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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

SOFTWARE IMPLEMENTATION

Now that Iýve explained most of the hardware, itýs time to take a look at the software used in the battery charger reference design and the explanation of the C code implementation for the ý2333. The same principles also apply for the assembly code for ATtiny15. For a complete description of the ATtiny15 assembly code, see the comments in the source code (download entire code package).

As long as the battery type to be charged is set at program compile time, the software can be extended to support charging more than one battery. The straightforward implementation is to charge batteries sequentially, allowing each battery a time slot during trickle-charge. SLA and Li-Ion batteries can be charged in parallel with constant voltage charging if the number of battery cells in each battery pack is the same. The charging current for each battery is limited and the charging voltage is limited to one cell.

In "Battery Characteristics" of b_car.h all values are calculated with all their scaling factors. These values are defined in the include files, calculated at compile time, and then handled as constants during program execution. All values taken from the A/D converter can be compared directly to these constants. This means that no time is spent recalculating values during program execution, which saves time and memory space. The values and formulas used to calculate the values can be found in Equations 4ý7 (Equations 13ý15 for the ATtiny15).

For NiCd batteries, the charge is started if the battery temperature is within the temperature range. The charge is always terminated with an error message if the temperature is higher than the maximum temperature, if the voltage exceeds the maximum battery voltage, or if the maximum fast-charge time expires.

The normal signs that the battery is fully charged are temperature rise (dT/dt) and voltage drop (ýdV/dt) methods. Therefore, a sample is taken every minute for the temperature and every second for the voltage. The values are compared to the sample taken 1 min./s ago. If the battery is fully charged, the charge status is automatically changed to trickle-charge, causing the program to jump into the trickle_charge() function.

The trickle_charge() function executes in a loop, checking for a change of the charge status, temperature, and voltage measurement and adjusting the current. If the temperature is outside the valid range or a voltage overflow is detected, an error flag is set and the function is terminated. If no error occurs and you do not change the charge status, the program loops forever, adjusting the charge current to the current defined at the top of the module.

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