No one has mentioned the memory effect of NiCd batteries. They should be
pretty well discharged before recharging because if you don't, they develop
a memory, and will not fully charge. NiMH do not, and you can charge them
even if you have only used them a little. It is better to discharge them
more to get a better recharge on them, but they do not have nearly the
memory effect of a NiCd.

James, you need to have your facts straight before stating things as Gospel.
Charge time not important? BWAHAAAAAAAAAAAA! That's rich.

Steve




On the contrary, you should do a bit more research, or maybe just read
my post more closely, as my statement of charge time not being important
reflects the specific condition of trickle or float charging. This isn't
really the best forum for this though, might wanna pop on over to one of
the EE groups, or candlepower forums, lots of discussion with a lot of
very knowledgeable people, both professional engineers and
hobbyists/enthusiasts.

Much debate has raged over the years on the topic of NiCD memory
effects, and to this day it remains controversial as to whether the
memory effect exists. My own experimentation suggests that the perceived
memory effect is due to poor quality chargers that severely overcharge
cells in the specified time unless they are discharged first, leading to
a dramatic reduction in lifespan. With any charger, there is a trade off
of charge time vs cell life and people like things to be fast. Most
inexpensive chargers have no onboard intelligence, they are simply a
constant current source and rely on the user to manually terminate
charging. If you want to charge a battery in a short amount of time with
one of those, you had better discharge it first so you know where you
start out, and then charge for a specific amount of time to prevent
overcharge. Better chargers are timer controlled and will charge at a
constant current of a few hundred mA in order to charge the batteries
relatively quickly, then revert to a 5-10mA trickle charge to maintain
them at a fully charged state, these will also overcharge if the
batteries are partially charged when you start out. The best chargers
are microprocessor controlled and monitor one of two things, cell
voltage vs time, or cell temperature rise to detect a fully charged
state and drop to a low current trickle charge. They are nice because
they are capable of individually charging multiple cells to full
capacity rather than charging groups in series and assuming all started
out the same.

Cheap items such as the emergency lights in question have very simple
chargers, they are a constant current source with no feedback, they
charge at a very low current which is essentially a trickle charge and
they do so constantly. The trade off here is long charge time, usually
close to 24 hours, but that doesn't matter since power outages usually
have more than that much time between them, as well as a somewhat
shorter cell life, given the resulting low cost of the item, this is
deemed acceptable. On a related note, years ago I got tired of the
backup battery in my digital clocks always being dead whenever I needed
it so I did a simple modification and installed a 1K resistor across the
isolating diode for the backup battery resulting in the power supply in
the clock providing a steady 3-5mA of current to a NiCD "9V" (really
7.2V) battery. This has been working great, and more than 6 years later
the original batteries in the four clocks I modified are still working
great despite being trickle charged steadily for those years.

In a nutshell, at high enough current level to charge a battery quickly,
180-500mA or more being typical, charge time is very important and
once the cell reaches full charge that energy will go into heating the
cell rather than the chemical reaction that stores energy. If you keep
that up, the electrolyte will vent and permanent damage will occur. On
the other hand, if you charge a cell at a low current of say 5-10mA, you
can charge it indefinitely without damage.

I stand by my claims that NiMH and NiCD are virtually identical from an
electrical standpoint and the differences only become important when
you're trying to charge them quickly or power loads that draw a very
high current. NiMH cells have a higher capacity generally speaking,
while NiCD cells have a lower internal resistance and are thus able to
dump that charge more rapidly. They both have a nominal cell voltage of
1.2V, they both have a recommended standard charge current of C/10,
where C is the mAh rating of the cell, and they both need to be charged
by a current limited source with an OCV higher than the nominal cell
voltage, and both can be trickle or float charged indefinitely at C/100
or so. This is based on my education, research, and years of personal
experience. If you insist that this is incorrect, please cite sources.