This article starts from the shallow to the deep, through the lithium battery charger circuit principle, gradually from understanding the charging principle of lithium battery charger and lithium battery-related knowledge, detailed analysis of how to set the appropriate charging voltage and current for lithium batteries, I hope that this article can give you an in-depth understanding of the basic knowledge of lithium batteries.
Lithium battery charging circuit principle
A lithium battery and NiCd, NiMH rechargeable batteries.
The negative electrode of the lithium-ion battery is a graphite crystal, and the positive electrode is usually lithium dioxide. When charging, lithium ions are embedded in the graphite layer by the movement of the positive electrode to the negative electrode. When discharging, the lithium ions are detached from the surface of the negative electrode within the graphite crystal and move to the positive electrode. Therefore, during the charging and discharging process of the battery, lithium always appears in the form of lithium ion, not in the form of lithium metal. Therefore, this kind of battery is called lithium-ion battery, referred to as lithium battery.
Lithium batteries have the following advantages: small size, large capacity, light weight, no pollution, high single cell voltage, low self-discharge rate, and more cycles of the battery, but more expensive. Nickel-cadmium batteries are being phased out due to low capacity, serious self-discharge and environmental pollution. Nickel-metal hydride batteries have a high performance-to-price ratio and do not pollute the environment, but the monomer voltage is only 1.2V, and thus is limited in the use of the range.
Second, the characteristics of lithium batteries.
1, has a higher weight-to-energy ratio, volume to 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 stored for a long time, which is the most outstanding superiority of the battery.
4, no memory effect. Lithium batteries do not have the so-called memory effect of nickel-cadmium batteries, so lithium batteries do not need to be discharged before charging.
5, long life. Under normal working conditions, the number of charge/discharge cycles of lithium batteries is much greater than 500 times.
6, can be quickly charged. Lithium batteries can usually be charged with a current of 0.5 to 1 times the capacity, so that the charging time is reduced to 1 to 2 hours.
7, can be used in parallel at will.
8, as the battery does not contain cadmium, lead, mercury and other heavy metal elements, no pollution to the environment, is the most advanced contemporary green battery.
9, high cost. Compared with other rechargeable batteries, lithium batteries are more expensive.
Third, the internal structure of lithium batteries.
Lithium batteries usually have two types of appearance: cylindrical and rectangular.
The internal spiral winding structure of the battery, with a very fine and permeable polyethylene film isolation material in the positive and negative electrodes are spaced. 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 composed of a sheet of carbon material and a current collecting electrode composed of a copper film. The cell is filled with an organic electrolyte solution. There are also safety valves and PTC elements to protect 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 is not infinite, therefore, single lithium batteries are 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, lithium battery charging: according to the structural characteristics of lithium batteries, the maximum charge termination voltage should be 4.2V, can not be overcharged, otherwise it will take away too much lithium ions due to the positive electrode, and the battery is scrapped. Its charging and discharging requirements are high, and can be charged with a special constant-current, constant-voltage charger. Usually 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 stopped.
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). Conventional charging current can be selected at about 0.5 times the battery capacity, and the charging time is about 2 to 3 hours.
2, lithium battery discharge: due to the internal structure of lithium batteries, lithium ions can not all move to the positive electrode when discharged, must retain a portion of lithium ions in the negative electrode to ensure that lithium ions can be smoothly embedded in the channel during the next charge. Otherwise, the battery life will be shortened accordingly. In order to ensure that the graphite layer after discharge to retain part of the lithium ion, we must strictly limit the minimum discharge termination voltage, that is, lithium batteries can not be over-discharged. Discharge termination voltage is usually 3.0V / section, the minimum can not be lower than 2.5V / section. The length of battery discharge time is related to the battery capacity, the size of the discharge current. Battery discharge time (hours) = battery capacity / discharge current. Lithium battery discharge current (mA) should not exceed 3 times the battery capacity. (Such as 1000mAH battery, the discharge current should be strictly controlled within 3A) Otherwise, the battery will be damaged.
