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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

CHARGE CURRENT

The charge current is measured by sensing the voltage over a 0.025-ohm shunt resistor. This voltage is amplified 20 times using the internal gain stage to improve the accuracy of the measurement before it is fed into the A/D converter.

The ADC input voltage output voltage is:

[16]

where V_Ibat is the analog input voltage to the A/D converter, I_shunt is the current through the 0.25-ohm shunt resistor, Ra and Rb are the resistors used to scale down the voltage on the shunt resistor with the same scale as the voltage measurement, Ra is equal to R9, and Rb is equal to R17.

The maximum current that can be measured is:

[17]

This gives a resolution of:

[18]

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

[19]

The current from a certain step number is:

[20]

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