What is 'memory effect'? How can we keep away from it?
'Memory effect' is primarily associated with the Ni-Cd batteries, and with Ni-MH batteries in a worse quantity. It means that if a battery is not fully discharged before recharging over a period of time, the battery then 'forgets' that it has capability for discharge further. To make it simple, suppose if you charge your battery fully and use just 45% of the total capacity before you charge it again, then the remaining 55% capacity of the battery remains unused. If this continues for long, then ultimately the battery will not remember its extra 55% capacity, which you left unused. Even though the battery works, it works only at 55% of its total capacity.
You should always follow the process of fully charging and then fully discharging your battery to avoid the problem of 'memory effect'. It must be done once in every two to three weeks. Batteries are discharged when you unplug the AC adapter device and let it run on the battery till the moment it stops to function. It provides the good state of your battery.
Mh and Li-Ion batteries remain unaffected by the memory effect.
The difference between Ni-Cd, Ni-MH and Li-Ion
Li-ion (Lithium Ion)
Li-ion is one of the latest types of cells which are available. It is one of the least heavy batteries in the market and provides better power output when compared with other cell types. This kind of cell is also free from 'memory effect'. Downside of this battery is the price since it involves the maximum engineering costs. This cell type is the most common cell type currently available for laptop computers.
Ni-MH (Nickel Metal Hydride)
It is the most common cell type available for laptop computers right now. This battery is cheaper while manufacturing and so it is cheaply available in the market. However, this cell type is vulnerable to memory effect, so it is imperative you take care of your Ni-MH battery well to avoid the goof-up.
Ni-Cd (Nickel Cadmium)
It is found in the oldest laptops, since it is one of the oldest cell types. Due to its lack of technological advances, its ability to deal with bigger loads is vulnerable. That's why it's always found in repair tools for the smooth functioning. It is highly prone to memory effect so it's better to get maximum out of this battery.