Here are three good ways to start, Vic:

1.  Check the PICLISt archives - several of us have just been working on
chargers last June or July.

2.  Check www.maximic.com  for their max797 charger chip.  They've got some
APP notes for several types of chargers, NiMH and NiCD.  You'll find NiMH
and NiCD chargers are very similar, with subtle differences in charging
sequence.  But in a pinch, one will work passably for the other.   Maxim
makes a chip for both technologies, can't remember which one the MAX797 is..

3.  Here's the principle:  Pump a constant current of 0.1 to 1.0 C (Your
battery's rated charge rate) into the baTtery, and measure the voltage while
you are doing this.  A NiMH will increase in voltage until the maximum
charge is reached, and then flatten out.  A NiCD will actually begin
dropping in voltage when it's full, but just a small amount.  Both types
will begin increasing temperature when they are full.  The really fancy
chargers look for voltage drop AND temperature rise, and limit the
temperature rise on the battery to a few degrees C.  These types will make
batteries last a long time, voltage based chargers will work fair.  You can
hack up a 5 volt regulator across a dropping resistor and your voltmeter and
test this scheme out in about 10 minutes, although I wouldn't recommend
using this method for anything but an experiment.

Many cheap NiCD chargers just use a constant voltage source and a resistor.
While this is simple and cheap, it is definitely not good for your
batteries, they won't last.

I'm slowly working on a charger for a robot project.  Alll the robot control
is built into a single PIC, so it combines everything in one controller.

--Lawrence Lile


----- Original Message -----
From: +ACI-Vic Lopez+ACI- +ADw-lopez+AEA-NETWORLD.COM+AD4-
To: +ADw-PICLIST+AEA-MITVMA.MIT.EDU+AD4-
Sent: Saturday, September 22, 2001 3:39 AM
Subject: +AFs-OT+AF0-: Need to build NiCd Battery charger


+AD4- Hi,
+AD4-      I have a 12 V 1200mAh NiCd battery pack. I need to build a battery
+AD4- charger for it. I'd appreciate any pointers. Thanks a lot. Vic
+AD4-
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