Lithium battery charging circuit principle
A. Lithium batteries and NiCd, NiMH rechargeable batteries.
The negative electrode of lithium-ion batteries is a graphite crystal, and the positive electrode is generally lithium dioxide. Charging lithium ions from the positive to the negative electrode movement and embedded in the graphite layer. When discharging, the lithium ions are detached from the negative surface of the graphite crystal and move to the positive electrode. Therefore, during the charging and discharging process of the battery, lithium is always presented in the form of lithium ions, not in the form of lithium metal. Therefore, this battery is called lithium-ion batteries, referred to as lithium batteries.
Lithium battery has: small size, large capacity, light weight, no pollution, high single cell voltage, low self-discharge rate, more battery cycles and other advantages, but the price is more expensive. Nickel-cadmium batteries are being phased out due to low capacity, serious self-discharge and environmental pollution. Nickel-metal hydride battery has a high functional price ratio, and does not pollute the environment, but the monomer voltage is only 1.2V, and is therefore constrained in the scope of application.
Second, the characteristics of lithium batteries.
1, has a higher fractional energy ratio, volume energy ratio.
2, high voltage, single lithium battery voltage of 3.6V, equal to the series voltage of three NiCd or NiMH rechargeable batteries.
3, small self-discharge can be long-term storage, which is the most outstanding superiority of the battery.
4, no recall effect. Lithium batteries do not have the so-called recall effect of nickel-cadmium batteries, so lithium batteries do not need to be discharged before charging.
5, long life. Under normal operating conditions, the number of charge/discharge cycles of lithium batteries is much greater than 500 times.
6, can be quickly charged. Lithium batteries can generally be used to charge 0.5 to 1 times the capacity of the current, so that the charging time is shortened to 1 to 2 hours.
7, can be used in parallel at will.
8, because the battery does not contain cadmium, lead, mercury and other heavy metal elements, no pollution to the environment, is the most advanced green battery in the world.
9, high cost. Compared with other rechargeable batteries, lithium batteries are more expensive.
Third, the internal structure of lithium batteries.
Lithium batteries generally have two types of appearance: cylindrical and rectangular.
Battery internal spiral winding structure, with a very precise and permeable polyethylene film isolation material in the distance between the positive and negative electrodes. The positive electrode consists of a lithium ion collector consisting of lithium and cobalt dioxide and a current collector consisting of an aluminum film. The negative electrode consists of a lithium ion collecting electrode made up of sheet carbon material and a current collecting electrode made up of copper film. The cell is filled with an organic electrolyte solution. In addition, there are safety valves and PTC elements to maintain the battery from damage in case of abnormal conditions and output short circuit.
The voltage of a single lithium battery is 3.6V, and the capacity may not be infinite, therefore, the single lithium battery is often processed in series and parallel to meet the requirements of different occasions. String 5
Fourth, the charging and discharging requirements of lithium batteries.
1, the charging of lithium batteries: based on the structural characteristics of lithium batteries, the maximum charging suspension voltage should be 4.2V, can not be overcharged, otherwise the lithium ion will take away too much positive, and the battery is ruined. Its charging and discharging requirements are high, you can use a special constant-current, constant-voltage charger for charging. General constant-current charging to 4.2V / section after switching to constant-voltage charging, when the constant-voltage charging current drops to within 100mA, charging should be suspended.
Charging current (mA) = 0.1 to 1.5 times the battery capacity (e.g. 1350mAh battery, its charging current can be controlled between 135 and 2025mA). The customary charging current can be selected at about 0.5 times the battery capacity, and the charging moment is about 2 to 3 hours.
2, the discharge of lithium batteries: caused by the internal structure of lithium batteries, lithium ions can not be fully moved to the positive electrode when discharged, it is necessary to save part of the lithium ions in the negative electrode to ensure that lithium ions can be unclogged in the next charge embedded in the channel. Otherwise, the battery life will be shortened accordingly. In order to ensure that the graphite layer after the discharge of some lithium ions, we must strictly limit the minimum discharge termination voltage, that is, lithium batteries can not be over-discharged. Discharge suspension voltage is generally 3.0V / section, the minimum can not be lower than 2.5V / section. Battery discharge time length and battery capacity, discharge current size. Battery discharge time (hours) = battery capacity / discharge current. Lithium battery discharge current (mA) should not exceed 3 times the capacity of the battery. (Such as 1000mAH battery, the discharge current should be strictly controlled within 3A) Otherwise, the battery will be damaged.
Lithium battery pack sold on the market are now sealed with a matching charge and discharge maintenance board. Only need to control the external charge and discharge current can be.
Five, the maintenance circuit of lithium batteries.
