Design of electric vehicle charger

1. Charging characteristics of sealed lead-acid batteries

　　Battery charging usually has two tasks to complete. The first is to restore the rated capacity of the battery as quickly as possible. The other is to use a small current to charge and replenish the energy lost by the battery due to self-discharge to maintain the rated capacity of the battery. During the charging process, the lead sulfate on the negative plate of the lead-acid battery gradually precipitates lead, and the lead sulfate on the positive plate gradually generates lead dioxide. When the lead sulfate on the positive and negative plates completely generates lead and lead dioxide, the battery begins to overcharge and produces hydrogen and oxygen. In this way, in non-sealed batteries, the water in the electrolyte will gradually decrease. In sealed lead-acid batteries, hydrogen and oxygen can recombine into water when a medium charging rate is used. The time when overcharging starts is related to the charging rate. When the charging rate is greater than C/5, the overcharge reaction begins before the battery capacity recovers to 80% of the rated capacity. Only when the charging rate is less than C/100 can the battery have an overcharge reaction after the capacity is restored to 100%. In order to restore the battery capacity to 100%, a certain amount of overcharge reaction must be allowed. After the overcharge reaction occurs, the voltage of a single cell rises rapidly. After reaching a certain value, the rate of increase decreases, and then the battery voltage begins to slowly decrease. It can be seen from this that after the battery is fully charged, the best way to maintain the capacitance is to add a constant voltage across the battery pack. Under the floating charge voltage, the charged current should be able to replenish the energy lost by the battery due to self-discharge. The floating charge voltage should not be too high, so as not to shorten the battery life due to severe overcharging. With an appropriate floating charge voltage, the life of a sealed lead-acid battery can reach more than 10 years. Practice has shown that when the actual floating charge voltage differs from the specified floating charge voltage by 5%, the life of a maintenance-free battery will be shortened by half. The voltage of a lead-acid battery has a negative temperature coefficient, and its single cell value is -4mV/℃. An ordinary (no temperature compensation) charger that works very well at an ambient temperature of 25℃ will not be fully charged when the ambient temperature drops to 0℃, and the battery will be shortened due to severe overcharging when the ambient temperature rises to 50℃. Therefore, in order to ensure that the battery is fully charged over a wide temperature range, the various conversion voltages of the charger must vary with the temperature coefficient of the battery voltage.

The common charging modes are:

1. Current-limited constant-voltage charging mode, its charging curve and conversion voltage are shown in Figure 1.

2. Two-stage constant current charging mode, its charging curve and conversion voltage are shown in Figure 2.

3. Constant current pulse charging mode, its charging curve and conversion voltage are shown in Figure 3.

These three charging modes are recommended by the industry. The conversion between the charging currents in each stage is controlled by the temperature-compensated conversion voltages Vmin (lowest allowable voltage for fast charging), Vbik (fast charging termination voltage) and Vflt (floating charge voltage). Many dedicated charging integrated circuits with the above functions have been developed abroad, such as UC3906, bq2031, etc.

