The Easy Guide to Rechargeable Batteries:
In an effort to take away the science from what is in fact quite complex technology we will liken the rechargeable battery to a
bucket! Sounds crazy? Well maybe, but read on and you will see…..
A rechargeable battery is just a storage device, it stores electrical charge, once that energy is used up it can be refilled again and so on. Just like a bucket can be filled with water again and again. However, to make the example a bit more realistic, the bucket has a hole in it, so as we are filling it up water is leaking out all the time, and so as we slowly pour water into the bucket it will leak away, meaning that we may need to pour 12 litres of water into a 10 litre bucket before it reaches the brim. In the same way, it takes more energy to charge a battery than it will actually store. Remember the hole in the bucket? Well once its full, the hole is still there and so the bucket will slowly empty, in the same way a rechargeable battery will slowly lose its charge.
So what do all the numbers mean?
The first figure that you will be given is the “Capacity” usually expressed in “mAh” meaning milli-Ampere hours. This is basicallthe amount of current the battery will give multiplied by the number of hours that it will give it for, so obviously a bigger number means that the battery will last longer. So from the following table you can see how many hours a 2700mAh battery would in theory last.
|Rated current for device||Equation||Theoretical battery life|
This all seems very simple, however it doesn’t quite work this way. The battery is less efficient when larger currents are being drawn from it
|Rated current for device||Equation||Actual battery life|
So our “2700mAh” battery will last longer than expected if it is used in a device which takes little current, but with a device using a lot of current it will not last as long as expected. Generally battery manufacturers will give the capacity based on “0.2C discharge” This means at a discharge of 2/10 its rated capacity i.e. 540mA for a 2700mAh battery, There are some cheap batteries where the manufacturer gives the capacity based on a much slower rate of discharge, and so the capacity seems very good, like with the example above, at a discharge current of 100mA the battery would seem to have the capacity of 3000mAh. At this moment (May 2008) there are no manufacturers producing 3000mAh or greater AA size rechargeable batteries, with a 0.2C discharge rate, the technology simply hasn’t got there yet, although there are plenty who have products advertised as “3200mAh” or higher. Be very wary of both these manufacturers and the companies who sell them. If they are being dishonest in this way about the capacity of the battery, you must ask what else are they being dishonest about. Sanyo areregarded as the world leaders in the development of this type of rechargeable battery, so if they cannot produce a battery with a capacity like this, you can be totally sure that no-one else can.
What about the Voltage?
This is the one area that rechargeable batteries do differ from their non-rechargeable cousins.
The standard throw-away batteries (Zinc-carbon, zinc-chloride, alkaline etc) have a typical voltage of 1.5V whereas rechargeable (NiCd / NiMH) batteries are given as 1.2V These voltages are governed by the laws of physics / chemistry for the different technologies and so the voltages are the same for all manufacturers, although some claim 1.25V this is true for all rechargeables immediately they are charged, but once in use this quickly falls to 1.2V. The vast majority of devices this voltage difference does not make any difference. Take a look at the following graph, which shows how the voltage of a set of Duracell alkaline batteries changed over time when used in a digital camera. The graph also shows the way the voltage of a set of rechargeable Vapex 2700mAh batteries changed in exactly the same application.
As you can see the voltage of the Duracell alkaline batteries steadily dropped as they were used, but the voltage of the Vapex rechargeables remained fairly constant until they were almost exhausted. What is interesting to see is that the voltage of the alkaline batteries when they are around half used is the same as that of the rechargeables, so rather than being a different voltage, they are the same as the voltage of a part used alkaline.
What about other types of rechargeable battery, such as rechargeable alkaline, or rechargeable lithium?
Rechargeable alkaline batteries will give you the same voltage as a standard alkaline battery, but that is about all they have going for them.
They are expensive,
Require special charger
Have quite low capacity,
Can only be recharged <100 times whereas the NiMH type can be recharged at least 10 times as many times.
Lithium rechargeable batteries are better, but still there are drawbacks to their use in place of standard AA or AAA batteries:-
They are expensive
Require special charger
Have lower capacity than NiMH types
Have a voltage of 3V and so 1 lithium battery would need to be used in place of a pair of NiMH ones, which is not always
That being said, lithium battery technology is advancing fast and so the future will probably see more & more devices being designed to be powered by lithium batteries.
Written by Karl, who has written a total of 202 posts.
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