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NiMH MEMORY

Old 04-29-2008, 12:04 AM
  #1  
celticflyer
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NiMH Memory, does anyone know if these have a memory like nicad??
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Old 04-29-2008, 12:08 AM
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GreenAce92
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Umm you talkin about stuff like charging cycles? I would think they are pretty much the same.
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Old 04-29-2008, 12:17 AM
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celticflyer
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Nicad's need to be totally discharged before re-charged. If you don't they will eventually develop a memory and you can ruin a battery. Lipo's do not have this problem at all. My question is NiMh or nickel metal hydride, do they display any of this problem?
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Old 04-29-2008, 12:31 AM
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Red Scholefield
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"MEMORY" EFFECT IN NICKEL CADMIUM BATTERIES

by Jon Eager and Marlene Dix, Gates Energy Products

"Memory" is a term which implies that a nickel cadmium battery
somehow remembers a particular depth of discharge and upon
subsequent use, will only deliver the capacity associated with
the memorized depth of discharge. Another phenomenon
associated with the "memory" effect is an apparent decrease in
capacity that is associated with extended overcharge (at the
overnight charge rate) of a nickel cadmium battery,
particularly at high temperatures. Both conditions actually
induce a characteristic more correctly identified as voltage
depression, and both conditions are completely reversible or
recoverable. Though the term was originally coined because it
appeared in a satellite application which involved shallow
discharges to the same voltage cutoff, it is rarely seen in
Gates Energy Products applications and has only been induced in
a laboratory environment.

WHAT IS VOLTAGE DEPRESSION?

Voltage depression refers to a step in a standard discharge
voltage profile which amounts to a reduction of approximately
150 millivolts. Referring to the attached figure, this voltage
depression does not actually reduce capacity. However, if the
application is designed to terminate the load at a voltage
between the midpoint voltage (mpv) and (mpv - 150 millivolts),
the runtime will be reduced significantly as can be seen in the
figure.

WHAT CAUSES VOLTAGE DEPRESSION?

There are two theories which have been proposed to explain
"voltage depression" in electrochemical terms. The first
theory states that APM (anti polar material) nickel in the
negative sinter electrode, either from nickel attack or from
the cadmium nitrate solutions, alloys with the cadmium
hydroxide to form a material which discharges at a lower
potential. The passivation and dehydration processes were
added to the Gates Energy Products impregnation process to
eliminate this problem. This theory is also consistent with
what happens in the pressed negative electrode. Nickel
hydroxide added to the negative pressed electrode can lead to
"voltage depression" at higher concentrations. This additive
acts like the APM nickel in the sinter negative electrode
alloying with the cadmium hydroxide and discharging at a lower potential. It is not seen with the concentrations that are currently used in the Gates Energy cell
designs.

The second theory is based on crystal growth of the cadmium
hydroxide. During extended overcharge, the cadmium hydroxide
forms larger crystals over time. These larger crystals reduce
the effective surface area of the active material and thus
increase the current density and effective internal resistance,
which in turn leads to a lower cell voltage. Discharging the
cell returns this less active form to its normal condition and
the cell will perform normally on the next discharge. This
theory explains how the term "memory" might have been chosen
for this phenomenon.

When a cell in this condition is partially discharged, only the
discharged materials will be returned to normal during the
subsequent charging operation. The next discharge will have a
normal voltage profile because the normal active material will
discharge first at a normal discharge voltage. The larger
crystal size active material will discharge later in the
discharge at a lower voltage. The cell appears to remember the
depth of the previous discharge, but in fact the user has only
converted a portion of the agglomerated active materials to
their normal form. This theory is consistent when applied to
the pressed cadmium electrode. Crystal growth in the pressed
electrode does not occur in the same fashion as in the sinter
negative electrode, and as a result "voltage depression" is not
seen in the pressed negative cell designs.

HOW ARE CELLS RECONDITIONED?

Some users of nickel cadmium products advocate a conditioning
process to correct "memory". These processes usually entail
frequent deep discharges to correct the problem.
Unfortunately, frequent deep discharges increase the risk of
cell reversal and can shorten the life of the battery. The
best approach is to understand that "memory" is a myth - it
does not exist in properly designed nickel cadmium batteries -
and that the "conditioning" processes are unnecessary and can
be detrimental to the battery. A normal charge and discharge
as identified in the attached troubleshooting guide should
completely restore 100% of the batteries rated capacity.

Avoiding the real problems of improper cutoff voltage design or
"voltage depression" requires selecting a well designed
rechargeable product using a properly designed nickel cadmium
battery. The Gates Energy Products nickel cadmium cell is
designed to prevent "voltage depression" and coupled with the
proper design of the rechargeable device will provide optimum
performance for the end user.
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Old 04-29-2008, 12:33 AM
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adhoc
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Short answer is yes, but not as much as NiCad...

A few web resources I found, take them with a grain of salt...

http://www.buchmann.ca/Article10-Page1.asp

http://www.dansdata.com/gz011.htm
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Old 04-29-2008, 04:18 PM
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Rodneh
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Originally Posted by celticflyer View Post
NiMH Memory, does anyone know if these have a memory like nicad??
Pretty much the same; i.e. neither have a memory problem in the way we use them in RC. An old myth that refuses to die.
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Old 04-29-2008, 09:27 PM
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Originally Posted by Rodneh View Post
Pretty much the same; i.e. neither have a memory problem in the way we use them in RC. An old myth that refuses to die.
Seconded.

Unless you discharge to the same point everytime, your batteries will not develop memory. Just enjoy the flight. Batteries don't last forever anyway, and they will last longer if you put your efforts into properly charging them, keeping them cool, and using correct long term storage.
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Old 04-29-2008, 10:54 PM
  #8  
celticflyer
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Thanx everyone, this has been interesting. I have a NiMH in my TX and wondered about toping it off every day.
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