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NiCad Charging
Basics
NiCad and NiMH batteries are amongst the hardest batteries to charge. Wheras
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 do
do this is not fixed in stone like it is for the other batteries.
This makes these cells and batteries 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 even when
they are full. 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.
The coulometric charging efficiency of nickel cadmium is about 83% for a fast
(C/1 to C/.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).
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.
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.
Nickel cadmium battery chargers should cut the charge off when the termperture
exceeds the maximum charging temperature, typically 45 degrees C for a controlled fast charge, and 50 degrees C for an
overnight or fast charge.
Overnight Charging
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 http://www.powerstream.com/NiMHWM.htm . This charger uses a microprocessor to report the state of charge via an LED as well as performing the timing
function.
Faster Charging
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 http://www.powerstream.com/NiMHWM.htm 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.
Fastest Charging
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 http://www.powerstream.com/product3.htm does this, as does the battery management board shown in http://www.powerstream.com/product5.htm. 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.
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.
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.
Trickle Charging
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.
Custom design and manufacture of state-of-the-art battery chargers, UPS, and power supplies for
OEMs in a hurry!
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NiCad/NiMH charger catalog page
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