Constant current battery charger

　　The battery is charged with a constant current, and the charging current is about one-tenth of the battery's capacity when calculated in ampere-hours. That is, for a battery with a capacity of 4.5Ah, the charging current is about 450mA.

　　This constant current battery charger has the following features:

　　1. Can charge 6v, 9v and 12v batteries. Batteries with other rated voltages can also be charged by simply changing the voltage values ​​of the two Zener diodes ZD1 and ZD2. 2. The constant current can be set at will according to the battery capacity by connecting a potentiometer and a multimeter in series with the battery. 3. Once the battery is sufficient, after it reaches a certain voltage (for example, a 12V battery reaches 13.5V~14.2v), the circuit can give instructions and automatically cut off the charger without removing the battery from the circuit. 4. If the battery is discharged until the voltage is lower than a certain value, the circuit will give a "deep discharge" indication. 5. The quiescent current of the circuit is less than 5mA. Most of its current consumption comes from the Zener tube. 6. The DC power supply voltage (Vcc) has a wide range of use, from 9V to 24V. 7. The charger has short circuit protection function.

　　Under normal circumstances, the minimum DC power supply voltage should be "D1 voltage drop + fully charged battery voltage + Vdss + R2 terminal voltage", which is close to "fully charged battery voltage + 5v". For example, for a 12V battery, if the fully charged battery voltage is 14V, the DC power supply voltage should be 14V+5V=19V.

　　The charger circuit can be divided into three parts: constant current source, overcharge protection and deep discharge protection.

　　The constant current source part is composed of MosFET field tube T5, transistor T1, diodes D1 and D2, resistors R1, R2, R10 and R11 and potentiometer VR1. D2 is a low temperature coefficient, high stability reference diode LM236-5. The gate-to-source voltage (Vcs) of T5 is set slightly higher than 4v with VR1. After setting Vcs, the charging current can be determined according to the battery capacity. First, determine the charging current (one-tenth of the battery's Ah capacity), and then calculate R2 (take the nearest value according to the resistance standard series): R2=0.7/safety failure current.

　　R2 and T1 limit charging current in the event of a fault or accidental short circuit at the battery terminals.

　　Overcharge and deep discharge protection circuits are marked with dashed lines in the circuit diagram. All component values ​​within the dashed box are based on maximum battery voltage, not DC supply voltage. so. The charger circuit can operate under a wide range of power supply voltage without affecting the charging current value.

　　in the overcharge protection circuit. When the battery is full (for example, the 12V battery voltage reaches 13.5V), adjust VR2 to set T5's Vcs to zero, so the charging current stops flowing into the battery. At this time, LED 1 lights up, indicating that the battery is fully charged. While LED1 is emitting light, the internal LED of optocoupler IC1 also emits light, turning on its internal transistor. As a result, T5's Vcs becomes zero and charging stops.

　　In the deep discharge protection circuit, ZD2 is turned on under normal circumstances. It drives transistor T3 to turn on. As a result, T4 is turned off and LED2 does not emit light. If the battery terminal voltage drops to 11V (for a 12V battery), adjust VR3 so that T3 is cut off and T4 is turned on. At this time, LED 2 lights up, indicating that the battery voltage is low.

　　The voltage regulation values ​​of ZD1 and ZD2 are the same for 6V, 9V and 12V batteries. For batteries with other voltages, the values ​​of ZD1 and ZD2 need to be changed appropriately. This circuit provides charging current from 1mA to 1A. T5 does not require a radiator. If the required maximum charging current is 5A, you need to replace D2 with two LM236-5, and change the value of R11 (10kΩ) to 1kΩ. In addition, two SB560s must be connected in parallel to replace D1. At the same time, T5 needs to add a heat sink. When T5 is packaged in TO-220, the maximum power consumption can reach 50w.