What about Nickel-
Metal Hydride Batteries
In the early
stages, battery
manufactures were quick
to point out the energy
advantages of Hydride
batteries ,but were
reluctant to mention
some of the drawbacks!
Advantages
Nickel-Metal Hydrides,
or Ni Mh, have more
capacity then Nickel
Cadmium
(NiCad) cells
for the same size.
They are environmentally
friendly , can deliver
higher current with less
voltage drop, ( lower
internal resistance )
and have the potential
for
even more power per
size then Ni Cads.
Disadvantages
Ni Mh cells do not
perform as well as Ni
Cads in higher or lower
temperatures.
Hydride
cells self-discharge two
to three times more then
Ni Cads and they have a
nasty habit of suddenly
stopping. A stored
fully
charged Ni Mh pack can
lose as much
as 25-35%
of it's charge in just a
few
weeks.
How are the charging
requirements different?
When a NiCad or Ni Mh
battery is charging and
becomes “ full”, the
excess
energy starts to
heat the battery. Early,
NiCad, quick chargers ,
let the batteries
get
very hot. The
Temperature Cut Off, or TCO, method resulted in
hot NiCad
and Ni Mh
packs when used as the
primary charge detection
method.
Peak chargers
were an improvement.
They stop charging
before the excessive
heating occurred by
switching off after the
battery voltage
peaks. When a battery
charges, the voltage
continually rises until
the battery starts to
over-charge and
the
battery voltage
decreases slightly. A
typical NiCad voltage
reduction is 20-25 mV
per cell. In other
words, if charging a
battery with 6 cells the
charger would look for,
and shut the high charge
off, when the voltage
dipped down 120mv .( 6 x
20 )
Nickel-Metal Hydride
batteries peak, but very
slightly, approximately
2-5mv per cell.
This is
very slight in
comparison to the NiCad
example and the
depression rate does
not
increase even if you let
the battery get very
hot. Battery chargers
made for Hydrides
can
detect the smaller
voltage depression and
cut the charging current
at the right time.
If our Hydride battery
were charged in
the NiCad peak-detecting
charger used in the
first example, the
charger would be looking
for a 100mv depression,
when the Hydride
pack is
only capable of a
20-30mv drop. Result,
the charger continues to
pump high
charge
current and the battery
gets very hot. The
high charge currents is
eventually
stopped by
thermal circuits
designed to stop
charging when extreme
conditions are
detected.
The cells endure
excessive heat each
time they are charged!
This repeated
overheating of the
cells, reduces the
operational life of the
battery by half, or
about 200
charge cycles
instead of the normal
400-450. For example, a
police chief buys
2100mah Ni Mh battery
packs and is unhappy
when the performance is
just slightly
better
then the 1500mah NiCad
packs he bought for
years. Why, excessive
heat terminates
charge
cycles early when
charged in the same
chargers used for the
old NiCad packs.
Early
termination , equals
reduced charge ,which in
turn results in loss of
performance.
Now, the
chief calls the dealer
back and complains. The
dealers solution is to
provide a
2700 mah
unit. Now, the chief has
spent twice as much
for a 20% improvement in
performance ! If that's
not bad enough, the
battery life for the big
Ni Mh pack is
half as
much as the 1500 Ni Cads
he used to buy!
Summary
Do not put Nickel Metal
Hydride batteries
directly into any
charger
unless you have
checked for
compatibility.
Many companies offer
single chargers that can
charge both Ni Cads and
Ni Mh cells .
A good
place to start is where
you bought your
batteries. Make sure you
are clear
on
any limitations the
dealer mentions. and
follow the advise given.
"I have been using the
wrong chargers for my
batteries , do I need to
scrap all of this
equipment if I want to
switch to Ni Mh?"
NO, Lteck manufactures
compensation circuits
into many Ni Mh packs
that eliminates
incompatibility and
charges the cells
properly.
C.G. Durda
Chief Eng. Lancaster
Consulting
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