8 August 2017
In this column, Kristen answers questions about all aspects of electronics, including computer
hardware, software, circuits, electronic theory, troubleshooting, and anything else of interest to
the hobbyist. Feel free to participate with your questions, comments, or suggestions. Send all
questions and comments to: Q&A@nutsvolts.com.
n WITH KRISTEN A. McINTYRE
More on NiCd Chargers
QI am an Ophthalmologist and a long time Nuts & Volts reader. I use several hand diagnostic ophthalmic instruments that are powered by rechargeable 3. 5 volt NiCd batteries that are
well charged constantly by a transformer, a resistor, and
a diode. They are under a constant voltage charge of 3. 5
VDC in a well charger. The batteries are made up of 3-1/2
size AA cells in series. Is there a better charger for the 3.5V
NiCds since they remain on constant charge?
John Haley
AThis is similar to another recent question I got about NiCd vs. NiMH charging. Just to repeat a bit of what I’ve written before: “For a NiCd, as you mention, a constant
current is generally adequate, as long as the current is kept
relatively low. Charging at a rate like 1C (or the capacity of
the battery in one hour) can cause overheating, but lower
rates are fine.”
Please keep in mind that everything I’m about to say
only applies to NiCd batteries, and not to nickel metal
hydride (or NiMH) batteries.
Your simple charger is probably doing a
reasonable job when the cells are healthy. The
resistor limits the current and the transformer
provides an upper bound on the voltage, given
the characteristics on how voltage is induced in
transformer windings (actually a consequence
of Faraday’s Law of induction).
Figure 1 reproduces this simple circuit. The peak
current is approximately expressed by (Vp - 0.6 - 3Vb)/R =
I, where Vp is the peak voltage from the transformer and
Vb is the cell voltage; typically 1.2V. The approximation
results from the diode drop voltage of ~0.6.
We don’t know the transformer voltage or the value
of the limiting resistor, but we can say with certainty that
if the battery voltage decreases, then the current flowing
into the batteries will increase. So, how could the battery
voltage decrease beyond discharge?
NiCd cells are prone to internal shorts resulting from
chemical filaments growing inside; particularly if they
are stored without charging. If one of the three cells you
have shorts internally, the current through the remaining
cells increases. The current equation changes to (Vp - 0.6
- 2Vb)/R = I. Notice the factor of 2 instead of 3 on Vb.
This might or might not be a really significant change,
depending on the resistor and transformer voltage. There is
the potential to overheat the remaining batteries, perhaps
damaging them.
What can we do better? The easiest approach is to
avoid charging the batteries in series. Instead, use three
separate resistors to
limit the current to
the cells individually,
making it so that one
cell’s failure wouldn’t
significantly affect the
next (Figure 2). This
could be impractical,
though, because the
batteries have to be
reconfigured from
their normal use
topology for charging.
One other
approach might
be to use a current
source that would
deliver roughly the
same current even
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Q & A
n FIGURE 1. Simple
NiCd trickle charger. n FIGURE 2. NiCd trickle charger for
individual cells.