Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

1. Conventional charging

Lead-acid battery charging has two ways: float (also known as constant voltage) and charge cycle.

This is the commonly used lead-acid battery charging, but there are some shortcomings of these two ways. In the process of charging, the battery voltage was gradually increased, the charge current decreases. As the constant voltage regardless of the actual state of the battery voltage, charging voltage is always constant, the beginning of the charging current is relatively large, and then decreased exponentially; using fast charging battery may overcharge, easily lead to cell damage. For the charge cycle, the use of small current charge, the charge is better. But for large-capacity battery, charging time will be delayed very long time ,low aging ,has a lot of inconvenience.

2. Process analysis of the intelligent charger charging

After analysis and comparison of both charging, to integrate the their advantages to design a intelligent lead-acid battery charger with fast charge and slow charge. The charger uses singlechip control, charging process is divided into three stages ,fast charge, slow charge and trickle charge, the charging results will be better. Figure 1 shows the charging current and voltage curves of the charger.

It can be seen from Figure 1: In the fast charge phase (0 ~ t1), battery charger is charging for battery with constant current of 1C, controlled by singlechip to control fast charging time ,to avoid overcharge ;in the slow charge (t1 ~ t2), single-chip output PWM control signal ,to control the chopper switch-off, charging for battery by a constant voltage, at this time charging current decrease exponentially, when the battery voltage rises to the specified value, finish the slow charge, and enter trickle charge stage; in trickle charger stage (t2 ~ t3), the single chip output PWM control signal ,to charge for battery by charging current of about 0.09C, in this condition, it can charging for battery in a long time, so that maximize battery life .

3. Working principle of intelligent charger

Based on the above analysis ,design the intelligent lead-acid battery charger, which is mainly consisted four parts of switching power supply, chopper switch, controller, and auxiliary power, and has overcurrent protection, overvoltage and overtemperature protection . Figure 2 shows the functional block diagram of the electrical equipment, Figure 3 shows charger circuit schematic.

3.1 Regulated switching power supply

In the circuit shown in Figure 3, the switching power supply use half-bridge PWM transfer circuit. Its working principle is: IC1 (TL494) 8 feet of and 11 feet switching power supply integrated controller to output the inverted PWM signal ,go through the transistor Q3, Q4 for complementary amplification, provide a driving signal for the transistor Q1 and Q2 base through the driver transformer T2 makes Q1 and Q2 turn off, primary winding N1of high-frequency transformer T1 will generate about 320V peak value square wave, in theT1's secondary winding N2, N3 have the induced voltage to generate, the voltage goes through D9 MUR1620 rectifier, C22 filter, into a DC voltage , charging for battery through the chopper switch. T1 secondary winding N4, N5 is the auxiliary winding, the induced voltage goes through the D10, D11 rectifier, C21 filtered, is connected to IC1's 12 feet, as its working voltage.

3.2 Chopper switch

Chopper switch circuit is consisted by the transistor Q5, Q6, Q7 and resistors R29, R30, R31, R32 and so on. Work process: PWM control signal which is outputed by IC3 (PIC16C54) 6-pin goes through resistor R32 to connect to the Q7's base, to control Q7 off, so that make Q5 and Q6 turn on or off, charging current is flowing through Q6  to charge for battery( BAT) . Changing the pulse width of PWM control signal, making the charging voltage adjustable.

3.3 Controller

Shown in Figure 3, the controller is consisted by IC2 (LM358) and IC3 (PIC16C54) as well as resistors, capacitors and so on. IC3 is produced by PIC16C54 microcontroller of Microchip.Over-temperature protection is achieved by the positive temperature characteristic thermistor RT2, R36, R37 attached to the battery.

The charger uses LED to indicate charging status. Namely the fast charge and slow charge phase, the green light emitting diode G is bright; trickle charge phase, yellow LED Y is bright. Figure 4 shows the program flow.

3.4  Auxiliary power supply

Auxiliary power is consisted by the frequency transformer T3, rectifier components B2, filter components C27, C28 and three-terminal voltage regulator integrated circuit IC4 (7805) , offer (5V) power supply voltage for the microcontrollers. Using the microcontroller charge separately , can enhance anti-jamming capability, improve reliability. At the same time providing the 50Hz timing pulse signal for microcontroller.

4. Comprehensive experiment

The circuit shown figure 2 can charge for 12V/4Ah lead-acid batteries, the maximum charge current limit is 4A, the maximum output voltage is 18V. When charging starts, the battery charger use 4A current to fast charge for battery about 25 minutes ; then use14.7V constant voltage to slow charge on the battery ,until the battery voltage goes up to 12.8V, finish the slow charge, finally charger use 14.1V voltage to trickle charge for battery. Temperature protection point is 45C °; when the battery temperature rises to 45C °, the chip control charge voltage drops to 14.1V, as the temperature dropped, charging voltage is restored to the state before protection and continue charging. The charger has a shorter charging time of 2/5 than that of common charger for battery.
Different types of lead acid batteries, charging requirements are not identical, set the fast charge time and the maximum charge current parameters, go through trial and error, in order to achieve the best charging results, the battery life get be extended. This charger go through a variety of comprehensive test, has a good effect, suitable for a variety of battery charging.