Using LM338 to make a battery charger

When designing a dual-purpose AC/DC test equipment controlled by a single-chip microcomputer, a 12V, 4Ah lead-acid battery was used. In order to make the test equipment more convenient to use, the equipment itself must have the function of charging the battery. In order to minimize the production cost and reduce the volume and weight of the equipment. The three-terminal voltage regulator LM338, which originally only works in the equipment test state to generate a constant current power supply , is used as a control tube for battery charging when the equipment is in the charging state. Because a single-chip microcomputer is used, the battery can be switched between the "charging" and "working" states. The following mainly introduces how to use LM338 to realize the battery charging function.

When charging the battery, the device does not need to output current . Can LM338 be used in the charging circuit through circuit switching? Practice has proved that it is feasible. The charging circuit for 12V, 4Ah lead-acid battery using LM338 is shown in the figure above. The battery in the figure is in the state of outputting current. When charging the battery, the device is connected to the mains. Control K1 to make J2 close, the positive pole of the battery is connected to BA, and the battery enters the charging state.

The key to this circuit is the LM338 adjustable three-terminal voltage regulator. According to the different states of the battery charging process, the external circuit of LM338 can be changed to make it have two functions: "constant voltage" and "constant current". In this circuit, when the battery voltage is lower than 15V, LM338 outputs constant current; when the battery voltage is charged to 15V, LM338 automatically changes to constant voltage output. Thus, the entire charging process can be completed well. How does this circuit achieve this purpose?

The constant current and constant voltage switching circuit is composed of the voltage comparator LM393 and the voltage stabilizing diode . The voltage stabilizing value of Z1 is 15V (in order to ensure the reliable switching of the comparator, the comparator IC1 (2) pin is slightly lower than 15V during debugging). When the battery voltage is higher than this value, the LM393 (3) pin outputs a high potential, and the J1 power-on relay is energized. When the battery voltage is lower than this value, the LM393 (1) pin outputs a low potential. J1 is released when power is lost.

When the button switch K1 is in the open position, it supplies power to the outside. When charging is needed, press K1, the positive pole of the battery is connected to the second group of common contacts of the relay, and the battery voltage is lower than 15V. The comparator|1 pin outputs a low potential, the transistor G1 is turned off, the relay J1 is released, the ⑾~⒀ pins are connected, and the ⑷~⑹ pins are connected. At this time, the LM338 is in a constant current output state, the current size = 1.25/(R5∥R6)A, and it provides a constant current of about 0.65A; when the voltage across the lead-acid battery reaches 15V, the IC11 pin outputs a high potential, the transistor G1 is turned on, the relay J1 is energized, the ⑼~⒀ pins are connected, and the ⑷~⑻ pins are connected. The equivalent circuit is shown in the figure below.

From the equivalent circuit, we can see that LM338 is a voltage-stabilized power supply. Since the current is very small during charging, the effect of R6 on the charging current can be ignored. The voltage-stabilized value is related to R2. By adjusting R2, a 15V voltage-stabilized output can be obtained. At this time, the lead-acid battery is in a slow charging state. It takes about 6 hours to fully charge a 12.4Ah lead-acid battery with this charger. Repeated use has proven that the expected effect is achieved.

Because the device itself uses LM338 as a 3A constant current source, it is equipped with a large heat sink. There is no problem of overheating damage in the charging state. If you make a charger separately, you must take the heat dissipation problem into consideration. If R6 is 2Ω, the power requirement is more than 2W. The 20V voltage input uses a switching regulated power supply. K1 uses a button switch with a self-locking function. When the switch is released after full charge, the lead-acid battery switches to the position of providing power to the outside.