Monday, February 7, 2011

Weak current College】 【lithium battery charging circuit theory and application of】.

Lithium-ion battery with its excellent characteristics, are widely applied in: cell phone, VCR, portable, cordless phones, power tools, electric toys, remote control or on the camera, and other portable electronic device.

I. lithium batteries and nickel-cadmium, nickel-metal hydride rechargeable batteries:

Lithium-ion battery anode to cathode typically graphite Crystal, as lithium oxide. Cathode ion rechargeable shili from embedded to negative campaign and graphite layers. Discharge, lithium-ion from graphite Crystal surface from within the anode to the cathode. Therefore, the battery charge and discharge lithium is always in the process of lithium-ion speciation occurs, rather than the form of lithium metal. Thus this type of battery is called lithium-ion batteries, or Li-ion battery.

Lithium batteries have: small size, large capacity, the weight is light, pollution-free, single voltage high and low self discharge rate, battery cycle number, etc., but expensive. Nickel-cadmium batteries due to low self-discharge capacity, and on the environment pollution, is gradually being phased out. Ni-MH batteries with high cost performance, and do not pollute the environment, but only a single voltage, thus using 1.2V scope be limited.

II. characteristics of lithium battery:

1, with higher weight energy ratio, volume energy ratios;

2, high-voltage, single-cell Li + battery voltage to 3.6V, equal to 3 only NI-CD or Ni-MH rechargeable battery-voltage;

3. low self discharge can be stored for a long time, this is the most significant advantages of the battery;

4, no memory effect. There is no lithium batteries NI-CD batteries of so-called memory effect, lithium battery charge before without discharge;

5, have long performance life. Under normal operating conditions, lithium battery charge/discharge cycle times larger than 500;

6, you can quickly recharge. Lithium battery usually uses 0.5 ~ 1 times the capacity of the charging current, so that the charging time shortened to 1 to 2 hours;

7, can be used in parallel;

8, since the battery does not contain cadmium, lead, mercury and other heavy elements, environmental pollution, the most advanced green battery;

9, the cost is high. And other rechargeable batteries, lithium battery compared to expensive.

3. the internal structure of lithium battery:

Lithium battery there are typically two shape: cylindrical and rectangular type.

Battery internal use spiral-wound structure, with a very fine and permeability of strong polyethylene film quarantine material in the interval between positive and negative. Positive electrode including lithium cobalt oxide and composed of lithium-ion collector and aluminum film composition of current collectors. Anode carbon by Flake material for lithium-ion collector and copper film composition of current collectors. The battery is filled with organic electrolyte solutions. Also equipped with a safety valve and PTC component, so that the battery in an unhealthy state and output short-circuit protection battery is not damaged.

Li + battery voltage to 3.6V, capacity is not likely to infinity, therefore, often a single lithium battery for series and parallel processing to meet the requirements of different occasions.

IV. lithium battery charge and discharge requirements;

1. rechargeable lithium battery: according to structure characteristics of lithium batteries, maximum charge termination voltage should be can't overcharge 4.2V, or they will take away the cathode of lithium-ion, and make too many battery scrap. Its charge and discharge requirements are higher, you can use the constant current, constant voltage dedicated charger for charging. Usually constant current charging to 4.2V/post-holiday to constant pressure and constant voltage charging, when the charging current drops to 100mA or less, you should stop charging.

Charging current (mA) = 0.1 ~ 1.5 times the battery capacity (such as 1350mAh battery, charging current can be controlled at 135 ~ 2025mA). Normal charging current can choose 0.5 times the battery capacity, charging time is about 2 to 3 hours.

2 lithium batteries discharge: due to the internal structure of lithium batteries, discharge shili ion cannot be all moved to the cathode, must remain part of the lithium-ion in negative for the next charge shili ion can easily embed channel. Otherwise, the battery life is reduced accordingly. In order to guarantee the graphite layer after leaving some discharge lithium ion, strictly limiting the minimum voltage discharge termination, i.e. lithium battery cannot discharge. Discharge voltage is usually 3 .0V/section, the minimum can not be lower than 2.5V/section. Battery discharge duration and battery capacity, discharge current size. Battery discharge time (hour) = battery capacity/discharge current. Lithium battery discharge current (mA) should not exceed three times the battery capacity. (Such as 1000mAH battery discharge current should strictly control within the 3A) or cause the batteries to be damaged.

Currently on the market within the lithium batteries are sealed with matching charge and discharge protection board. As long as you control the external charging and discharging current.

5. lithium battery protection circuit:

Two lithium battery charge and discharge protection circuit shown in Figure 1. Consists of two FET and private protection manifold S-8232, overcharge control tube FET2 and discharge control tube FET1 series to protect the circuit from IC monitors battery voltage and control, when the battery voltage is increased to 4.2V, overcharge protection tube FET1 due to stop charging. To prevent incorrect operation, generally with a delay detection tester for electronic circuit. When the battery is discharging State, the battery voltage drops to 2.55V, over discharge control tube FET1 due to load, the power supply. Overcurrent protection is in when the load on a large current flows, the control of the globe, FET1 stop to load discharge in order to protect the battery and FET. Overcurrent detection is to use the MOSFET on-resistance as the detection of resistance, voltage drop monitoring it, when the voltage drop over the set value when you stop discharge. In General, also in circuit with delay circuitTo differentiate the surge current and short circuit currents. The circuit of functional, reliable, professional and dedicated to buying, manifold amateurs are not easy to clone.

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