Circuit Project Electronic

Introduction
With this mAh meter you can check the mAh capacitance of your NiCad or NiMH rechargeable batteries

Normally the cheaper NiCad AA-type cells have a capacitance of 500-750mAh (NiCAD) the more expensive
NiMH AA-cells types are up to 1800mAh.

I built this circuit because the capacitance of some of my older NiCad batteries seemed to be quite low, now I can
check their capacitance. It is also nice to check if the new cells meet the specified capacitance!

Circuit description
IC1/TR1 builds a constant current sink which will discharge the battery with a constant current of 500mA.
IC1 must be a so-called rail-to-rail opamp like CA3140 of Intersil; a 741 will not work here!
TR1 can be any big- low ohmic mosfet. (The BUK455 in the schematic is 100mohm).
The current “sink” must be adjusted to 500mA with P1; TR1 must be put on a small heat sink.

IC2 checks the battery voltage; when it is too low the output (pin 7) will become high. The trip level can be adjusted by
P2; I set the level on 800mV. The value is not very critical because the voltage of a NiCAD cell drops very quickly
when it is almost empty.

IC3 is a pulse generator. It will generate pulses as long as the battery voltage is higher than the trip level of comparator IC2.
When the battery is discharged, MR (master reset) of IC3 will be high and the IC will quit generating pulses.
The pulse frequency is set by P3; the frequency must be set at 500 pulses per hour; with this setting the counter will display
the capacitance in mAh.

The network of diodes resistors and capacitors around Q2 creates a 200ms pulse for the 24V-electro-mechanical 4-digit counter
which is connected to the “counter” terminals.
D4 protects Q2 against the high voltage spikes from the counter magnetic coil.
The LED will blink at a fast rate when the counter is running

Thank you Source :
http://www.geocities.com/tjacodesign/mah/mah.html

This entry was posted on Wednesday, August 1st, 2007 at 1:28 am and is filed under battery charger. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.