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'[EE]: Cell Phone NICAD Battery Conditioning'
2001\03\05@114651 by Jess Hancock

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Our local Red Cross has received a donation of 20 new Motorola cell phones.
These phones will be put on the shelf and activated in the event of a
disaster/emergency.  Activation details are to be coordinated with local
cell phone providers, hopefully with donated air time.

The instruction manual indicates that the NICAD batteries should be charged
overnight, and then run through five discharge/overnight charge cycles
before being put into regular use and again if the phone (battery) is stored
for an (unspecified) "extended period" of time without use.

I assume an "extended period" would be 2-3 months or would it be longer?

I am aware of the Nicad memory effect but have heard modern Nicads do not
suffer from memory problems.

I am attempting to plan a periodic battery recharge schedule and would
appreciate comments regarding the best way to keep the phones ready and in
good battery condition for extended periods of (hopefully) no emergency
usage.  Comments from managers (or others) having experience using (and
activating) cell phones under similar usage patterns would be especially
helpful.

I do not own or use a cell phone so this is all new to me.  You may contact
me via the [EE]: list or off list at: spam_OUTjhancockTakeThisOuTspamcwv.net .

Thanks, Jess

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2001\03\05@122849 by Bill Westfield

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   I am attempting to plan a periodic battery recharge schedule
   and would appreciate comments regarding the best way to keep
   the phones ready and in good battery condition for extended
   periods of (hopefully) no emergency usage.

I would be very tempted to get some of those "emergency" battery packs
that allow most cell phones to be run on "standard" alkaline batteries.

NiCds suck; especially old NiCds.  I have several old laptops computers
that are perfectly usable other than having completely unusable batteries.

BillW

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2001\03\05@141349 by Dale Botkin

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My solution to that problem is usually an X-Acto knife and new cells.
You're right, though; NiCds do suck.

On Mon, 5 Mar 2001, William Chops Westfield wrote:

{Quote hidden}

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2001\03\05@161606 by Gordon Varney (personal)

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What you are referring to is called memory effect. NiCd Batteries have a
problem with retaining the min max voltages applied.

Example: if you discharge the batteries to the 70% point and then recharge
them every time you use the device that the battery is used in. The battery
will begin to retain this pattern.

The trick to using a NiCd battery is to completely discharge the battery
about every 3rd or 4th cycle. Then recharge the battery under high current,
back to the battery voltage rating, and trickle charge until fully charged.
Even a very bad battery can be restored if charged to battery voltage under
high current then discharged. Repeat until battery is restored. 5 - 10
cycles will bring a NicD back to life and will be usable for many years to
come, if properly discharged and charged. If the battery goes dry then it is
useless.

I personally have restored hundreds of NiCd using this technique.

Gordon Varney



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2001\03\05@171141 by hard Prosser

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Gordon
I get a bit confused about this.
When you say "completely discharged" do you mean 0V terminal voltage or
just  <1V ? The problem I have is that to discharge a series bank of cells
completely will mean that some of them will be reverse biased - which I
believe is not a good thing to do. On the other hand I have had very
limited success in recovering Nicads from this condition.
Voltage depression seems to be my most common problem where the battery
fully (?) charges to a lower voltage than when new, which results in
equipment with battery charge monitors (e.g. cellphones) reporting the
battery as low capacity almost immediately.

Thanks,
Richard P





What you are referring to is called memory effect. NiCd Batteries have a
problem with retaining the min max voltages applied.

Example: if you discharge the batteries to the 70% point and then recharge
them every time you use the device that the battery is used in. The battery
will begin to retain this pattern.

The trick to using a NiCd battery is to completely discharge the battery
about every 3rd or 4th cycle. Then recharge the battery under high current,
back to the battery voltage rating, and trickle charge until fully charged.
Even a very bad battery can be restored if charged to battery voltage under
high current then discharged. Repeat until battery is restored. 5 - 10
cycles will bring a NicD back to life and will be usable for many years to
come, if properly discharged and charged. If the battery goes dry then it
is
useless.

I personally have restored hundreds of NiCd using this technique.

Gordon Varney



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2001\03\05@171740 by Robert Rolf

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www.a1.nl/phomepag/markerink/battery.htm
has a good discussion/explanation of the so called 'memory' effect.

I also found the below link. The key point is the last line.
"One cell may become fully discharged, but the remaining cells continue
to force current through it, causing permanent damage."

