The BQ24013 is a simple-to-use charge controller suitable for use with lithium-ion and lithium-polymer batteries. A major advantage it has is that it includes integrated power MOSFETs capable of working with charge currents of up to 2 A. Its switching frequency is high, at 1.1 MHz, and so only a small external coil is needed. In comparison to linear charging circuits the switching topology offers a much higher degree of efficiency. A further benefit is that it is capable of charging battery packs consisting of either a single cell or of two cells wired in series. Two LEDs indicate when the battery is being charged (D1 lights) and when the battery is fully charged (D2 lights). The charge current is set by the choice of external resistors [1]. There are three currents to set: the initial (precharge) current, the charge current and the charge termination current. With the component values given the precharge current is 67 mA, the charge current is 667 mA and the termination current is also 67 mA.
The IC of course ensures that the charging process is carried out correctly and in particular that the maximum permissible cell voltage is never exceeded: this is extremely important for lithium chemistry cells. Even more important is to note that jumper JP1 should be fitted only in the case where two cells are being charged. When charging a single cell the jumper must not be fitted, or there is a risk of explosion or fire as the charging voltage will be too high. The minimum supply voltage for charging a single cell is 5 V; for charging two cells it is 9 V. According to its datasheet, the IC is specified for supply voltages of up to 16 V. Unfortunately the IC is only available in a QFN20 package, which is rather tricky to solder.
In compensation, the tiny package does make it possible to build a complete 2 A charging circuit on less than 2.5 cm2 of printed circuit board. For the prototype, with a charging current of 670 mA, we selected for L1 a 4.7 μH inductor with a DC resistance (DCR) of 0.082 Ω (82 mΩ) rated for a current (DCI) of 1.72 A. If a charge current of up to 2 A is wanted, an inductor with a DCR of less than 0.025 Ω (25 mΩ) and a current rating of 4 A or more should be chosen. For R5 we used a Vishay 150 mΩ SMD resistor in an 0805 package (available, for example, from Farnell), and for C3 a ceramic barrier-layer capacitor with a working voltage of 25 V. If an electrolytic capacitor is used it must have a very low ESR. An overview of the various versions of the IC that are available can be found at [2]. For our prototype we used a type BQ24103A.
