New circuit for fully charging lithium-ion batteries

　　Previously, many manufacturers produced a wide range of fairly standard Li-Ion battery products with a maximum charging voltage of 4.2 V ±1%. As a result, most existing ICs for charging Li-Ion batteries were designed to charge to a tight tolerance of 4.2 V ±1%.

　　However, in the past few years, a new generation of lithium-ion battery technology has entered the market. They offer higher power density, accept greater charge and discharge rates than previous generations of batteries, and are equipped with different charging terminal voltages depending on the manufacturer. The design concept is to modify a standard, high-end IC charger application circuit to provide different terminal voltages and higher charging current rates, while maintaining all the original functions of the charger.

　　In this case, the battery to be charged is the ANR26650m1 manufactured by A123 Systems. It accepts standard charging at 3A (1.3C) and can fast charge at 10A (4.34C) with a charging terminal voltage of 3.6V. Therefore, it represents a new type of battery with a terminal voltage in the range of 4.2V to 3.6V. The circuit shown in Figure 1 modifies the application circuit of an IC ( MAX1737 ) designed (originally) to charge 1 to 4 lithium batteries. By adding a micropower dual op amp (MAX4163) and some resistors, this modification allows you to charge 3.6V batteries.

Figure 1: The dual op amps shown here and associated peripheral components enable this Li-ion battery charger to accept new, higher voltage Li-ion batteries.

　　Additionally, the modification changes the current sensing resistor value (RCS), thereby increasing the limit on the charging current accepted by the A123 battery in standard charging (3A). The power components N1, N2, D1, D4 and L1 shown are suitable for charging currents up to 3A.

　　For currents above 3A, external switches N1-N2 should be rated for higher drain currents, but with similar drain voltages. They should not produce a greater total switch current than recommended in the MAX1737 datasheet. If the charge current exceeds 3A, the maximum current rating of diodes D1 and D4 should also be increased.

　　The MAX1737 charger is internally set to switch from constant-current mode (CC) to constant-voltage mode (CV) at 4.2 V ±0.8% tolerance. The dual op amp MAX4163 is configured to modify that threshold. Op amp A2 is connected as a non-inverting amplifier with a gain of 1.16, so when its input is 3.6 V, it produces a 4.2 V output. The output of op amp A2 is connected to the charger's BATT terminal (usually used to sense the battery voltage), so the charger now switches from CC to CV at a battery voltage of 3.6 V.

　　The input of op amp A2 is connected to the positive terminal of the battery to be charged. If the resistors associated with op amp A2 have a 1% tolerance, the error in the terminal voltage is 3.62 V -1.1%/+1.2%. With smaller tolerance resistors, this error can approach the error of the charger (0.8%). You can also achieve higher accuracy by using the charger's Vadj function (pin 8).

　　Op amp A1 is configured as a differential amplifier with a gain of 1, and its reference voltage (the output voltage when the differential input voltage is assumed to be zero) is the output of A2. The output of A1 is connected to the CS terminal of the charger. (The IC senses the charging current as the voltage difference between BATT and CS.) When the voltage drop across Rcs is zero, the voltage difference between BATT and CS is also zero. The differential input of A1 is connected across Rcs, so the voltage across it is copied by the gain-of-1 circuit as the voltage difference between BATT and CS, as required by the IC. By setting the ISETOUT terminal to half VREF, the batteries are charged to a constant voltage of 3.6V/cell, delivering the charging current at 0.100 V/RCSΩ at the output of A1.

　　These modifications to the charger sense input affect other parameters, one of which is the voltage at which full charging is allowed to begin (2.5V/cell for this charger when unmodified). Amplifier A2 scales this voltage down (to 2.14V) by the same factor as applied to the CC/CV switching voltage. When the connected battery voltage is less than 2.14V, the charger enters a pre-qualification mode where it charges at 1/10 the IOUT setting until the voltage rises above 2.14V. It is then suitable for the full charge rate.

　　The maximum supply voltage of the dual op amp limits the maximum number of cells that can be charged by this circuit to two. Figure 2 shows the voltage-current curve obtained by modifying the circuit of Figure 1.

Figure 2: Charging current versus battery voltage for the circuit of Figure 1.