This is a condition that affects Nickel Cadmium (NiCd) batteries. Because of the chemistry involved the NiCd battery was required to be fully discharged in an appliance before it was charged - failure to do this brought on a shortening of the useable working time or a reduction of 'capacity'.

The 'memory effect' implies that a NiCd cell can forget how to deliver total capacity charge if continually partially discharged to a shallow level. In other words, if a rechargeable 2 hour use battery only runs 5 minutes a day and is then regularly recharged, it will turn into a 5 minute battery.

With the technology of today's better quality manufacturers the 'memory effect' has largely been eliminated by the design of the internal battery anode plates to increase their oxidation efficiency during discharge.

Cheaply made NiCd cells are known to suffer problems of short life and consumers should always purchase well-known brands, particularly the Japanese technology.

Firstly there is no difference in the voltage of the cells; they both produce 1.2 volts per cell.

NiCd batteries have less capacity but around three times more cycle life than NiMH Batteries.

NiCd batteries are more suitable for high drain and fast charge appliances such as Cordless Power Tools and appliances running motors. Large capacity NiCds are still in use for Aviation, Solar Storage appliances, Emergency Generator Set Start and Theatre Lighting. The reputation of NiCds in these applications is one of reliability and longevity.

NiMH batteries have a higher capacity to size ratio but only have around one third of the cycle life of a NiCd battery. If a NiMH battery is treated like a NiCd and deep discharged regularly it will not have a long lifespan.

NiMH batteries are not known for having the 'memory effect' problems of NiCds. They are very suitable where higher capacity is required in appliances such as Mobile Cellular phones, Laptop Computers, Video Cameras and Hand Held Radios. A recent development of Japanese NiMH technology has seen the emergence of NiMH batteries for Cordless Power Tools, which has been the traditional area for NiCd chemistry.

The life of a battery depends on its use and abuse.

If it is a NiCd or NiMH battery one can expect:

For Lithium Ion (Li-ion) batteries the expected life is two years or less.

Sealed Lead Acid (SLA) batteries can last from 2 to 6 years depending on the application and the charging methods.

Here are some tell tale signs:

• It only runs your appliance for a short time before it indicates 'low battery' or it turns the appliance off.
• It acts erratically - eg. one day it seems OK - the next day it is not OK (especially NiMH batteries)
• The charger does not seem to charge the battery. (the problem could also be the charger)
• It won't run your appliance or even fire up

The answer is complicated. It depends on the amperage draw of your appliance (i.e. how much petrol per Kilometre it uses). You should be able to get some information from the manufacturer's instruction book.

Batteries will draw more 'petrol' from the tank according to the load that they are put under. For example a laptop computer will draw much more power whilst the hard disk is spinning that when the computer is idle with just the screen lit.

Battery size is a major factor in this instance. Both NiCd and NIMH cells are rated at 1.2 volts per cell but Lithium Ion cells are rated at 3.6 volts per cell (for the same size). Thus you need three times the number of NiMH and NiCd cells to meet the required voltage of one Lithium Ion cell.

Size-wise NiCd and NiMH cells are interchangeable and many appliance manufacturers start with NiCd cells and switch to NiMH batteries with no size design problems. Lithium Ion batteries are smaller because of the voltage and capacity advantage but they are not interchangeable with NiCd and NiMH batteries. As 'smaller is better' becomes the catch cry of customers there will be more Li-ion batteries being designed for new mobile and portable appliances.