Using the HT46R53 to charge Nickel-Metal Hydride Batteries

Transcription

1 Using the HT46R53 to charge ickel-metal Hydride Batteries Using the HT46R53 to charge ickel-metal Hydride Batteries D/:HA0097E Introduction The units for battery capacity are specified in mah. As an example, if a battery is specified with a capacity of 500mAh, then this means that the battery can supply a current of 500mA continuously for 1 hour. If the current was only 50mA, then it could be supplied for 500mAh/50mA = 10 hours. Regarding charge rate, a unit specified using the term C is used. An an example, for a 500mAh capacity battery, if charged at a current of 500mA, then this will be known as 1C. ickel Metal Hydride batteries exhibit a memory effect although not as pronounced as ickel Cadmium batteries. The memory effect is not pronounced, as these batteries can be charged many times, and even if not fully discharged, will not exhibit too much of a memory effect influence. Therefore, when charging ickel Metal Hydride batteries, it is not necessary to first discharge them before charging. However if it is requied to extract maximum life out of the batteries, after every charge and discharge cycles, a deep discharge cycle is recommended, to restore the battery maximum capacity. The meaning of a deep discharge would be to discharge the batteries down to a voltage between 0.85~0.9V. With a low current discharge, allowing the batteries to reach this low voltage range value, the capacity of ickel Metal Hydride as well as ickel Cadmium batteries, can be restored to their maximum capacity, and the life of the battery extended. However if the battery continues to be discharged beyond this point, once the battery voltage is allowed to fall below 0.8V, the battery may be irreversibly damaged, and the life of the battery reduced. A single cell Metal ickel Hydride battery has a cell voltage of 1.2V, however just after charging, the actual voltage will exceed this and can have a value of between 1.4~1.5V. 1

2 Using the HT46R53 to charge ickel-metal Hydride Batteries Charging a battery is the process of transferring energy into the battery. The energy is stored via way of a chemical reaction. However not all of the electrical energy is converted into chemical energy. Some of the electrical energy will be converted into heat, causing the battery temperature to rise. If fast charging takes place, then the battery temperature will quickly rise and if charging is not quickly stopped, the battery will be damaged. Description Methods of Measurement In the process of charging ickel Metal Hydride batteries, a method of detecting when the battery is fully charged is an especially important consideration. If there really is no way of detecting the fully charged status of the battery, then there will be no way of automatically stopping the charging process at the correct point. In this case the battery will be easily damaged and may even be dangerous. The following shows several ways of detecting when a ickel Metal Hydride battery is fully charged: Time control: Using a 1.25 charging rate, the battery can be fully charged in 1 hour. With a 2.5C charging rate, the battery can be charged in 30 minutes. Therefore according to the battery s capacity and the charging current, it is easy to determine the charge time. This method of control is very simple, but because the amount of charge already in the battery when the charging process begins will differ between batteries, some batteries may be undercharged and some may be overcharged. Therefore only a charging rate of less than 0.3C can be used for this method. Maximum Voltage: During the charging process, after the voltage reaches VMAX, fast charging should be immediately ceased. The disadvantage of this method is that the maximum voltage will vary with the environmental temperature and with the charging rate. In addition, the maximum charging voltage of each cell in the group of cells being charged, will not all have an equal value. Therefore this method cannot guarantee that the batteries are fully charged. egative Delta Voltage: Because the negative delta voltage has no relationship with the cell group absolute voltage, and also because it is not influenced by the environemental temperature or charge rate factors etc, makes this method a reasonably reliable means of determining 2

3 Using the HT46R53 to charge ickel-metal Hydride Batteries when the baterries are fully charged. The disadvantages of this method, is that after the battery exhibits a negative delta voltage, the battery will already have been overcharged, and the battery will consequently have reached a high temperature. In addition, after ickel Metal Hydride batteries have been fully charged, the battery voltage will only exhibit this negative delta voltage after a considerable amount of time has elapsed, leading to the batteries being excessively overcharged. For this reason therefore, this method is chiefly suitable only for ickel Cadmium battery types. Zero Delta Voltage: When ickel Metal Hydride batteries are charged, in order to prevent the problem of wating for too long for the negative delta voltage to be exhibited, another method is to look for a zero delta voltage. The disadvantage of this method is that, before reaching the fully charged state, the battery voltage in a certain period of time will only exhibit minute voltage changes, thereby having the risk of stopping too early, the fast charging process. For this reason, most of the present ickel Metal Hydride fast battery chargers, use high sensitivity negative delta voltage detection methods, to detect when the battery voltage shows only a minute reduction, at which point the fast charging process can be terminated. Temperature Control: In order to prevent battery damage, if the battery temperature is too low, then fast charging should not be implemented. Only when the temperature rises to the specification value can fast charging begin. Charger Description This project uses a Holtek HT46R53 MCU device as the controller to charge up four cells. The HT46R53 contains an 8-channel 12-bit A/D converter and a single PWM output. The 8 A/D input channels are pin-shared with the PB I/O pins. With Vref = 5V, the 12-bit ADC can distinguish voltage differences of 1.22mV. The device uses the PWM and A4 pins on the HT46R53 device to control the current, and A0 to check the Battery 1 positive voltage, A1 to check the Battery 2 positive voltage, A2 to check the Battery 3 positive voltage and A3 to check the Battery 4 positive voltage. PA0~PA3 are used to control the charging of Battery 1 ~ Battery 4, PA4~PA7 are used to contol the discharging of Battery 1 ~ Battery 4. A maximum of four batteries can be charged at the same time. Each battery has a yellow LED to indicate the battery status. If a good battery is inserted into the charger then its corresponding yellow LED will illuminate. If the green LED flashes, this indicates that the battery is discharging. When the green LED extinguishes, and the yellow LED flashes, this indicates that the corresponding battery is being charged. 3

