Charger for Li-lon batteries. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells
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Currently, lithium-ion (Li-Ion) and lithium-polymer (Li-Pol) batteries are widely used. Their distinguishing feature is a large capacity with less weight and dimensions. A fairly large range of such batteries is used in cell phones.
The specificity of charging such energy storage devices lies in the strict observance of charging modes, non-compliance with which leads to failure and sometimes explosion of batteries.
To charge Li-Ion batteries, specialized microcircuits are produced to ensure safe charging. I offer a charger based on the MC34674 chip. The IC is a fully integrated ready-made device for charging single-cell Li-Ion and Li-Pol batteries. This microcircuit provides a maximum charging current of 1 A at an input voltage of 4,3 ... 10 V.
The microcircuit provides protection against high input voltage (but not more than 28 V), overheating of the microcircuit and battery The IC allows charging completely discharged batteries with low current. The chip dimensions are 2x3 mm, so the assembled charger takes up very little space and allows it to be integrated into a wearable product.
The charging process consists of three stages.
1. The so-called "drip charging", the battery is completely discharged, the voltage on it is below 2,7 V. Charging is performed with a varying input voltage, approximately 60 mV higher than the voltage on the battery.
2. Battery voltage is higher than 2,7V. DC charging.
3. Battery voltage - 4,2 V.
Recharging at a constant voltage, the current gradually drops to zero.
During the charging process, the magnitude of current and voltage is controlled.
If an energy consumer is connected in parallel with the charger, and during its operation the voltage on the battery drops to 4,1 V, the DC charging mode (maximum possible) is switched on again. When the temperature of the crystal rises to 110°C during the charging process, charging stops in order to avoid failure of the microcircuit. The microcircuit allows you to adjust the battery charge current depending on its temperature (for this, an NTC thermistor must be built into the battery.
Cell phone batteries usually have such a thermistor).
In the charger (Fig. 1), the supply voltage via the USB interface or from an external source is supplied to the X2 connector, passes through the FU1 fuse, is filtered and smoothed by the L1 choke and the C3 capacitor and is fed to the input of the D1 microcircuit. Capacitor C4 and varistor R6 are used to dampen surges in the power circuit.
The charge voltage from pin 8 D1 is supplied to the "+" battery GB1.
Using the R1-R2-Rt-C2 chain, a "temperature window" is set in which charging is performed.
(click to enlarge)
With these values of resistors (at Rt+R2=120 kOhm) the upper cut-off temperature is +55°C, the lower one (at Rt+R2=290 kOhm) is -40°C.
The EN input (pin 4) D1 is used to externally turn on (logic "0") or turn off (logic 1) the microcircuit. Outputs GRN (pin 2) and RED (pin 3) are used to indicate the mode of operation of the charger.
In this circuit, these pins are connected to the free ports of the microcontroller of the portable device.
If the circuit does not provide for the use of a microcontroller, then pin 4 must be grounded, the resistances of resistors R3 and R4 should be reduced to 470 ohms, connected instead of +5 V to pin 1 D1 and turn on the LEDs in series with the resistors to indicate the operating mode of the charger. If Uvx_norma=0 and Zar_Bat=1, the battery is charged. If Uvx_norma=1 and Zar_Bat=0, the battery is charging.
The device is assembled on a single-sided printed circuit board with dimensions of 9x13 mm (Fig. 2). It uses chip components: C1, C2, C4 - size 0402, C3 - 1206, R1, R5 - 0402, R6 - 0805, L1 - 3216, FU1 - 0402.
Author: S.Abramov, Orenburg
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