Model Forum / Radio Controlled / Land Models / February 2006

NiCad has less internal resistance and provides more oomph for capacity. -
Poisonous and now virtually banned from Europe hence increased use of NiMH
which is "Greenee" friendly. Power difference is especially noticed when
using smaller AA packs with EP aircraft.

NiMh releases the power more slowly and must be cycled at least three times
before hard use to build up the power.
Note NiMH also suffer from the dreaded "black wire" =
quote     Following from the "Duratrax USA" webpage

"My switch stopped working, and when I checked it, I found the black wire
all corroded. The copper part looked dark and was brittle. What happened?
This is a case of what's been called "Black Wire Corrosion". Over time, the
negative lead from the battery pack, through the switch harness, to the
receiver will corrode until the copper wire becomes dark, almost black, and
brittle. It no longer has the bright "coppery" look, and is no longer
flexible.

The cause is storage of the system in a damp environment with the battery
installed. The effect of the wire being connected to the battery pack, and
the environmental moisture, will cause an electrical effect to promote
corrosion of the wire. The corrosion usually starts at the battery pack and
works its way towards the switch harness.

A "damp" environment does not necessarily mean that it's particularly humid.
Storage in a garage or shed provides enough humidity to allow the corrosion
to happen. The wet that gets brought into a garage from your car is enough.
It will happen faster if the battery pack is not maintained and allowed to
go flat. Keep the battery charged and cycle it regularly to prevent or slow
down the corrosion.

The net result of black wire corrosion is to make the battery lead act like
a resistor, which will prevent proper current flow from the battery pack to
the receiver and servos. In some cases, the resistance can be high enough
that during aerobatics, with all servos moving, the voltage at the receiver
can drop enough to cause the receiver to quit. The model crashes as a
result.

What can make this baffling is the fact that the R/C system may operate
normally when tested. That's because the tests don't involve high loads upon
the servos, so the voltage drop caused by the resistance of the corroded
wire isn't enough to cause the receiver to quit.

Transmitter batteries can also be affected, but usually not as much because
transmitters are usually stored in a friendlier environment. They still need
to be checked periodically, though. When the corrosion gets bad enough, the
transmitter will just not turn on. It's not likely that the transmitter will
fail during a flight.

The effects on the corrosion would also be seen on the transmitter's power
meter as low output. The battery pack gets blamed, gets replaced, and the
problem goes away. That's because a new battery pack comes with new wires.
Corrosion may never even be suspected or found in these cases.

There is no cure once black wire corrosion starts. You can only replace the
wires. Prevention requires that your equipment be stored in a clean, dry
environment, and maintain your batteries. Store them fully-charged and cycle
them regularly. If you can't do that, then at least remove the batteries
from your models, and store them, along with your transmitters, inside,
where the temperature and humidity are fairly stable, compared to a garage
or shed."

[see also
"Hobbico" webpage = http://www.hobbico.com/faq/product-faq.html#q599
and under comments toward end of NiMH charger review at
=  http://www.imaging-resource.com/ACCS/C204W/C204WA.HTM]

Much more info on my web page under batteries and chargers.

regards
Alan T.
Alan's Hobby, Model & RC Web Links
http://homepages.ihug.co.nz/~atong/
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