| |
VK3ABK > INFO 04.05.03 19:44l 74 Lines 3974 Bytes #999 (0) @ WW
BID : 55395_VK3KAY
Read: DB0FHN GUEST
Subj: Re: Nicads and Memory.
Path: DB0FHN<DB0ZWI<DB0CHZ<DB0ERF<DB0MRW<DB0RGB<OK0PPL<RZ6HXA<SP7MGD<VK3KAY
Sent: 030428/0436Z @:VK3KAY.#WEV.VIC.AUS.OC #:55395 [Wendouree] $:55395_VK3KAY
From: VK3ABK@VK3KAY.#WEV.VIC.AUS.OC
To : INFO@WW
Hello All.
The question of Nicad memory will not go away readily. After many years of
promotion in 'amateur' literature, it continues to gain some credibility.
However, information from reliable professional sources can be found which
either does not support 'memory', or doesn't even mention it. Battery
manufacturers haven't had much to say about 'memory'. Cynics would say, 'why
would they'; realists would say, 'why should they'? In this category, I would
like to offer the following.
I first read about a 'memory effect' in 1976, when reading the manual for a
new portable instrument. Among other notes about caring for the internal
Nicad battery, was a statement that 'charge capacity may be affected by the
charge-discharge routine, due to a memory-type phenomenon'. ( Words taken
from my notes of the time). As I had not heard of this effect, I contacted
Nicad cell suppliers, and manufacturer's agents, who knew nothing of this
Nicad 'memory.
To try the strange claim, I set up a simple trial of charge-discharge, using
a strip-chart recorder to plot the discharge curve of a Nicad cell. The
cell was discharged to about 50% of it's rated capacity, and charged to
the manufacturers specification. After many months of daily cycling the cell
charge-discharge, no evidence of lost capacity was found.
From that time, many articles in magazines commented on 'Memory Effect'.
Constructional articles for Nicad battery chargers, invariably cautioned
about the dreaded memory effect and gave various cures! However, eventually,
in Ham Radio magazine for January 1983, an article titled, 'Forget Memory',
appeared. In this, the author quotes from a General Electric Handbook which
mentions memory. The HR author writes, 'Two paragraphs (in the handbook)
state and restate that "memory effect does not manifest itself when the
battery is discharged to random depths of discharge or overcharged for random
amounts of overcharge time, as is typically the case in most applications."
In addition, "Two General Electric engineers...told how the effect was
discovered in satellite test programs,...after many hundreds of cycles. Even
then, the effect did not always appear in all types of cells." They concluded
that, "though sintered plate nickel-cadmium batteries can remember, the
conditions necessary are almost *never* encountered in practice".
I have a copy of 'All About Niclel Cadmium Batteries' by T R Mishra, in which
he describes the make-up and chemistry of Nicads, their charge and discharge
requirements, and presents many pages of circuits for chargers, from 'simple'
to 'fast and sophisticated', but at no time does he even mention 'MEMORY'!
After thinking about the cause of the un-deniable short comings of Nicads,
I came to the conclusion that the problem of charging to full capacity is
the cause of low output energy. A Nicad is fully charged at 1.2 volt. It is
regarded as 'discharged at 1.0 volt. Only 0.2 volt (200 mV) separates the
charged and discharged state. This is not a very 'noticeable' margin, yet
it is the only way we have of telling how our Nicad cell is fairing.
T R Mishra describes Nicad chargers that monitor the temperature of a cell
during charging. The temperature rises near full charge to the extent that it
can be used to control the charge rate, and then cut off at full charge.
This, or a voltage detecting controller should result in better charging, and
therefore a full capacity battery.
I believe this is the answer to our Nicad battery problems; at least in
respect of capacity short-fall. I suspect that the Nokia phone, and Andy's
laptop has a 'smart' charger with heat sensing. But, my main concern has been
the 'short circut effect'! Forget memory. How do you stop the growth of
spikes on the sintered plates of a Nicad cell, which then causes the whole
*sealed* battery to fail!
I hope this has been interesting, and perhaps, helpful.
73. Dick. VK3ABK.
Read previous mail | Read next mail
| |
