Edward Knobloch wrote:


Chuck Harris wrote:


NiMH cells don't like being trickle charged. If you want to
charge them at a "trickle" charge rate you need to use a smart
charger that will cut off the current after they have reached
full charge.


Hi, Chuck -
Yes, I agree that one must be careful about using a NiCad charger
with a NiMH battery, the NiMH can be in danger of overcharging.

In the Racal receiver memory protect circuit,
there is a silicon diode in series with a 270 Ohm resistor
from the 5V supply to the 2.4V battery, which means about 7 mA
trickle charge rate. I looked at the Varta site, for a 110 mA-H
button cell (2/3 capacity of the 150 mA-H Racal battery suggestion),
they say either 11 mA continuous trickle or 22mA for 1 year
would be overcharging. So, there should be no problem in the Racal.

73,
Ed Knobloch

Hi Ed,

There is alot of pressure on the NiMH battery makers to pretend the
NiMH cells are direct replacements for NiCD cells. If you are willing
to accept drastically shortened cell life (compared to NiCD), they are.

What I have found to happen (during my research phase for a series of
rapid chargers I designed for the US Army) is NiMH cells, once they
achieve full charge, will outgas if continued on even a 1/20th C
charge rate. Pressure builds, and the safety valve spits... Soon,
the cell is bone dry, and has no capacity. I found that you could keep
the cells on a 1/100th C rate indefinitely, and it did a good job of
keeping the cells from self discharging... well new cells anyway.

So, a good compromise for a trickle charger is to charge at 1/10th C
for 14 hours, and switch over to 1/100th C. The Racal cannot do this.

-Chuck Harris