Circuit diagram and design of lithium battery charger

The lithium battery commonly used in Yu Han machine, fixed wing, model aircraft, helicopters, etc. It has discharge stability, wide working temperature, larger charging current, fast charging speed, low self-discharge and long storage life, high energy, large stored energy density, etc. Let me show you 11.1 V lithium battery charger design, and take a look at the battery. What does the charger circuit diagram and schematic diagram look like?

【Lithium battery charger 】 lithium battery charger circuit diagram and schematic diagram lithium battery charger design scheme

1, the introduction

11.1 V lithium batteries commonly used Yu Han machine, fixed wing, model aircraft, helicopters and so on has the discharge stability, wide working temperature; Allow larger charging current, charging speed, can be filled with only 1 ~ 2 hours; No memory effect; Low self-discharge rate, long storage life; Energy is high, the stored energy density; High output voltage (single section of lithium battery rated voltage is 3.6 V, commonly and single section nickel-metal hydride and nickel cadmium battery voltage is 1.2 V), etc. Charging of lithium battery to prevent excessive charging, if the voltage is above voltage charging or charging current is greater than the rated current, will damage the lithium battery or scrapped. In the case of charging, the excess energy lithium battery temperature rising, the electrolyte decomposition to produce gas, make the voltage rise caused by spontaneous combustion or within the risk of rupture. On lithium battery when use to prevent excessive discharge, also can cause excessive discharge battery characteristics and durability, rechargeable number reduced.

2, the structure of the charging circuit design and analysis

Lithium batteries need to control it in the process of charging battery charging voltage and charging current and accurate measurement of voltage, according to the lithium battery voltage charging process can be divided into four stages. Stage one is charging, with small current 0.1 C for lithium battery charging, when the battery voltage is 2.5 V or higher to the next phase. Phase 2. Constant current charging, with constant current of 1 c to the lithium battery quick charging, point when the battery voltage is 4.2 V or higher to turn to the next stage. Phase 3 for constant voltage charging, gradually decrease the charging current, ensure constant = 4.2 V battery voltage, when the charging current 0.1 C or less to the next phase. Phase four for trickle charging, constant voltage charge after the battery has been basic, in order to maintain the battery voltage, can use 0.1 C or less to supplement the battery charging current, to the end of the lithium battery charging process.

3, the charger hardware circuit design

This system mainly include micro controller, the voltage detection circuit, current detection circuit, battery charge status indication circuit and control circuit, the circuit principle diagram is shown in figure 1.

The principle diagram of the lithium battery charger

3.1 the main control chip

This system adopts the ATmega8 as control core.

ATmega8 AVR is a high performance and low power consumption microprocessor. It adopts advanced RISC structure, a total of 130 instructions, most of the instruction execution time for a single clock cycle, with 32 eight general working registers, work in 16 MHZ up to 16 MIPS performance; Only need two clock cycles hardware multiplier;8 k bytes within the system of programmable Flash; Independent lock an optional Boot code area;512 bytes of E2PROM chips was introduced;1 k bytes of on-chip SRAM; Two independent preassigned frequency eight timer/counter;23 programmable I/O ports;8 road 10 bit ADC; Three-channel PWM. Real time counter RTC; Byte-oriented two line interface; Two USART interfaces; Can work at host/from machine model SPI interface; Watchdog timer in pieces; Inside the internal resources such as analog comparator.

3.2 voltage detection circuit

As a result of the ATmega8 ADC reference voltage is set to 3.072 V, the battery in the process of charging voltage can be as high as 12.6 V, so you need to battery voltage scaling down into the ATmega8 to the ADC acquisition of mouth.

This circuit consists of a phase detector and 20 k adjustable resistance, battery voltage input to the composed of LM324 in phase, the phase detector output after isolation buffer adjustable resistance to 20 k, by adjusting the adjustable resistance to input into the mouth of the ADC ATmega8 one 5 of the voltage of the battery voltage.

3.3 current detection circuit

When charging current flows through the sense resistor will produce pressure drop, with measurement testing resistor voltage can know the size of the charging current, phase, have the effect of isolation buffer.

3.4 battery status indicator circuit

Battery status indicator circuit consists of a green and a red light-emitting diodes (leds), used to indicate status of the battery. Red and green leads are lit for standby, not connected to the battery; Red light for charging status leads separate; Red led is flashing fast charging status; Green led is flashing constant-voltage charging status; Green leads light alone for trickle charging status, at the moment that the battery is fully charged.

3.5 charging control circuit

Charging control circuit adopts PWM method of controlling the charging voltage and charging current, the resolution of the PWM is nine, switching frequency is 2 KHZ. When not connected to the battery, adjusting the R1 that BAT + voltage of 12.975 V, said when the BAT + voltage is less than 12.8 V battery has access.

4, control software design of the charger

In this control software, the control program USES multistep structure, the whole charging process can be divided into five state, in each state after a certain condition, moved to another state.

5, conclusion

This paper presents a high ratio of 11.1 V li-ion battery charger design, it discussed the charger circuit structure and software design thought, this paper introduces the control method of lithium battery, with ATmega8 as control core, the comprehensive management on the charging process, through to automatically detect and adjust the charging current, voltage, complete the accurate control of different charging stage and full automatic stop filling.

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