-MaH meter-

-Check the milli-Ampere-hour capacitance of your rechargeable NiCad batteries-

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

mAh meter  schematic:

Circuit adjustment & application

• First set all potentiometers in the center position, then switch on the power.
• Connect a power supply set at about 5V to the battery terminals, the LED starts blinking.
• Adjust the discharge current to 500 mA using P1.
• Adjust the cuter frequency to 500 pulses/hour (25 pulses/ 3 minutes) with P3; it will take some time
  to do this accurately!
• Reduce the power supply voltage to 800mV and set the trip level with P2 (use the blinking LED as indicator)

Now the circuit is ready for use: when you insert a freshly charged battery, the LED will start blinking and the counter
will start counting.  After some time (1-3 hours) the battery is discharged ad the counter will stop. The readout of the counter
indicates the capacitance of the battery in mAh

When you use a different current setting the pulse frequency must be changed accordingly.
The circuit can also be used to check other batteries ;you will have to change the trip level and current sink setting to the
size and the voltage of the battery.

Note that the current sink remains active when the trip level is reached. This is not a problem for NiCAD / NiMH cells,
but  lead based batteries (car batteries) can be ruined when they are discharged too deep !
To prevent this you can put about 10 power diodes in series with the car battery, this way the voltage will always remain above 6-8 Volts.

→ Download specifications of key components at the  Component specs page

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