Design Ideas: August 1, 1996

Smart chargers for rechargeable batteries often use terminal voltage and cell temperature to optimum time to terminate fast charging. In the circuit in Figure 1, the active temperature sensor in the rechargeable battery pack does not drain the battery, and it needs only one terminal for its power-supply input and temperature output. IC₁, an LM235 temperature sensor, behaves as a two-terminal shunt voltage reference with a 10-mV/K temperature coefficient. Although IC₁ is located in the battery pack, it derives its power from the charger and, thus, cannot discharge the battery.

Resistor R₁, connected to a positive voltage within the charger, develops a current to power IC₁. You can choose an ADC with the resolution that provides the desired sensitivity to temperature changes. With an 8-bit ADC, one LSB corresponds to 0.5°C change in temperature. The 10-bit converter used in Figure 1 provides a 1-LSB transition every 0.125°C. R₁ provides approximately 1 mA to the LM235, as well as an additional 350 µA (at a temperature of 350K) to the R₂-R₃ divider. In this example, R₁ connects to the ADC's 5V power supply. Other supply voltages can work, provided you select R₁ to provide a nominal supply current of approximately 1 mA to IC₁.

Voltage-reference IC₃ provides the 1.225V reference voltage to the ADC10732 10-bit ADC. The attenuation provided by R₂ and R₃ reduces the LM235's 10-mV/K scale to 9.57 mV/K, resulting in the desired temperature-to-digital sensitivity of 8 LSB/K. The second voltage reference, IC₂, stacks on IC₃ to provide a 3.725V (389K, or 116°C) reference point against which to differentially measure the sensor's output. Because the ADC10732 can measure negative or positive input-voltage differentials, the circuit can measure battery-pack temperature from below 0°C to the 125°C temperature limit of the sensor.

In charging nickel-metal-hydride (NiMH) batteries, the rate of temperature rise is the most reliable indicator of full charge. Depending solely on terminal voltage in charging NiMH batteries results in reduced capacity and cell lifetime. For NiCd batteries, the terminal-voltage rate of change provides the most useful charge-termination data, and the rate of temperature increase provides a "backup" input to the charging system. Lithium-ion battery packs use temperature sensors to detect excessive cell temperatures. The packs do this to cut off charging or discharging to avoid damage.

Optimal charging profiles and safe operating limits vary from one chemistry to another and even from one cell model to another. You should, therefore, always consult the cell vendor for up-to-date information. The ADC in this circuit also analyzes battery-pack terminal voltage for optimal charge termination. Select R₄ and R₅ to attenuate the terminal voltage, so that it is approximately 4.5V at the ADC's input at charge termination. Mount the sensor in close thermal contact with the cells for best indication of cell temperature. (DI #1903)