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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

VOLTAGE REFERENCE AND BATTERY TEMPERATURE

The voltage reference is supplied by a TL431 CPK voltage reference. A_REF is set by resistors R34 and R10 and can be calculated by:

[1]

This value is a trade-off between a high-resolution (low A_REF value) and a high signal-to-noise ratio (high A_REF value). The voltage reference is common for both battery charger designs.

Temperature is measured by a negative temperature coefficient (NTC) resistor that has an approximate resistance of 10 kilohms at 25ýC. The NTC is part of a voltage divider, which is powered by the reference voltage.

The resolution, in respect to the voltage measured across the NTC, is the same as for the voltage measurement circuit:

[2]

The steps can be calculated by the following equation:

[3]

The NTC resistance does not follow a linear curve, which makes it difficult to calculate the temperature from the ADC value. Using a table to look up the temperature solves this (see Table 1).

Table 1ýCheck out the NTC steps according to temperature.

Table 1 indicates the steps equal 0.5° C for ADC values 400 to 675. ADC value 400 is approximately 37ýC and 675 is 8.6ýC. Using this table and doing some minor changes in the header file B_DEF.H will make it easy to implement any NTC resistor.

The ATtiny15 battery charger design assumes that the linearity of the thermistor is sufficient to detect a temperature increase. Therefore, it uses a constant compare value to monitor the temperature.

The values in the table are calculated from the NTC voltage divider and datasheet.

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