[sci.electronics] NiCad "Deep Discharger"

myers@hpfcdj.HP.COM (Bob Myers) (06/26/90)

>How bad?  Are these the battery packs typically used for cordless
>drills and charging flashlights?  I've got a flashlight which came with
>instructions to discharge completely to avoid a memory effect, which
>sounds like the NiCd effect being discussed.  The instructions had no
>warning against complete discharge.  They also did not avoid constant
>charging -- the trickle charger is obviously intended to store the
>flashlight whenever it's not being used.

Most of the rechargeable flashlights I've come across use cylindrical lead-acid
"gel-cells", presumably because they're expected to be left plugged in (and
so, trickle-charging) basically "forever" - something a lead-acid's good at.
I'd be surprised if anything but a small pocket rechargeable light was using
NiCds - but then, I've been surprised before, too! :-)  However, don't
forget that there are a lot of people out there writing instruction sheets,
and there's no reason to expect them to be completely current on battery
technology.  Hence the mention of "memory."



Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers%hpfcla@hplabs.hp.com               |  sentient life-form on this planet.

pasik@motcid.UUCP (John Pasik) (06/26/90)

emcguire@cadfx.ccad.uiowa.edu (Ed McGuire) writes:

>In article <3653@sienna9.UUCP> pasik@motcid.UUCP (John Pasik) writes:
>> The typical maximum discharge voltage reccomended for Ni-Cad battery packs
>> is 1v per cell.  The 1v/cell value is chosen to minimize the chance of
>> electrically reversing the polarity of any cell in the pack (very bad)!
>                                                               ^^^^^^^^
>How bad?  Are these the battery packs typically used for cordless
>drills and charging flashlights?  I've got a flashlight which came with
>instructions to discharge completely to avoid a memory effect, which
>sounds like the NiCd effect being discussed.  The instructions had no
>warning against complete discharge.  They also did not avoid constant
>charging -- the trickle charger is obviously intended to store the
>flashlight whenever it's not being used.

>The battery would not take a charge after the flashlight was
>accidentally stored "on" for several months.  If this is due to cell
>reversal, is there a way to restore the battery?  Is it dangerous to
>charge a reversed battery?
>---
>peace.  -- Ed
>"He looked again.  No, these cows were strangers."  (Milo and Otis)

Trickle charging is OK as long as the current level doesn`t cause cells
to overheat or outgas.  You can discharge a single cell completely without
fear of "reversal", many small cordless devices only use one or two cells.
In such cases it is harder to cause a cell reversal.  If a cell is reversed,
it typically begins to short-out.  It is possible to remove cell shorts
with brief high-current pulses (>10C, were C is the amp/hr rating of the
battery).  The typical batteries used in small flashlights and power tools
are lead acid.  Sealed lead acid batteries are "starved electrolyte" systems.
This means the plate capacity is greater than the electrolytes ion capacity.
If such batteries are deep discharged, they will lock all the free ions up
in the plate material.  The result is basically water as the electrolyte.
Water is not a good conductor so the battery will not take a charge easily.
It is sometimes possible to recharge such a battery by charging it for an
extended interval (10 to 100 times normal).  If the battery can be connected
to a higher than normal voltage charging source, the process can be speeded
up.  Monitor the charging current and reduce the voltage as the current begins
to ramp up.

ron@vicorp.com (Ron Peterson) (06/27/90)

What is the shelf life of a nicad battery?
Are there any ways to improve it? (Store
in refridgerator, store charged/discharged,
store it in helium, store in a vacuum, ???)

parnass@cbnewse.att.com (Bob Parnass, AJ9S) (06/27/90)

In article <1990Jun26.194638.12467@vicorp.com>, ron@vicorp.com (Ron
Peterson) writes:

	> What is the shelf life of a nicad battery?
	> Are there any ways to improve it? (Store
	> in refridgerator, store charged/discharged,
	> store it in helium, store in a vacuum, ???)

No NiCd battery is perfect -- all have a finite shelf life.
NiCd batteries will discharge by themselves, through spontaneous
chemical decomposition, even when not connected to a load.
The General Electric Company indicates that the average
NiCd will lose about 1% of its capacity per day at 70 degrees F.

