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A BETTER BATTERY CHARGER

Part 2: Hardware and Software Implementation
by Thomas Richter

Start ý The Buck Converter ý Voltage Reference and Battery Temperature ý AT90S2333 Battery Charger ý ATtiny15 Battery Charger ý Charge Current ý Software Implementation ý User Settings ý Source Code ý INT Battery Function ý The Stable_ADC Function ý BC.H ý B_DEF.H ý Charge MethodýSLA.C ý Charge MethodýNiCd.C and NiMH.C ý Charge MethodýLilon.C ý Suggested Improvement ý Sources and PDF

AT90S2333 BATTERY CHARGER

This section describes the theory specific for the battery charger design based on the ý2333. Figure 2 gives you the parameters for layout and the equations for determining inductance and the duty cycle.

Figure 2ýHere you can see the layout parameters for the ý2333.

The charging voltage is monitored using an op-amp to measure the voltage difference between the positive and negative poles of the battery. In order to select a suitable measurement range for the charger, decide how many battery cells and what type of batteries to charge, select a suitable input voltage (V1ýV2), and scale resistors for the voltage measurement.

The op-amp circuit for measuring the battery voltage is an ordinary differential op-amp circuit. The ADC is capable of measuring the voltage range from A_GND to A_REF (3.67 V). The output voltage (V_BAT2) from the op-amp has to be within this range:

[4]

where V_BAT2 is the output voltage from the op-amp to the AVR A/D, V1 is the positive pole of the battery, V2 is the negative pole of the battery, Ra and Rb are the resistors in the resistor network used to set the gain for the op-amp, Ra is equal to R10 and R12, and Rb is equal to R6 and R7.

The maximum charge voltage will be:

[5]

which makes the gain in the op-amp:

[6]

And, the resulting battery measurement resolution is determined by:

[7]

The charge current is measured by sensing the voltage over a 0.033-ohm shunt resistor (R1). This voltage is amplified using an op-amp to improve the accuracy of the measurement before it is fed into the A/D converter.

This voltage is amplified by factor:

[8]

The op-amp output voltage is therefore:

[9]

The maximum current that can be measured is:

[10]

The step number for a given current can now be calculated from:

[11]

The current from a certain step number is:

[12]

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