4 Using the HT46R53 to charge ickel-metal Hydride Batteries Charger Characteristics Supports the charging of 4 independent ickel Metal Hydride batteries Uses 5 LEDs to indicate the battery status Uses fixed current for charging, the slow charging current is 180mA, and the fast charging current is 700mA. Supports cell discharging to reduce memory effects. The discharge current is 50mA. Supports automatic detection for charging or discharging. When charging a corresponding yellow LED will illuminate. Supports battery short circuit protection. If the battery is short circuited, the corresponding yellow LED will not be illuminated. VMAX control. When charging, if the battery voltage exceeds 1.55V then charging will cease. Timer protection. If the charge time exceeds 3.5 hours then charging will automatically cease. egative Delta Voltage Method. If the battery voltage falls by 5~10mV, then charging will automatically terminate. 4

5 Application Circuit Using the HT46R53 to charge ickel-metal Hydride Batteries 5

6 Using the HT46R53 to charge ickel-metal Hydride Batteries Flowchart Main Flow Chart MAI Initial Register Battery Present? Discharge Required? Charge Process Discharge Process Fully Charged? Fully Discharged? Jmp Charge Process 6

7 Using the HT46R53 to charge ickel-metal Hydride Batteries Discharging Flow Chart MLP_DISCHARGIG CHECK BAT1 STATUS F_BAT1O=1 F_OLDBAT1=1 F_BAT1DISCHAR GIGOK=1 CLR PA.4 SET PA.4 CHECK BAT2 STATUS F_BAT2O=1 F_OLDBAT2=1 7

8 Using the HT46R53 to charge ickel-metal Hydride Batteries F_BAT2DISCHAR GIGOK=1 CLR PA.5 SET PA.5 CHECK BAT3 F_BAT3O=1 F_OLDBAT3=1 F_BAT3DISCHAR GIGOK=1 CLR PA.6 SET PA.6 CHECK BAT4 STATUS F_BAT4O=1 8

9 Using the HT46R53 to charge ickel-metal Hydride Batteries F_OLDBAT4=1 F_BAT4DISCHAR GIGOK=1 CLR PA.7 SET PA.7 R_FLAG Low bble=0 JMP MLP_MAI R_FLAG Low ibbleibble JMP L_CHARGIG RETUR MLP_DISCHARGIG 9

10 Using the HT46R53 to charge ickel-metal Hydride Batteries Charging Flowchart L_CHARGIG R_FLAG Low ibble = 0? JMP MLP_CHARGIGEXIT R_FLAG Low ibble =R_FLAG1 low nibble JMP MLP_CHARGIGEXIT F_BAT1CHARGE=0? ext page Judge bat2 Check BAT1 DELTA V SET F_OCBAT1 Check BAT1 TIMEOUT SET F_OCBAT1 F_QCBAT1= 0? L_QUICKCHARGIG L_SLOWCHARGIG RETUR L_CHARGIG 10

11 Using the HT46R53 to charge ickel-metal Hydride Batteries ext page Judge bat3 F_BAT2CHARGE=0? Check BAT2 DELTA V SET F_OCBAT2 Check BAT2 TIMEOUT SET F_OCBAT2 F_QCBAT2 = 0? L_QUICKCHARGIG L_SLOWCHARGIG RETUR L_CHARGIG 11

12 Using the HT46R53 to charge ickel-metal Hydride Batteries ext page Judge bat3 F_BAT3CHARGE=0? Check BAT3 DELTA V SET F_OCBAT3 CHECK BAT3 TIMEOUT SET F_OCBAT3 F_QCBAT3 = 0? L_QUICKCHARGIG L_SLOWCHARGIG RETUR L_CHARGIG 12

13 Using the HT46R53 to charge ickel-metal Hydride Batteries F_BAT4CHARGE=0? RETUR L_CHARGIG CHECK BAT4 DELTA V SET F_OCBAT4 CHECK BAT4 TIMEOUT SET F_OCBAT4 F_QCBAT4 = 0? L_QUICKCHARGIG L_SLOWCHARGIG RETUR L_CHARGIG 13

14 Using the HT46R53 to charge ickel-metal Hydride Batteries ISR_TIMER Flow Chart ISR_TIMER Has time exceeded 200ms? ISR_TIMEREXIT CHECK BAT1 STATUS F_BAT1O = 0? CLR PC.1 SET F_OLDBAT1 F_OCBAT1 = 0? CHECK BAT2 STATUS F_BAT2O = 0? CLR PC.2 SET F_OLDBAT2 F_OCBAT2 = 0? CHECK BAT3 STATUS F_BAT3O = 0? CLR PC.3 14

15 Using the HT46R53 to charge ickel-metal Hydride Batteries SET F_OLDBAT3 F_OCBAT3=0? CHECK BAT4 ATUS F_BAT4O=0? CLR PC.4 SET F_OLDBAT4 F_OCBAT4=0? L_CHECK GIGFLAG ISR_TIMEREXIT 15