.
If some equipment requires a 3.6 volt power supply it can use a single Li-ion cell. How many NiCd or NiMH cells would it need, and how should they be connected?
To get 3.6 volts from NiCd or NiMH cells, which are each 1.2 volts, three cells would need to be connected in series, as shown in Figure 16.
When the different voltages have been taken into account, the capacities of battery packs using Li-ion batteries are greater than packs using NiCd and NiMH batteries for a given size and weight.
There are other pros and cons to Li-ion batteries, and a particular disadvantage is the need to control more carefully the charging and discharging of the batteries, both to maximise the battery life and for safety reasons.