maAccuMeter – battery meter

maAccuMeter it is a simple meter of 1.2V accumulators capacity (R6, R14, R20, AA, AAA, etc.). It allows to measure two accumulators at the same time. During projecting we focused on simplicity and total cost of the device. Due to these assumptions it is an adapter to the computer and communicate with it via USB interface. For communication V-USB library is used. Additionally C# application was written (required .NET 4.0 or above), installation of any additional drivers is not required.

Features:

displaying accumulator voltage
constantly current load
displaying capacity of accumulator
chart of voltages and currents
internal voltage reference (Vref)
minimal voltage of accumulator; protection against deep discharge

How it works:

two independent test tracks
3 parallel connected MOSFETs BS170 are responsible for the load current
transistors are controlled by PWM signal which was smoothed by RC filter
power dissipated across the resistor 1Om is measured and based on this value the capacity of accumulator is calculated
two ADC channels measure voltages before and after resistor, these values are enough to calculate current
based on the current value we can modify width of pulse of PWM and we can modify opening of transistors

Additional information:

device can be build based on ATtiny461 or ATtiny861 microprocessor (recompilation of firmware could be required)
in the prototype quartz 16MHz was used, is possible to use other quartz which is supported by V-USB library
internal reference voltage is used, is possible to measure it on pin 17 and this value should be set by windows application
suitable value of FuseBit should be set (FuseLowByte=0xef, FuseHighByte=0xdd)