🚀 go-pugleaf

>In article <BzD705.C94@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes:
>>In article <1992Dec15.103954.6299@quando.quantum.de> henigan@quando.quantum.de (Kevin Henigan) writes:
>>>I believe you can tell how full a nicad is from it's Voltage...
>>
>>No, you can't.  This is one of the fun problems of building a NiCd charger.
>>The output-voltage curve of a NiCd is essentially flat from pretty nearly
>>full charge to pretty nearly empty.

Ah, but there *is* a way to determine when full charge is reached.
The November, 1983 issue of the UK magazine Wireless World (now called
Electronics World) had a construction article for a device that would
safely charge any nicad battery pack in just one hour.  Here is the
basis of that circuit:
 "If a moderately heavy charging current is applied to a NiCd cell for
a short period, the terminal voltage will quickly rise to a level
somewhat higher than the voltage marked on the cell.  Discharging the
cell a little will reduce the terminal voltage once more.  A repeating
cycle of rapid charge and light discharge will produce a succession of
slowly rising `peak' and `trough' voltages, as shown in diagram.
 If the cycle is repeated for long enough, there will come a stage at
which the peak voltage is increasing much more steeply than the trough
voltage.  At the point the cell is losing the ability to accept
further charge and the charger can be shut off.  By measuring the
trough voltage as a fraction of the peak voltage it is possible to
make an end-point detector which works independently of the number of
cells under charge, since the proportions will be constant whether
there is just one cell or a dozen.  If the charge and discharge
currents are selected to correspond with the rated capacity of the
battery, the same setting of the detector should be able to cope
effectively with all cell sizes."

                                               ..........peak
                                         ...~~~
v|                                 ...~~~
o|                         ....~~~~
l|            ........~~~~~
t|....~~~~~~~~                       ....................trough
a|            .............~~~~~~~~~~
g|....~~~~~~~~
e|___________________________________________|_______________
                           time            60 min

For 500 mah batteries, the device typically produces 650 ma charge
periods for 30 seconds, alternating with 50 ma discharge for 10
seconds. 

--Ken Carver