Teardowns of electronics in 2019.
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<TD HEIGHT="19">November 17, 2017</TD>
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<H1>NiCad Battery Charging Basics</H1></TD>
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<H2 ALIGN="Left">NiCad (NiCd, Nickel Cadmium) Battery Charging</H2>
<H2 ALIGN="Left">Nickel Battery Charging Basics</H2>
<P ALIGN="Left">NiCad and NiMH batteries are amongst the hardest
batteries to charge. Whereas with lithium ion and lead acid batteries you can
control overcharge by just setting a maximum charge voltage, the nickel based
batteries don't have a "float charge" voltage. So the charging is based on
forcing current through the battery. The voltage to do this is not fixed in
stone like it is for the other batteries. </P>
<P ALIGN="Left">This makes these cells and batteries especially
difficult to charge in parallel. This is because you can't be sure that each
cell or pack is the same impedance (or resistance), and so some will take more
current than others <I>even when they are full.</I> This means that you need to
use a separate charging circuit for each string in a parallel pack, or balance
the current in some other way, for example by using resistors of such a
resistance that it will dominate the current control. </P>
<P ALIGN="Left"><FONT COLOR="#000000">The coulometric charging
efficiency of nickel cadmium is about 83% for a fast (C/1 to C/0.24) charge,
and 63% for a C/5 charge. This means that at C/1 you must put in 120 amp hours
in for every 100 amp hours you get out. The slower you charge the worse this
gets. At C/10 it is 55%, at C/20 it can get less than 50%. (These numbers are
just to give you an idea, battery manufacturers differ).</FONT></P>
<P ALIGN="Left"><FONT COLOR="#000000">When the charge is complete
oxygen starts being generated at the nickel electrode. This oxygen diffuses
through the separator and reacts with the cadmium electrode to form cadmium
hydroxide. This causes a lowering of the cell voltage which can be used to
detect the end of charge. This so-called minus delta V/ delta t bump that is
indicative of end-of-charge is much less pronounced in NiMH than NiCad, and it
is very temperature dependent. Many of the chargers listed here use a
sophisticated algorithm that uses the -deltaV to accurately charge NiMH and
NiCad packs <A
HREF="NiCad-NiMH-Catalog.htm">/NiCad-NiMH-Catalog.htm</A></FONT></P>
<P ALIGN="Left"><FONT COLOR="#000000">As the battery reaches
end-of-charge oxygen starts to form at the electrodes, and be recombined at the
catalyst. This new chemical reaction creates heat, which can be easily measured
with a thermistor.. This is the safest way to detect end-of-charge during a
fast charge. This method is often used with multi-cell packs and the 20, 30,
and 40 cell pack chargers here use a thermistor
<A HREF="NiCad-NiMH-Catalog.htm">/NiCad-NiMH-Catalog.htm</A></FONT></P>
<P ALIGN="Left"><FONT COLOR="#000000">Nickel cadmium battery
chargers should cut the charge off when the temperature exceeds the maximum
charging temperature, typically 45 degrees C for a controlled fast charge, and
50 degrees C for an overnight or fast charge.</FONT></P>
<H2 ALIGN="Left">Overnight Battery Charging</H2>
<P ALIGN="Left">The cheapest way to charge a nickel cadmium battery
is to charge at C/10 (10% of the rated capacity per hour) for 16 hours.. So a
100 mAH battery would be charged at 10 mA for 16 hours. This method does not
require an end-of-charge sensor and ensures a full charge. Cells can be charged
at this rate no matter what the initial state of charge is. The minimum voltage
you need to get a full charge varies with temperature--at least 1.41 volts per
cell at 20 degrees C. The best charging practice is to use a timer to prevent
overcharging to continue past 16 hours. An example of this kind of charger is
shown at <A HREF="Ni-6-200.htm">/Ni-6-200.htm</A> . This charger uses a
microprocessor to report the state of charge via an LED as well as performing
the timing function. </P>
<H2 ALIGN="Left">Faster Charging</H2>
<P ALIGN="Left"><FONT COLOR="#000000">Some nickel cadmium cells are
designed to be "quick chargeable." This is just a timed charge at C/3 for 5
hours, or C/5 for 8 hours. This is risky because the battery should be fully
discharged before charging. If the battery still has 90% of its capacity when
the timer starts you would have a good chance of venting the battery. One way
to ensure this doesn't happen is to have the charger automatically discharge
the battery to 1 volt per cell, then turn the charger on for 5 hours. The
advantage of this method is to eliminate any chance of battery memory.
PowerStream does not currently have such a charger, but the microprocessor
board used in the C/10 charger <A
HREF="NiMH-NiCad-solar-charge-controller.htm">/NiMH-NiCad-solar-charge-controller.htm</A>
could easily be modified to do the discharge. A power dissipating package would
be needed in order to dissipate the energy from a partially charged battery in
a reasonable amount of time. </FONT></P>
<H2 ALIGN="Left">Fastest Charging</H2>
<P ALIGN="Left"><FONT COLOR="#000000">If a temperature or voltage
monitor is used NiCad batteries can be charged at rates up to 1C (in other
words 100% of the battery capacity in amp-hours for 1.5 hours). The PowerStream
battery charge controller shown in <A HREF="Product3.htm">/product3.htm</A>
does this, as does the battery management board shown in <A
HREF="Product5.htm">/product5.htm.</A> </FONT></P>The termination can be done
with minus delta V, when the battery voltages drops -10 to -20 mV per cell. To
terminate the charge on temperature requires a temperature slope
measurement.<BR><BR>This board also has the ability to sense voltage and
current for more sophisticated algorithms required for ultra-fast charging.
These algorithms require constant monitoring of the voltage, temperature, and
sometimes pressure, to actively determine the amount of current a battery can
take without damage. This is sometimes called a smart charge, or a controlled
fast charge.<BR><BR>A cheaper version of the fast charger can be made by just
monitoring absolute temperature. The charge rate is set at C/2 until 45 degrees
C is reached, then switched over to a C/10 charge to complete the charge. This
is the most common NiCd fast charger of the 1960's through the 1980s because it
could be controlled by a simple bi-metallic thermostat switch mounted on the
battery.<BR><BR>
<H2>Trickle Charging Nickel Batteries</H2><BR>In a standby mode you
might want to keep a nickel cadmium battery topped up without damaging the
battery. This can be done safely at a current of between 0.05 C and .06 C. The
voltage required for this is dependent on temperature, so be sure to regulate
the current in the charger.<BR>
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