Average self discharge at 100 degrees is about double (2% per day)
the discharge at 70 degrees, which is a good reason to avoid
storing NiCd batteries in a warm automobile.
-- 
============================================================================
Bob Parnass, AJ9S - AT&T Bell Laboratories - att!ihuxz!parnass (708)979-5414

phil@ux1.cso.uiuc.edu (06/28/90)

The reversal of a cell in a battery (battery = many cells) of NiCd cells
happens when 1 of the cells is weaker than the others.  The high forward
current causes a voltage drop across that cell that, if higher than the
charged level of the cell if it has any charge left, will appear to be
in the opposite direction of ordinary charging.

The stated figures for the lowest voltage to discharge a battery to are
some attempts to prevent a cell reversal.  If your battery is SUPPOSED
to have N*1.2 volts, then discharging it NOT LOWER than (N-1)*1.2 volts
will assure you of not actually reversing a cell since the weak cell
will have to pass 0 before becoming reversed, and at that point the
voltage of the bettery must be (N-1)*1.2 volts or less.

I have a radio that can operate on voltages from 6 to 15 volts.  I have
a bettery pack that is 11 cells and rated at 13.2 volts.  I can run this
pack until the radio dies at about 6 volts.  By this time, if I am going
to be reversing any cells, I may have ALREADY done so.

Many electrical devices manage to function over a significant range of
voltage.  Alkaline batteries have a gradual voltage drop, so many devices
are designed intentionally to work under these conditions.  The normal
capacity of alkaline cells is rated in terms of how long for their voltage
to drop to 1.0 volts (from 1.5).

So it may be moot to even attempt to discharge your NiCd battery DOWN to
(N-1)*1.2 volts since it may in fact already be BELOW that level.  So what
can you do?

I have for a long time suspected that the "memory effect" is caused by the
internal discharging of a cell.  This is a chemical reaction that is very
different than the reactions to chanrge or normally discharge a cell.
This reaction apparently increases the internal resistance.  Think of a
single cell as an array of cells in PARALLEL.  The chemical surfaces of
the cell are electrically in parallel, so this is valid.  When the cell
is PARTIALLY discharged in normal use, half of the cell is used up, and
half (the weaker half) remains charged.  The first time around this is
not a distinct boundary.  The cell is then recharged.  As this cycling is
repeated, part of the cell is constantly discharged and recharged while
the rest of the cell is NOT.

When this abused cell is used for longer than the cycles it is used to,
it is forced to start discharging from the lesser used portion since the
rest of the cell is discharged.  This portion, having a higher resistance,
conducts less current and appears to have a significant voltage drop.
This may even be so severe as to prevent the device to function, and the
capacity of the cell will apparently be diminished.  I also believe that
higher discharge current bring this condition on faster.

Also, cells are not manufactured equal in capacity.  Typically you might
see as much as a 10% variation in capacity and/or internal resistance.
So your battery may already have a weak cell fresh out of the box before
any abuse gets to happen.

Given that it is not really practical to discharge battery packs down to
the (N-1)*1.2 volt level, and that cells can get weaker if they are
allowed to only be partially discharged, and that weak cells can become
reversed in discharging UNDER NORMAL USE, my practice is to TOTALLY
discharge my battery packs to ZERO volts EACH AND EVERY cycle they are
used.  After I am done with a battery pack, I set it up to be fully
discharged AT A LOW CURRENT for 24 hours or so, then put it on charge.
The idea is to prevent a cell from becoming noticeably weaker than the
other cells in the pack.  What I am doing is keeping the battery pack
BALANCED.

I have been doing this for over 120 cycles so far on 3 packs, and a 4th
was added a few months ago which has had about 40 such cycles.  Ordinarily
NiCd's last much longer than 120 cycles, so my testing period is not over
yet, but neither has this caused an untimely death.  One pack was
inadvertenly subject to sitting in a very hot car for a day a couple
of weeks ago, but it seems to have survived the ordeal just fine.
I also have carried and used the packs in deeply freezing (below -15 C
or +5 F) weather many times this past winter, without any apparent
problems.

--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.