2. Production Example of DB3616C Electric Bicycle Charger

At present, most electric bicycles on the domestic market use 36V or 24V sealed lead-acid battery packs. In order to reduce costs, most of the chargers that match them use a simplified constant current and constant voltage mode. The charging curve is shown in Figure 4. Compared with Figure 1, this solution omits the replenishment charging stage (i.e., the Vlk high voltage constant voltage overcharging stage), so the battery capacity can only be restored to 80% to 90% of the rated capacity. At the same time, its charging conversion voltage has no temperature compensation. In winter and summer, it is easy to be undercharged or overcharged. Furthermore, since the self-discharge rate of each battery in the series battery pack is not the same, if a constant floating charge voltage is used, it will affect the charging state of the single battery.
This charger example uses the charging mode of Figure 3, and the schematic diagram is shown in Figure 5. This machine uses the AC/DC resonant high-efficiency converter component DBX6001 as the front-stage isolation and voltage reduction. The efficiency of this component is as high as 92%. The 60V DC output of the component enters the rear-stage charging circuit from the c and d terminals. The power components of the latter stage use low conduction voltage drop devices. Considering portability, this machine adopts a miniaturized design with a built-in automatic small fan. The volume of the whole machine is 75mm×130mm×50mm. IC and Q1, L, D1, etc. form a fast constant current charging system. IC uses SG3842, R1, DZ1, C3, C4 are the power supply circuit of IC, R4, C6 determine the oscillation frequency of IC, C5, R3 are compensation components. At the beginning of charging, the battery voltage is low and PC is not turned on (the principle will be described later). IC pin ① is pulled to the ground potential by R3 and R4, and pin ⑥ outputs about 100 kHz pulses, which are added to the gate of Q1 through R8 to control the on and off of Q1. During the conduction period of Q1, the charging current output from pin ③ of DBX6001 passes through the energy storage inductor L, the external battery E, Q1, R6 to pin ④. While charging the battery, the inductor L also stores energy. The charging current increases linearly and generates a detection voltage drop on R6, which is transmitted to IC pin ③ through R5 and C7. When the voltage on pin ③ reaches 1.1V, pin ⑥ turns off the pulse and Q1 is turned off. At this time, the magnetic field in the inductor L is released, and the generated current continues to supply power to the battery. D1 provides a freewheeling channel for L. The size of the average charging current is determined by R6. After the battery is fully charged, PC is turned on, and the 5V voltage output by pin ⑧ is added to R2 through PC. When the potential of pin 1 is higher than 2.5V, pin ⑥ turns off the output and the charger stops charging.

　　DBM36 is a dedicated charging detection and control module for 36V lead-acid battery packs, with two internal charging modes.

　　The working principle of DBM36 is:

　　When the battery voltage is connected to the ② terminal of DBM36, it works in the constant current pulse charging mode, that is, when the potential of the ② foot is less than 45V, the ④ foot outputs a high potential, the optocoupler PC is not turned on, the charging circuit composed of the IC starts to work, and Q2 is turned on, and the fan FS is powered on. When the battery voltage gradually increases and the potential of the ② foot reaches 45V, the trigger a flips, the ④ foot outputs a low level, the primary of the optocoupler PC flows through, the secondary is turned on, the IC ① foot is higher than 2.5V, the ⑥ foot stops outputting pulses, Q2 is turned off, and the charger stops charging. At the same time, the fan stops. Then the battery voltage gradually decreases. When the voltage drops to 41.5V, the trigger a resets, the ④ foot outputs a high level, the optocoupler PC is turned off, the IC is unblocked, and the charger outputs current again. Repeating the cycle, the charging time becomes shorter and shorter, and the self-discharge time of the battery voltage from 45V to 41.5V becomes longer and longer, and the power gradually recovers to 100%. This state is indicated by the charging indicator LED, which turns off when charging and turns on when charging is stopped, while the fan works in the opposite way to the LED: it rotates when charging and stops when charging is stopped. R9, C10 and DZ2 form the power supply circuit of DBM36.

　　When the battery voltage is connected to the ③ terminal, the DBM36 works in the constant current and constant voltage charging mode. At the beginning, the charger outputs 1.6A constant current to continuously charge the battery. When the battery voltage rises to 45V, the DBM36 ③ foot detection reference voltage automatically switches from 45V to 41.5V and remains unchanged. Through the feedback of the optocoupler PC, the charger switches from constant current charging to constant voltage floating charging. It should be noted that if the charging current is too large and the battery temperature increases significantly, the self-discharge current may exceed the charging current. The temperature continues to rise, causing Vblk to continue to drop, and a serious overcharge reaction will occur, affecting the battery life.

In addition, when working in constant current and constant voltage charging mode, the charger should be connected to the battery first, and then connected to the 220V mains. Otherwise, the 45V voltage output by the charger will cause the DBM36 to misjudge and directly switch to the 41.5 constant voltage floating charge state, resulting in insufficient battery charging. For charging measurement and control of 24V battery packs, the DBM24 module is required.