Re: What is the memory effect of rechargeable batteries?

The short answer is that some rechargeable NiCd batteries (pronounced ni-
cad with a long i) seem to be unable to take a full charge if they have 
been repeatedly discharged to the same level. There is quite a bit of 
misunderstanding about this phenomenon. When you look at some of the 
sources I found, you should be able to see that not everyone agrees on how 
important this effect really is. It does seem agreed that there are other 
problems that may be more important in the poor performance of NiCd 
batteries than the famous memory effect.

Memory: Myth or Fact? 

The word memory was originally derived from cyclic memory, meaning that a 
NiCd battery can remember how much
discharge was required on previous discharges. Improvements in battery 
technology have virtually eliminated this
phenomenon. Tests performed at a Black & Decker lab, for example, showed 
that the effects of cyclic memory were
so small that they could only be detected with sensitive instruments. After 
the same battery was discharged for different
lengths of time, the cyclic memory phenomenon could no longer be detected. 

The problem with the modern NiCd battery is not so much the cyclic memory 
but the effects of crystalline formation. In
most cases, however, there is a combination of the two phenomenon (from now 
on when memory is mentioned we
refer to crystalline formation.) The active materials of a NiCd battery 
(nickel and cadmium) are present in finely divided
crystals. In a good cell, these crystals remain small, obtaining maximum 
surface area. When the memory phenomenon
occurs, the crystals grow and drastically reduce the surface area. The 
result is a voltage depression which leads to a loss
of performance. Some of the capacity may still be present but cannot be 
retrieved because of the battery’s low voltage
table. In advanced stages, the sharp edges of the crystals grow through the 
separator, causing high self-discharge or an
electrical short.

Another form of memory that occurs on some cells is the formation of an 
inter-metallic compound of nickel and cadmium
which ties up some of the needed cadmium and creates extra resistance in 
the cell. Reconditioning by deep discharge
helps to break up this compound and reverses the capacity loss.

2.Reconditioning is a slow deep discharge applied below the end-of-
discharge voltage threshold. During this gradual depletion of the remaining 
energy, the crystalline build-up on the cell plates dissolves and the NiCd 
battery is commonly restored. Typically the End-of-Recondition voltage is 
0.2 - 0.4 volts per cell. 
3.There are two types of memory:
       a. Crystalline Formation (what we call memory), caused by lack of 
          proper exercise. The active material (chemicals) grow 
          crystalline structures which limit the capacity of the battery.
       b. Cyclic Memory, meaning the NiCd remembers how much discharge was 
          required on previous discharges and  will not discharge below 
           that level . With the modern NiCd technology this form of memory
           is almost immeasurable. 

Hope this helps. It sure gives me alternate explanations for the poor cost-
benefit experience that I have had with NiCd's.

David Winsemius