acid batteries, in particular maintenance free version with an electrolyte in gel form, due to their relatively low cost and the operating conditions are still finding many uses in the buffer supply schemes. The described power supply circuit / Charger is designed to charge the battery 1,2 … 7 BUT·hours with a voltage 12 V and current of up to 0,5 BUT.
This scheme buffered power supply or battery charger is used to charge batteries on 1,2 … 7 BUT·h with typical voltage 12 V and current of up to 0,5 BUT. Underlying circuit device is an integrated circuit from Texas Instruments BQ24450, containing the full charging controller. The block diagram shown in Figure BQ24450 1, a schematic diagram of the charger based on it – on the image 2.
The charger is powered by a conventional power transformer, the supply voltage 16 … 18 VAC. AC voltage is connected to the connector (wire 1 and 2), From where, after rectification in a Graetz bridge, consisting of diodes D1 … D4 and filtering through a capacitor CE1, he feeds a charging circuit.
Also, charger can operate from a source of constant voltage in the range 18 … 24 AT, Connect the power supply to terminals 1 («+») and 3 («-»). Despite the wide range of supply voltage, necessary to limit the value of the input voltage due to the power loss. The charger is linear regulator, and any unnecessary excess power is lost as heat, especially at the beginning of the charging process, when the battery voltage is the lowest. For proper operation of the circuit the voltage difference between the input and the output of U1 3 AT, when the battery is fully charged.
Chip U1 is configured to run in a two-stage charging mode with a pre-charge current. Operation BQ24450-spread begins, when the voltage exceeds 4,5 AT, If the battery voltage drops below, than the reference voltage VTH (measured by removing the CE, usually, Vref = 2,3 V):
VTH = VREF × (OUT + RB + RC // RD) ÷ (RB + RC // RD)
In the prototype, the author has been set to VTH 10,5 AT. Charging current is provided U1, The transistor Q1 is turned off, and the current value is set via resistor R12 to about 80 mA (for Vin = 18 AT):
IPRE = (VIN - 2 – VBAT) ÷ R12.
This prevents heating of the battery greatly planted, will take place when charging at the maximum current. After exceeding the threshold low battery voltage internal circuit, responsible for charging, disconnected, i Q1, controlled from the comparator circuit, measuring the decrease in the resistors R1 and R2, connected in parallel, responsible for regulating the charging current. The charging current is determined in accordance with the expression: IMAX CHG = VILIM ÷ (R1 // R2). for resistance, indicated in the diagram, Charging current is 0,5 BUT (We glue = 250 mV). If the charger is used in a buffer system, its value must be selected for charging the battery, loaded system within a certain time. Charging with maximum current lasts until, until the battery reaches apryazheniya 14,7 AT (Boost), which is supported by a charger. Vboost voltage value calculated by the formula: Vboost = V REF × (OUT + RB + RC // RD) ÷ RC // RD.
Battery power consumption decreases if the voltage drop across the resistor R1 / R2 reaches 25 mV (10% from the charging current). The circuit goes into hold state (floating point). charged in the battery 100%, when the voltage at its terminals is 13,8 AT. Vfloat voltage value calculated by the formula: VFLOAT = VREF × (OUT + RB + RC) ÷ RC. If the current consumption does not exceed the charging current, the charger is fully charged. If current consumption above, low battery voltage decreases to, and charging cycle is repeated. Two float mode / acceleration allows recharge the battery quickly and maintain it in good condition for the entire service life. LEDs ST1 and ST2 indicate the operating state of the system, the optocoupler separates the signal circuit and changes the charging thresholds, without losing the ability to signal the internal states of the system using the diode ST1.
The charger signals three charging states:
- Stock, ST1 on, battery voltage below threshold 95 % Boost.
- Boost, both LEDs are on.
- Float, both diodes go out.
The circuit provides a cyclic charging process depending on the state of the battery. Charging Voltage Compensation System, built in U1, provides correction of charging voltage depending on temperature, corresponding to the characteristics of the gel battery.
Author's model is assembled in a small-sided PCB, arrangement of components which is shown in Figure 3. Transistor Q1 is mounted on the radiator. Location of contact with the heatsink should be coated with thermal grease, device installation requires no description. Due to the compensation of the charging voltage must be provided with similar temperature conditions for the battery, U1 system and radiator Q1, that it did not affect the temperature measurement. If you want a larger charging current (to 1,5 BUT), place the transistor Q1 to the external radiator with a correspondingly larger surface. The board also allows you to adapt the other modes, by appropriately mounting the elements Ra-Rd. Each of the voltage setting resistors consists of two elements, connected in series. This was done to facilitate the selection of the typical values of resistors with a tolerance 1%, For example, when setting the operating conditions for a 6-volt battery or by setting the voltage for cyclic operation.