That is why electronic equipment has a 'low battery' cut off. To prevent
damage to battery packs (battery=collection of cells).
==========================================
www.straightdope.com/columns/990312.html
For years I've heard that nickel-cadmium batteries which are not fully
discharged will "develop a memory" upon recharging, and--after a
time--if they
are only partially used each time before recharging, eventually the full
capacity of the battery will not be available. Is this true?
--Hobberstad

Cecil replies:

It's a myth, bud, though one with a basis in fact. Many years ago in the
space program it was discovered that a computer-monitored nicad battery
repeatedly
discharged to exactly 25 percent capacity did develop
"memory"--eventually a quarter of its charge became permanently
unavailable. But this kind of thing seldom
if ever occurs in earthbound applications. More commonly one sees a
condition that mimics true memory, called voltage depression. In some
overcharged
batteries the available voltage may drop partway through the discharge
cycle, spoofing a low-battery monitor (on a laptop computer, say) into
indicating that the
battery is low. But there's still plenty of good juice left.

Fully discharging a single nicad cell (e.g., a flashlight battery) is
usually harmless and may sometimes be desirable. But fully discharging a
true nicad battery--that
is, a series of cells wired together, which you typically find in your
higher-voltage devices--is definitely a bad idea. One cell may become
fully discharged, but the
remaining cells continue to force current through it, causing permanent
damage. So the best advice is, don't fully discharge your nicad
batteries, and don't
overcharge them either.

"Gordon Varney (personal)" wrote:
{Quote hidden}

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2001\03\05@173358 by Jess Hancock

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Thanks Bill W for your comments.  I will check and see what I can find.

Gordon, I have seen Nicad battery packs damaged by "complete" discharge.  I
hesitate using the term "complete" discharge because it suggests (to me)
zero volts.  If I remember,  the safe, non-damaging "complete discharge"
should occur somewhere near the knee of the discharge curve which is well
above zero volts.  The weakest cell in a battery pack will become reverse
charged (and ruined??) by the discharge current if the discharge process is
allowed to go too far.

How do you define "completely discharging" and how do you accomplish it - in
the device or by using a separate resistive load?  When I learn more about
PICs, I may try my hand at a battery monitor/charger.

These cell phones and batteries that Red Cross has are new/never used and
have a battery voltage monitor built in which turns the cell phone off at
some point in the discharge.  The manual does not clearly specify the low
voltage cutoff point.  The battery is rated at 7.5 volts and the battery
monitor "kind of" suggests that 6 volts may be the low cutoff point.  I've
just charged one of the phones and have not discharged it yet to "low
battery" warning or to cutoff.

I'm trying to get some ideas on periodic charge/discharge procedures others
who supervise use of a large number of Nicad battery operated devices or
cell phones may use so that we can maximize the useful battery life.  I'm
afraid, the answer is that not much management actually takes place with
most users.

Thanks for your input.

Jess

{Original Message removed}

2001\03\06@102743 by Gordon Varney (personal)

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> Thanks Bill W for your comments.  I will check and see what I can find.
>
> Gordon, I have seen Nicad battery packs damaged by "complete"
> discharge.  I
> hesitate using the term "complete" discharge because it suggests (to me)
> zero volts.  If I remember,  the safe, non-damaging "complete discharge"
> should occur somewhere near the knee of the discharge curve which is well
> above zero volts.  The weakest cell in a battery pack will become reverse
> charged (and ruined??) by the discharge current if the discharge
> process is
> allowed to go too far.
>
> How do you define "completely discharging" and how do you
> accomplish it - in
> the device or by using a separate resistive load?  When I learn more about
> PICs, I may try my hand at a battery monitor/charger.


You are correct, There is a specific point of reference for completely
discharged. Just as in logic zero is a point below a specified level based
on TTL or CMOS. A NiCd has a point where the battery voltage will level out
and the battery will provide a small current. A discharge is when the
battery voltage will stay below this level with no load. I worked for
Motorola for 6 years and wrote one of the original papers on memory effect
about 18 years ago. I have to disagree with an earlier comment on there is
little or no memory effect on batteries under normal use. This is in fact
the place that memory effect is at its greatest problem.

Gordon Varney

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2001\03\06@135428 by hard Prosser

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Gordon,
You stated earlier that part of the fix was a partial recharge at "high
current". How high  - relative to the C of the battery ?
Thanks,
Richard P





{Quote hidden}

about
> PICs, I may try my hand at a battery monitor/charger.