The lithium battery pack currently on the market are sealed with a matching charge and discharge protection board inside. As long as the external charge and discharge current can be controlled.
V. Protection circuit of lithium battery.
Two lithium battery charge and discharge protection circuit as shown in Figure 1. By the two field-effect tubes and special protection integrated block S - 8232 composition, overcharge control tube FET2 and overdischarge control tube FET1 in series in the circuit, the protection IC to monitor the battery voltage and control, when the battery voltage rises to 4.2V, overcharge protection tube FET1 cut off, stop charging. To prevent 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 stops supplying power to the load. Overcurrent protection is when there is a larger current flowing on the load, the control FET1 to cut off and stop discharging to the load, the purpose is to protect the battery and the field effect tube. Overcurrent detection is to use the on resistance of the FET as a detection resistor to monitor its voltage drop and stop discharging when the voltage drop exceeds a set value. A time delay circuit is also generally added to the 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, amateurs are not easy to imitate.
Six, simple charging circuit.
Now there are many businesses selling single-cell lithium batteries without charging board. Its superior performance, low price, can be used for homemade products and lithium battery pack maintenance 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: the use of constant 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 when the battery is full, thus making Q3 cut off and the LED will go off. Please put a suitable heat sink on Q2 and Q3 when using. The advantages of this circuit are: simple production, easy to purchase components, charging safety, display intuitive, and will not damage the battery. By changing W1 can charge multiple lithium batteries in series, and changing W2 can adjust the charging current in a wide range. The disadvantage is: no over-discharge control circuit.
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 when the individual cells have problems. You can choose a suitable single lithium battery for replacement.
2, the production of high brightness miniature torch
I have used a single 3.6V1.6AH lithium battery with a white ultra-high brightness luminous tube to make a miniature flashlight, easy to use, small and beautiful. And because of the large battery capacity, the average use of half an hour per night, has been used for more than two months still do not need to charge. The circuit is shown in Figure 4.
3, instead of 3V power supply
As a single lithium battery voltage of 3.6 V. Therefore, only one lithium battery can replace two ordinary batteries, to the radio, walkman, camera and other small household appliances power supply, not only light weight, and long continuous use time.
Eight, the lithium battery storage.
Lithium batteries need to be fully charged and stored. In 20 ℃ can be stored for more than six months, it can be seen that lithium batteries are suitable for storage at low temperatures. It has been suggested that the rechargeable battery into the refrigerator freezer for storage, is indeed a good note.
Nine, the use of precautions.
Lithium batteries should never be disassembled, drilling, piercing, sawing, pressurization, heating, otherwise there may be serious consequences. No charging protection plate lithium battery can not be short-circuited, not for children to play with. Can not be close to flammable objects, chemical substances. End-of-life lithium batteries should be properly disposed of. Fourth, the charging and discharging requirements of lithium batteries.
1, lithium battery charging: According to the structural characteristics of lithium batteries, the maximum charge termination 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, and can be charged with a special constant-current, constant-voltage charger. Usually 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 stopped.
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). Conventional charging current can be selected at about 0.5 times the battery capacity, and the charging time is about 2 to 3 hours.
2, lithium battery discharge: due to the internal structure of lithium batteries, lithium ions can not all move to the positive electrode when discharged, must retain a portion of lithium ions in the negative electrode to ensure that lithium ions can be smoothly embedded in the channel during the next charge. Otherwise, the battery life will be shortened accordingly. In order to ensure that the graphite layer after discharge to retain part of the lithium ion, we must strictly limit the minimum discharge termination voltage, that is, lithium batteries can not be over-discharged. Discharge termination voltage is usually 3.0V / section, the minimum can not be lower than 2.5V / section. The length of battery discharge time is related to the battery capacity, the size of the discharge current. Battery discharge time (hours) = battery capacity / discharge current. Lithium battery discharge current (mA) should not exceed 3 times the battery capacity. (Such as 1000mAH battery, the discharge current should be strictly controlled within 3A) Otherwise, the battery will be damaged.