Two lithium battery charge and discharge maintenance circuit as shown in Figure 1. By the two field-effect tubes and special maintenance integrated block S - 8232 composition, overcharge control tube FET2 and overdischarge control tube FET1 in series in the circuit, the maintenance IC to monitor the battery voltage and control, when the battery voltage rises to 4.2V, overcharge maintenance tube FET1 cut off, suspend charging. To avoid misoperation, a time delay capacitor is generally added to the external circuit. When the battery is in the discharged state, the battery voltage drops to 2.55V, the over-discharge control tube FET1 cuts off and discontinues power supply to the load. Over-current maintenance is in when there is a large current flow on the load, the control FET1 to cut it off, suspend the discharge to the load, the intention is to maintain the battery and the field effect tube. Overcurrent detection is the use of field effect tube on resistance as a detection resistor, monitor its voltage drop, when the voltage drop exceeds the set value to suspend the discharge. In the circuit is also generally added to the delay circuit to distinguish between inrush current and short-circuit current. The circuit is functional and reliable, but professional, and special integrated blocks are not easy to buy, and amateurs are not easy to imitate.
VI. Simple charging circuit.
Now there are many businesses selling single-cell lithium batteries without charging board. Its function is superior, inexpensive, can be used for the repair of gram products and lithium battery pack replacement, and therefore loved by the majority of electronic enthusiasts. Interested readers can refer to Figure 2 to create a charging board. The principle is: choose a stable voltage to charge the battery to ensure that it will not be overcharged. R1, Q1, W1, TL431 precision adjustable voltage regulator circuit, Q2, W2, R2 constitute an adjustable constant current circuit, Q3, R3, R4, R5, LED for the charging indicator circuit. As the voltage of the charged battery rises, the charging current will gradually decrease, and the voltage drop on R4 will decrease after the battery is filled, thus making Q3 cut off and the LED will rest, to ensure that the battery can be enough, please continue to charge for 1-2 hours after the indicator rests. Please install suitable heat sink to Q2 and Q3 when using. The advantages of this circuit are: simple manufacturing, easy to purchase components, charging safety, intuitive display, and will not damage the battery. After changing W1 can charge multiple lithium batteries in series, change W2 can be a wide range of charging current adjustment. The disadvantage is: no over-discharge control circuit. Figure 3 is a printed board diagram of the charging board (perspective view from the component side).
Seven, the application of single-cell lithium battery example
1, as a battery pack repair substitutes
There are many battery packs: such as the kind used in notebook computers, the repair found that the battery pack is damaged only a single battery has problems. You can choose the appropriate single lithium battery for replacement.
2, the manufacture of high brightness micro torch
I have used a single 3.6V1.6AH lithium battery with a white ultra-high brightness luminous tube made of a miniature flashlight, easy to use, compact and beautiful. And because of the large battery capacity, evenly used for half an hour each night, has been used for more than two months still do not need to charge. Circuit as shown in Figure 4.
3, instead of 3V power supply
Because the voltage of a single lithium battery is 3.6 V. Therefore, only one lithium battery can replace two general batteries, to the radio, walkman, camera and other small household appliances power supply, not only light weight, and the continuous use of a long time.
Eight, the preservation of lithium batteries.
Lithium batteries need to be stored after enough electricity. In 20 ℃ can be stored for more than six months, it can be seen that lithium batteries are suitable for storage at low temperatures. There are advocates of rechargeable batteries into the refrigerator freezer for preservation, is indeed a good note.
Nine, the use of attention.
Lithium batteries can never be disassembled, drilling, piercing, sawing, pressurization, heating, or there may be serious consequences. No charging maintenance plate lithium battery can not be short-circuited, not for children to play. Can not be close to flammable items, chemical substances. Waste lithium batteries should be properly disposed of. Fourth, the charging and discharging requirements of lithium batteries.
1, the charging of lithium batteries: according to the structural characteristics of lithium batteries, the maximum charging suspension voltage should be 4.2V, can not be overcharged, otherwise the lithium ion will take away too much positive, and the battery is scrapped. Its charging and discharging requirements are high, you can use a special constant-current, constant-voltage charger for charging. General constant-current charging to 4.2V / section after switching to constant-voltage charging, when the constant-voltage charging current drops to within 100mA, charging should be suspended.
Charging current (mA) = 0.1 to 1.5 times the battery capacity (e.g. 1350mAh battery, its charging current can be controlled between 135 and 2025mA). The customary charging current can be selected at about 0.5 times the battery capacity, and the charging moment is about 2 to 3 hours.
2, the discharge of lithium batteries: caused by the internal structure of lithium batteries, lithium ions can not be fully moved to the positive electrode when discharged, it is necessary to save part of the lithium ions in the negative electrode to ensure that lithium ions can be unclogged in the next charge embedded in the channel. Otherwise, the battery life will be shortened accordingly. In order to ensure that the graphite layer after the discharge of some lithium ions, we must strictly limit the minimum discharge termination voltage, that is, lithium batteries can not be over-discharged. Discharge suspension voltage is generally 3.0V / section, the minimum can not be lower than 2.5V / section. Battery discharge time length and battery capacity, discharge current size. Battery discharge time (hours) = battery capacity / discharge current. Lithium battery discharge current (mA) should not exceed 3 times the capacity of the battery. (Such as 1000mAH battery, the discharge current should be strictly controlled within 3A) Otherwise, the battery will be damaged.
Lithium battery pack sold on the market are now sealed with a matching charge and discharge maintenance board. Only need to control the external charge and discharge current.
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