From: elmore@rap.ucar.edu (Kim Elmore)
Subject: Re: NiCd Batteries, Part 3/3
Organization: NCAR, Research Application Program
Date: Sun, 3 Jan 1993 17:08:36 GMT

In article <ljulsbINNo6a@exodus.Eng.Sun.COM> falk@peregrine.Sun.COM (Ed Falk) writes:

>Here's what I do:  When the batteries in a device are low, I remove
>them and put them into a discharger I built.  This discharger is simply
>a light bulb in series with a battery holder that holds 1 battery.  I
>discharge the batteries individually and then put them into the
>charger.  Is this good or bad?  I've toyed with adding a diode to
>the circuit so that the batteries only discharge to .7 volts instead
>of to 0 volts.

	From what I've been able to gather, *this* form of complete
discharge is relatively benign.  Because there is no way to reverse
charge the cell, you can't damage it.  However, it really shouldn't be
necessary.

>> Now, temperature... that's another problem.  I talked to four
>> applications engineers at Gates and each one stressed that temperature
>> is the killer.  They say that each 10 degree C over the life of the
>> cell reduces cell life by one half.  TEMPERATURE, THAT'S THE BIG THING!
>
>That's a qualitative answer; how about a quantitative answer?  How warm
>is too warm?  How long is too long?  I keep my batteries at 20 degrees
>C (room temperature), does that mean I've reduced the batteries to
>1/4?  That doesn't quite make sense to me.  Should I put my charger in
>the freezer?

	An excellent observation and a poor example of not
proofreading what I post :-( From what I've been able to gather,
battery manufacturers don't like to see cell temps much above about 40
deg C simply because the vapor pressure of the electrolyte becomes
high enough that some can vent and be lost; NiCds will vent if they
reach 45 deg C (113 deg F), even if it's just sitting in your airplane
on the ramp.  There is nothing *chemical* that precludes pretty warm
cell temps (within limits, of course); the cell temp limits are more
mechanical in that what causes loss of the cell -- as far as heat is
concerned -- is loss of the water part of the electrolyte.  Heat is
bad, whether or not it occurs due to charging of sunshine.  As NiCds
warm up, there are other problems with charging, called thermal
runaway.  More on this later.
	To finish this up, cold cells (like around freezing) will
work, but at reduced capacity.  The cell voltage will be lower and
charging should be done with care because of this; unless chargers
have built-in cell temperature sensing, they can force too much charge
current through the cell, resulting in electrochemical gas formation
and venting.

>
>> 1.  Nicads are very easily damaged due to gas pressure caused by
>> over-charging at high charge rates.  Unfortunately, Nicads do not have
>> a positive indicator for indicating the state of charge.  It is up to
>> the user to estimate the relative charge state.  Typically, the charge
>> rate is calculated at C/10 where C equals cell capacity and the 10
>> represents hours for charging.  At C/10, a nominal or normal charge
>> would occur.  A fast charge can be accomplished, cell design
>> permitting, by changing the ratio, i.e., 5C/2.
>
>I can't parse this.  I can't figure out the units.  What is C?  Volts?
>Milliamp-hours?  If I have a 1.25 volt battery, is C/10 .125 volts/hour?
>How do I find out the capacity of a cell?  What exactly does C/10 or
>5C/2 mean?

	Sorry... In this case C is the cell capacity in Ampere hours
(Ah).  Further, I *AGAIN* failed to proofread this part because the
information is old.  There *is* a reliable method for telling the
charge state of a NiCd as it is being charged by monitoring the cell
voltage.  Cell voltage is highest at peak charge; further
(over)charging causes the cell voltage to drop a small amount, about
25 to 50 millivolts if I remember right.  After this drop, the cell
voltage remains relatively constant as overcharging continues.  The
newer "quickchargers" have circuitry built in that sense this and
shut off the charge cycle when this point is reached.  Older
quickchargers had temperature sensors in the pack that shut off the
chargers after the cells got warm which required considerable
overcharging.  These tended to destroy packs because they got them
pretty hot (45 to 50 deg C), enough to damage them.  Also, the newer
chargers usually charge NiCds with short, high-current pulses, which
reducees the amount of free gas that is formed.
	How to determine cell capacity?  If it's not printed somplace
on the cell, you'll have to do some test discharges.  Pick some low
current for the cell -- AA cells are typically 500 mAh capacity, D
cells are about 1200 mAh capcity, so 100 mA discharge rate is a good
one to choose -- and monitor the cell voltage with time.  When the
cell under test drops to about 1 V, consider the cell discharged.
Multiply the discharge current by the number of hours it takes to
discharge the battery and you have the capacity, C.  If it takes 10
hours to discharge a cell at a nomibal current of 100 mA, then you
have a 1000 mAh cell to deal with.
	Now, about thermal runaway: the overcharge voltage drop I
mentioned can lead to horrific charge currents if the charge supply
isn't current-limited somehow.  Years ago, wet-cell NiCds were
available for aircraft -- they may still be.  These were, however
quite expensive.  The idea was to have lots of capacity in a little
space, not as heavy as lead-acid cells, that no one would have to
replace for a loooooong time.  But the darned things kept causing
problems like electrical fires, destroying alternators and exploding
in flight with enthusiasm sufficient to damage airframes.  The problem
was found to be thermal runaway, wherein the cell voltage dropped when
overcharge occurred, causing an increase in charge current.  This
heated up the batterry, whose internal impedance began to decrease due
to the nature of the chemistry.  Of course, the charge current went
up, causing the battery to get hotter, causing...  Sooner or later
something would give.  Either the battery would boil and explode or
the electrical system would burn up.  On exciting days, you might get
both to happen.  All NiCds exhibit this feature, but it requires a
healthy current supply for spectacular examples :-)

>> 3.  Pulse charging of Nicads is most preferred as it is more efficient
>> than a steady-state current value.
>
>How do I pulse-charge a Nicad?

	Ham magazines have featured articles on home-brewing NiCd
pulse chargers in the past.  The ARRL Handbook has an article (page
27-10 in the 1992 edition) that describes a non-pulsed charger that
won't damage batteries, but isn't as good as a pulse charger, which is
a bit more trouble to build.  Most libraries carry QST (a ham
magazine) and you could probably find articles in back issues on pulse
charging.  If you're really interested and can't find the article,
e-mail me and I'll dig up a reference

>		-ed falk, sun microsystems
>		 sun!falk, falk@sun.com
>	"Towards the end, the smell of their air began to change"

	Hope all this helps and sorry for the confusion!


					Kim Elmore
					N5OP, PP ASEL/Glider 2232456
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