🚀 go-pugleaf

On Sun, 09 Jan 2000 16:18:53 -0500, nxyrva@ivyyntrarg.pbz (Al Klein)
wrote:

>On Sun, 09 Jan 2000 11:01:51 +1000, Matthew McDonald
><matthew@matthewmcdonald.com> wrote:
>
>>On Sat, 08 Jan 2000 16:45:13 -0700, James Weber <mattheww@uswest.net>
>>wrote:
>
>>>You need to be careful discharging NiCd's. The phones will shut off
>>>when one of the cells is completely flat. If you go beyond that, you
>>>start reverse charging the  fully discharged cells, and that does
>>>destroys batteries with truly amazing speed.
>
>>Ahh no. The phone will shut off when a certain voltage level is
>>reached. The battery never get's even remotely close to being really
>>flat.
And if you check, you will discover that the voltage at which the
phone shuts down, corresponding to one of the cells in the NiCd having
reached zero volts. For instance an Ericsson 6 volt pack for a 300
series, there are 5 Nicd's in the pack. Each NiCd cell is 1.25v
nominal.
 Nominal output of 5 cells  is 6.25 volts (it is higher than that,
more like 7.5 volts immediately after charging, but won't stay there
for long). At 5.0 volts, one of the cells is full discharged (niCd's
have very flat voltage discharge characteristic), and that is the
point at which the phone shuts down to prevent reverse charging of a
cell.


>
>Most of the rest of Weber's post is along the same lines - it sounds
>good, but it's mostly bunk.\
Bunk? Do you have an EE degree? Are you an IEEE member?

How much do you know about the making of NiCds? The electrodes are
made by sintering. Taking powdered metal, and heating it just enough
so it sticks together. The result is immense surface area. It is like
comparing the surface area of sand to rocks. A pound of sand (fine
particles) has immensely more surface area than a rock of identical
weight. It is the manufacture of electrodes this way that has raised
the capacity of the AA NiCd from the original 450mah to 1100mah in
some commercial products today, without a corresponding increase in
battery weight.

In terms of peak output capacity the larger surface area is magic
. Perhaps one of the best demonstrations is the polapulse battery used
in Polaroid film packs. Not much capacity, but each cell  is made with
a pair of electrodes that are several times the area of AA alkaline
batteries. That little battery can put out 10 Amperes! It won't do it
for long, because the actual capacity isn't all that high.And it
doesn't have to, but it does take about 10 amps for 1 second to run
the camera when it ejects the picture.

Try to get 10 amps out of AA sized Alkaline sometime! Lots of luck.
The reason NiCd's were used initially in mobile phones was their
tolerance for overcharging, and the ability to put out very high peak
current. How high is very much a function of electrode surface area.
If the charging process reduces the effective area, the peak current
output is reduced along with it, which causes the battery voltage to
appear low due to internal resistance in the battery. Once the current
demand returns to standby levels, the battery voltage comes back up,
and the battery state indicator on the phones comes back up as well.



>
>It sounds as if his "knowledge" comes from reading a few incorrect
>magazine articles.
No, but I suspect your source is urban legend...