Dual comparator thermally protects lithium-ion battery

Most manufacturers recommend that you don't change lithium-ion batteries at temperatures lower than 0°C or higher than 50°C. You can monitor both thresholds by adding a thermistor and dual (window) comparator to a lithium-ion battery charger (Figure 1). Set the low-temperature trip point at 2.5°C and the high-temperature trip point at 47.5°C. A precision voltage reference is unnecessary, because the comparator's resistor network is ratiometric, so variations on the supply voltage, V_BUS, do not affect the trip thresholds. By driving the charger's enable input, EN, the comparators' open-drain outputs ensure that charging is inhibited when the battery temperature is out of range. As an alternative, you can substitute a dual comparator with push-pull CMOS outputs, such as the MAX9032, if you also add a tiny, SOT-323 dual diode (the dashed lines in Figure 1). The dual comparator and the MAX9032 are available in SOT-23 packages, and both offer built-in hysteresis of 2 or 4 mV, respectively.

IC₂ is a single-cell lithium-ion battery charger that can derive its power directly from a USB port or from an external supply as high as 6.5V. The 0.5% accuracy of its battery-regulation voltage allows maximum usage of the battery's capacity. The charger's internal FET delivers as much as 500 mA of charging current, and you can configure its SELV input for charging a 4.1 or 4.2V battery. The SELI input sets the charge current to either 100 or 500 mA, and an open-drain output, CHG, indicates the charge status. For near-dead batteries, a preconditioning capability soft-starts the cell before charging. Other safety features include continuous monitoring of voltage and current and initial checking for fault conditions before charging.