Note: The lithium battery pack currently on the market is sealed with a matching charge and discharge protection board inside. As long as the external charge and discharge current can be controlled. Lithium battery charging circuit design.
1. Trickle charge stage. (In the battery transition discharge, the voltage is low state)
3.0V or less. The internal medium of the lithium battery will undergo some physical changes, resulting in bad charging characteristics, reduced capacity, etc.. At this stage, the lithium battery can only be charged slowly by trickling, the internal dielectric of the lithium battery is slowly restored to its normal state.
2. Constant current charging stage. (The battery is restored from the over-discharged state to the normal state)
IC external a pin external a resistor to determine. The size of the resistance is calculated according to the formula on the datasheet of the charge management IC.
3. Constant-voltage charging stage (already full 85% or more, in the slow replenishment)
When the capacity of the lithium battery reaches 85% (about the value), it must enter the slow charging stage again. Make the voltage slowly rise. Eventually reach the maximum voltage of the lithium battery 4.2V.
BAT pin output, this BAT is connected to the end of the lithium battery. At the same time, this pin is also the lithium battery voltage detection collection pin. The Li-ion battery charge management IC detects this pin to determine each state of the battery.
5V is sent to switch SW2 through D2 and to the lithium battery through charge management IC MCP73831. The voltage at the left point of SW2 is 5V-0.7V=4.3V. Since the voltage of the lithium battery is lower than the voltage at the left point of SW2 when it is fully charged or not, D1 is cut off. The charge management IC charges the Li-ion battery normally.
D2 and D1, after the LDORT9193 directly connected to the BAT pin output, it will be charging IC in the time of power on, will produce a misjudgment. There will be connected to the external power supply of 5V, but the lithium battery will not be charged, and the LED indication of the charge management IC is not correct. The rear load LDO will also not get the normal input voltage (the input voltage is very small). In this case, as long as the voltage input pin of the charge management IC is connected directly to the BAT pin with a short circuit, all states are normal again, charging can be carried out, and the rear load LDO works normally.
The moment the IC is connected to the power, it has to detect the state of BAT, and the input pin of LDO is also connected to the branch connected to the positive terminal of BAT and Li-ion battery, which will affect the working state of BAT pin, causing the charge management IC to enter the trickle charging stage. Connect the BAT pin to the voltage input of the charge management IC with a short circuit, so that the voltage of the BAT pin is compulsorily increased, which makes the charge management IC judge that the Li-ion battery has entered the constant current charging stage, so it outputs high current. Able to drive the rear load LDO, etc.
D1 and D2 to use a small voltage drop diode. Such as germanium diodes, Schottky diodes, MOSFET switching tubes. In a design that requires battery switching, a diode with a 10mV forward voltage drop and no reverse leakage current is a "luxury" for the designer. But so far, Schottky diodes are still the best choice, with a forward voltage drop between 300mV and 500mV. But for some battery switching circuits, even the choice of Schottky diodes can not meet the design requirements. For an efficient voltage converter, the portion of energy saved may be completely wasted by the forward voltage drop of the diode. To efficiently preserve battery energy in a low-voltage system, a power MOSFET switch should be selected instead of a diode. MOSFETs with SOT packages and on-state resistance of only a few tens of milliohms can ignore their on-state voltage drop at the current levels of portable products.
MOSFET to switch the power supply, it is best to compare the diode on-state voltage drop, MOSFET on-state voltage drop and battery voltage, and consider the ratio of voltage drop to battery voltage as an efficiency loss. For example, using a Schottky diode with a forward voltage drop of 350mV to switch a Li+ battery (nominal 3.6V) results in a loss of 9.7%, while if used to switch two AA batteries (nominal 2.7V), the loss is 13%. In a low-cost design, these losses may be acceptable. However, when a high efficiency DC-DC is used, the cost of the DC-DC has to be weighed against the cost of the efficiency improvement from upgrading the diode to a MOSFET.
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