You are correct, There is a specific point of reference for completely
discharged. Just as in logic zero is a point below a specified level based
on TTL or CMOS. A NiCd has a point where the battery voltage will level out
and the battery will provide a small current. A discharge is when the
battery voltage will stay below this level with no load. I worked for
Motorola for 6 years and wrote one of the original papers on memory effect
about 18 years ago. I have to disagree with an earlier comment on there is
little or no memory effect on batteries under normal use. This is in fact
the place that memory effect is at its greatest problem.

Gordon Varney

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2001\03\06@200951 by Gordon Varney (personal)

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Richard,
       Motorola used 12 battery cells rated at 1.2 Vdc and 1.5 Amp per hour. When
placed in to a battery pack, these batteries were rated at 14.4 Vdc at 1.5
Amp per hour.
       The Battery packs would drop in voltage with each charge. I have seen these
batteries come in on equipment with voltages around 6 - 8 Vdc, totally
unusable.
       I would use a power supply and drive the battery voltage up slowly from
about 6 Vdc, watching the current rise, as the voltage started to rise, I
watched the current climb as high as 10 Amps. The voltage would remain at
about 8 Vdc, then all of a sudden the voltage would jump up, and the current
would drop. This process would take only about 30 seconds. I then set the
power supply to constant current of about 3 Amps and set the voltage to
about 15 Vdc. As the voltage would rise the current would drop. If the
current dropped below 3 Amps I would adjust the battery voltage up further
until the current was at 3 Amps, or twice the current rating of the battery.
I charged the battery until the temperature of the battery increased 20F
above the ambient temperature. then dropped to a slow charge for an hour.
       I discharged the battery pack  to a point where the voltage leveled. There
are two corners to the charge cycle. The battery voltage will drop slowly
supplying high current, till it reaches the first corner. The battery
voltage will drop fast as will the current,  to a point where the voltage
will hit the second corner. The battery voltage will level at about 2-3 Vdc
and will supply current at this level for a while. When the battery pack
voltage will remain at or below this level with out the discharge load, it
is discharged.
       Cycle through this process several times, to restore a bad battery.

Note: I have never had a battery explode while doing this.
If the voltage did not jump up after 30 seconds, then wait a few minutes and
try again. If still not there, then through it away.

If the power supply voltage rises above the battery voltage, more than about
25% with no increase in current than it is a bad battery. If the Voltage
rises above the battery voltage and the current starts to rise , then reduce
the power supply voltage to the battery nominal voltage and allow the
current to rise. Keep the current no more than twice the rated current for
any length of time. It is more about temperature than anything.

Gordon Varney
Director of Engineering
Translectric Inc.



>
> Gordon,
> You stated earlier that part of the fix was a partial recharge at "high
> current". How high  - relative to the C of the battery ?
> Thanks,
> Richard P
>
>

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2001\03\07@021420 by Dave Bell

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Robert Rolf <Robert.Rolfspamspam_OUTUALBERTA.CA> wrote:

>www.a1.nl/phomepag/markerink/battery.htm
>has a good discussion/explanation of the so called 'memory' effect.

>I also found the below link. The key point is the last line.
>"One cell may become fully discharged, but the remaining cells continue
>to force current through it, causing permanent damage."

>That is why electronic equipment has a 'low battery' cut off. To prevent
>damage to battery packs (battery=collection of cells).
>==========================================
>http://www.straightdope.com/columns/990312.html

 As usual, Uncle Cecil does have the straight dope... There is a little
more to it in practice, though. As pointed out, the big problem with deep
discharge of a NiCd (or most any other type) battery, is that one or more
cells run the risk of being reverse charged, before the battery is fully
exhausted. This is why, when reconditioning spacecraft batteries, great
care is taken to 1) monitor each cell's voltage, and 2) for full
recondition discharge, provide a resistive load for each series cell, NOT
the series string. Condition 1) is adequate for normal preparation of a
battery for full charge and testing capacity. It triggers the end of
discharge when the first cell reaches some cutoff voltage, say +50 mV.
Condition 2) is used for maximal discharge and for storage of a discharged
battery. Note that when using 2), you want the make sure there is no
significant load across the end terminals. If there is, current around the
loop will still cause one cell to go reversed, before they all reach zero!
As for what size resistor, I've seen as low as 1/4 Ohm shortdowns usd for
40 to 80 Amp-Hr cells. Proportionately larger values for smaller capacity.

Dave

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