The principle of the meter is very simple:You hookup a fully charged battery and load resistor to the meter, enter the value of the resistor and the cutoff voltage and start a discharge cycle. The meter will switch on the load using a power mosfet and continiously measure the battery voltage. To precisily measure the voltage a reference diode is used. During the discharge you can select to display the current voltage and current or the time expired and capacity so-far. If the cutoff voltage is reached the load will be disconnected and the capacity will be shown on the LCD display.source

With this mAh meter you can check the mAh capacitance of your NiCad or NiMH rechargeable batteries.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 and the counter will stop. The readout of the counter indicates the capacitance of the battery in mAh.source

The circuit acts like a magnifying glass on part of the scale of an ordinary meter, namely the part of the scale we are interested in. For example, an ESV for testing a single NiCd cell would range from 1.17V at one end of the scale to 1.28V at the other end of the scale. With the full scale covering this narrow range, it’s easy to see voltage changes within the range.The circuit described here uses a bar graph display consisting of eight LEDs.source

Each LED in the bargraph indicates an interval of this size. In practice (and with practice), the resolution is higher. When an RPM reading is taken that falls between two of the LEDs, both LEDs will flicker, and you can interpolate between the two values by the relative amount of time that each LED is on. source

The circuit is inserted between the supply and the load, therefore, until a trickle-charging current of at least 100µA is flowing towards the load, D1 and D2 will conduct. The forward voltage drop (about 1V) available across the Diodes, drives Q2 into conduction and, consequently, Q1 will be cut-off.If no appreciable load is connected across the circuit’s output, Q2 will become cut-off, Q1 will conduct and the Piezo-sounder will beep.source