QUESTIONS and ANSWERS
six hours and then shut down. The
circuit is in Figure 2.
My problem is that NiCad
batteries work fine, but are getting
hard to find. So far, I can get them
from Mouser, but I wonder how long
that will last. NiMH batteries work for
a few days, then quit. It may be that
they run until the original charge is
used up and never charge at all.
In the sun, the input to a NiCad
pair is 3. 62 volts after two-thirds of a
day's charging. It would not be
practical to add another solar panel if
I need more voltage to charge NiMH
batteries. The diode has the lowest
forward voltage drop I know of.
Is there anything else I can do to
accommodate NiMH batteries? I
would like to know what is going on.
I thought that NiMH batteries were
different somewhat from NiCads, but pretty close — at
least close enough for a circuit as simple as this one.
— Alan May (still using a Heathkit 1500 amp)
AI am glad to see someone else using older but proven equipment (I still use a Simpson meter — for those who don't know what this is, Google it). At the electronics program at the community
college where I taught, we had several Heath power
supplies dating to the late ‘70s and they kicked the butts
of the newer, slicker digital supplies.
As to your question, NiCad (Nickel-Cadmium) and
NiMH (Nickel-Metal Hydride) batteries are very different
"animals." Besides the differences in chemistry (for those
non-chemistry readers, I won't bore you with this
discussion, but for those interested in the chemistry check
the Internet because it would take more space to explain
that I am allowed here), the batteries have different charge
and discharge characteristics. NiMH batteries self-discharge at twice the rate of NiCads of the same ratings.
(In plain speak, if the NiCad lasts six weeks on the shelf,
the NiMH in an unpowered device will only last three
weeks before it needs a recharge.)
NiMH charge rates must be lower than that of NiCads
to avoid damaging the NiMH battery. NiMH batteries will
last longer, but have a lower maximum discharge rate than
NiCADs. NiMH batteries are very sensitive to over-charging, which I suspect your charger is doing. Charging
NiMH batteries requires a circuit that detects state-of-charge so the battery is not over-charged, and the solar
flashlight may not have enough room to install this
A low value resistor in series between the diode and
battery may reduce the charge rate sufficiently, but you
will have to experiment to find a suitable value. As to
NiCAD batteries being hard to find, that’s a new one to
■ FIGURE 2.
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