🚀 go-pugleaf

totty@flute.cs.uiuc.edu (Brian Totty) writes:

>	I just bought a set of rechargable batteries and a charger for them.
>	To my surprise, I found out that the 9v batteries were actually only
>	7.2v in a "nine-volt size".  I was wondering why they couldn't make
>	NiCd batteries in the same size that could store 9v.  Other brands on
>	rechargeables also seem to be 7.2v.  This seems to be a pretty serious
>	degradation from 9v, and I am surprised that these batteries would
>	even work in some applications.

I just read through the responses to date and found most of them are just
addressing why 6 NiCd cells produce only 7.2 volts and that 7 cells could
be used to get 8.4 volts would would be a little closer to 9 volts.

Here is my response:

The so called "serious degradation" is in fact NOT a serious degredation.
Alkaline cells (the conventional high capacity batteries we see in stores
these days) as well as more classical cells (LeClanche) have an open circuit
fresh voltage typically around 1.5 volts per cell.  This is what we tend to
think of the voltage as being.  In actual practice we are drawing current
from these cells and introducing a voltage drop.  Further we are running
these cells down in capacity and will be seeing even further voltage drops.

Electronic devices do typically manage to work quite well over a wide voltage
range.  Their specs might not be accurate, but few consumer items ever come
close to their published or design specs anyway, and few consumers ever
complain.

It is not unusual to see alkaline cells drop in voltage in use to as low as
0.9 volts or less.  For a 6 cell 9 volt battery, this means a voltage as low
as just 5.4 volts.  So a "9v" battery is in fact not really 9 volts.

A NiCd cell has a nominal open circuit voltage usually stated as 1.2 volts.
I typically see 1.25 volts for a freshly recharged cell.

During the course of the use of these cells (Alkaline vs. NiCd) the voltage
will drop.  However the NiCd cell differs from the Alkaline in the way it
drops.  First of all as the capacity runs down, it drops at a slower rate
than the Alkaline does.  Secondly when operated under load, the NiCd drops
less due to its lower internal resistance (read warning below).

While the capacity term of a fully charged NiCd will be shorter than that
of an Alkaline cell (1/4 by my experience) the NiCd cell will actually have
the higher voltage at the end of its useful charge life.

Any electronic device that cannot operate on the equivalent of 1.0 volts per
cell or less is just not going to give you your money's worth in battery
capacity.  You will be throwing away Alkalines that still have half their
capacity left if the device quits at, say, 1.1 volts.  I have seen some, but
not many, devices that will do that.

So a 6 cell NiCd battery used to replace a 6 cell Alkaline battery should
work out just fine for most electronic devices.  The best thing to do is to
try it and see.  Perform a careful scientific test of the device with both
kinds of cells.  Use the device as you normally expect to as well as under
extremes (heavy use, lack of use, etc) that you might happen to use it.

Remember that a NiCd cell will last much shorter than an Alkaline cell, so
don't immediately dismiss the cells you have as "short lived".  That is the
tradeoff for rechargeability.  The net total life with proper recharging is
going to be much longer for the NiCd over the Alkaline which cannot be safely
recharged.

+------------------------------------------------------------------------------+
|WARNING:  Be careful with ALL battery types.  NiCd batteries (due to their    |
|low internal resistance) can deliver a whopping level of current if shorted.  |
|I have melted connectors from accidental shorts of NiCds.  Alkalines are also |
|very dangerous.  I have had one explode on me before.  Fortunately it was     |
|nearly dead already and it just blew its guts out over to the kitchen floor   |
|for me.  I had also accidently shorted it because I foolishly assumed it was  |
|dead and was not taking proper precautions.  NEVER mix cell types.  This can  |
|end up charging a primary cell and there can be a resultant fire or explosion.|
+------------------------------------------------------------------------------+
-- 

--Phil Howard, KA9WGN-- | Individual CHOICE is fundamental to a free society
<phil@ux1.cso.uiuc.edu> | no matter what the particular issue is all about.