From: ray@anaconda.cis.ohio-state.edu (william c ray)
Subject: Re: Morris Slaves and Nicads
Organization: The Ohio State University Dept. of Computer and Info. Science
Date: Tue, 15 Sep 1992 15:30:16 GMT

In article <BuL3xC.LoM@ecf.toronto.edu> chang@ecf.toronto.edu (CHANG Rong Fawn Teresa) writes:
>In article <1992Sep14.183603.19930@enterprise.rdd.lmsc.lockheed.com> harrison@force.ssd.lmsc.lockheed.com writes:
>>at first.  The same problem applies to using nicads.  Nicads have very low
>>internal resistance, so they can produce a lot of current into a short 
>>circuit.  They get very hot when they do this.  In addition, using two 
>                         ^^^ 
>                         That's the way to give out heat!  But how come
>                         only one cell got very high temperature and not
>                         the others?  
(someone)>>>  Thermal Runnaway...
>>nicads instead of two akalines will make it very hard to get the capacitor
>>filled.  Nicad cells have a nominal voltage of 1.2V, akalines are 1.5V 
>>(the actual voltage varies according to charge and load).  Those 2 nicads 
>>were delivering 2.4V when the circuitry expected 3V.  It is doubtful that 
>>they could have ever charged the Morris slave fully, especially when you 
>>consider that many akaline cells will produce 3.3V from two cells, and many
>>flashes probably don't work too well below 3V.


I was going to stay out of this but I've just got to stick my nose in here
before someone gets _totaly_ confused...

Fact:  Yes NiCads have very low internal resistance.  This means that they can
source one heck of a lot of current if they so choose.  As a matter of fact,
it means that they can usually do it without overheating, since it is
after all I^2*R heat generated.  Now if you hook them up to a short, 
a NiCad is very happy to push tens of amps through it, and then they
do heat up, because even with a very small R, that I^2 starts to turn into
some very large numbers.

Fact:  Yes NiCads have an output voltage of about 1.2V, While Alkalines are
nominally 1.5V.  However, since Nicads have very low internal resistance,
unless you insist on drawing more than a half dozen amps or so, you can still
consider them to be 1.2V.  Alkalines on th other hand have an internal
resistance of some tenths of an Ohm  (testing has indicated anywhere from
.35 - .85 Ohm, but I haven't checked all brands).  This means that if you draw 
as much as 1 amp out of the things, you've now got a battery that is worth
less than 1 Volt.  

Fact: Alkalines have a fairly linear discharge curve, dropping from
slightly over 1.5V very quickly to 1.5V, and then decreasing in output
voltage continually through use until exhausted.  Most people would consider
somewhere between 1.1-1.2V time to throw them out, and put in fresh
batteries.  (this is about when you start to notice unreasonable
recharge times, etc... NOT due to the 1.2V open circuit voltage, but due to
the sudden increase in internal resistance when the cells are almost
exhausted).  NiCads on the other hand have a very flat discharge curve,
showing almost no loss in voltage, or increase in internal resistance until
the very point of exhaustion.

Now some comments, and complaints...

Flash circuitry "expecting" 3V... Hah...  Consider the fact that your flash
is willing to charge on a set of 1.2-1.3V Alkalines, that under load are
good for maybe 1V apeice, if that.  We won't get into the mechanics of 
the HV circuit in a flash, but consider what it is... it trades _current_
for voltage...  High Current-Low Voltage is converted into Low Current-
High Voltage.  So long as the input voltage is high enough to drive
sufficient current thought the charging circutry, the flash will charge.
If the NiCads had the capacity to drive the one of them into "Thermal
Runaway"  (Nice term, why dont you explain it to the rest of them? 
I'm having too much fun with the rest of this to detour here...  If you've got
a NiCad at say, .1 Ohm, (much too high, but works for the sake of argument)
which is dissipating enough energy to get "very hot", (say 5 Watts,
for the sake of argument?), how much current does that make?  Now, 
remember what we just pushed that current though... does the flash
still work?)  then they certainly had the voltage to push (aparently more
than enough) current through the circutry.

Doubting that they could charge the morris fully...most flashes won't 
work on less than 3V...  double Hah!  Have you ever met a flash that
wouldn't charge on Nicads?  Have you ever even TRIED to charge a flash
on NiCads?  The problem with Flashes and NiCads isn't not being able
to charge them, it's the fact that the NiCads can drive the charging
circuitry with _so_ much current that it toasts some flashes quite
nicely.  You're perfectly welcome to doubt something all you like,
but don't tell other people "doubtful"s and "probably"s as fact, unless
you _have_ some facts, in which case you'll no longer need to doubt
or probably anything...

Now a (perhaps?) helpful hint...

Check the positive end of your NiCad batteries.  Many Nicads have
the entire outer shell directly connected to the negative terminal.
This is all fine and well, except that on the positive end, 
where the outside casing is rolled over and squashed down around
the lip to hold the positive plate in.  On some devices the battery
holders are designed in such a way (poorly) that the contact for
the positive end can brush against that rolled rim, causing a
direct short between the (negative) casing, and the positive plate.

A diagnostic question...

When you used the NiCads did the flash _try_ to charge?  i.e., could
you hear it charge, but it just never managed to "top-off" the charge?
This would be indicative of the NiCads having too low voltage.  
This is contradicted by the fact that one of your NiCads overheated.
If the NiCads had too low voltage, they would have partialy charged
the flash, and beyond that point would not have been able to source
any more current into the flash, thus no overheating. (yes I know 
that's an oversimplification...)
If you didn't hear it try to charge, and just sort of "never make it"
for lack of a better description, then something else was wrong, and
you shouldn't give up on the NiCads. 

Whew!!!
If you've got any other questions, feel free to email me.

Will Ray
ray@cis.ohio-state.edu
wcr@rsch.oclc.org