Rechargeable button battery 2025 technology and charging methods

　　Batteries have never been more widely used than they are now. Batteries are getting smaller and lighter, holding more energy per unit volume. The main driving force for button battery 2025 development comes from the development of portable devices (mobile phones, laptops, camcorders, MP3 players).

　　This article provides an overview of charging methods and modern button battery 2025 technology to better understand batteries used in portable devices. This includes descriptions of nickel-cadmium (NiCd), nickel-metal hydride (NiMH) and lithium-ion (Li+) button battery 2025 chemistries. This article also describes protection devices for single-cell lithium-ion and lithium-ion polymer batteries.

　　button battery 2025 definition

　　The button battery 2025 is called the energy storage system, which also includes freewheeling and clock sources. From a modern technology perspective, a button battery 2025 is usually a portable device that is a self-storage chemical system that produces electrical energy.

　　Disposable batteries (called non-rechargeable or primary batteries) generate electrical energy from chemical reactions that constantly change the button battery 2025. The discharge of a disposable button battery 2025 causes permanent and irreversible changes in the button battery 2025's chemical composition. On the other hand, rechargeable batteries are called secondary batteries, which are charged by a charger and discharged during use. Therefore, secondary batteries can generate energy multiple times and store energy multiple times.

　　Charge or discharge current (in amps) is usually expressed as a multiple of the rated capacity (called the C-rate). For example, a button battery 2025 rated for 1 amp for one hour (1Ah) has a C/10 discharge current of 1Ah/10 = 100mA. A button battery 2025's rated capacity (Ah or mAh) is the amount of electricity it can store (produce) when fully charged under specific conditions. Therefore, the total energy of a button battery 2025 is the capacity times the voltage, which is measured in watts per hour.

　　button battery 2025 performance test

　　The button battery 2025's chemistry and design together limit the amount of current the button battery 2025 can provide. Without practical limitations on performance, batteries can produce infinite amounts of current. The main factors that limit button battery 2025 performance are the reaction rate of the chemical components, the button battery 2025 design and the area where the reaction occurs. Some batteries have the ability to produce large currents. For example, nickel-cadmium batteries produce currents large enough to melt metal and cause fires. Other batteries can only produce weak current.

　　The net effect of all chemical and mechanical factors in an electric current can be expressed as a single mathematical factor - equivalent internal resistance. Lowering the internal resistance results in greater current.

　　No button battery 2025 can store energy permanently. Inevitably, the button battery 2025's chemical reaction capacity gradually decreases, causing the button battery 2025 to store less charge. The ratio of button battery 2025 capacity to weight (or size) is called the storage density of the button battery 2025. In a button battery 2025 of a given size and weight, high storage density means more energy can be stored.

　　If both primary and secondary batteries can achieve the same purpose, why not always choose secondary batteries? This is because secondary batteries have the following disadvantages:

　　Charging batteries

　　A new rechargeable button battery 2025 or button battery 2025 pack (several cells in a button battery 2025 pack) is not guaranteed to be fully charged. In fact they are probably almost discharged. Therefore, the first thing to do is to charge the button battery 2025/button battery 2025 pack according to the manufacturer's guidelines regarding chemical composition.

　　Each charging operation sequentially adds voltage and current according to the chemical composition of the button battery 2025. Therefore, chargers and charging algorithms meet the different requirements of button battery 2025 chemistries. Terms often encountered in button battery 2025 charging are: constant current (CC) for NiCl and NiMH batteries and constant current/constant voltage (CC/CV) for lithium-ion and lithium polymer batteries (see Figure 1-6).

　　Nickel cadmium button battery 2025 charging

　　Add constant current (0.05C-1C) to charge the NiCd button battery 2025. Some low-cost chargers rely on absolute temperature to terminate charging. Although simple and low-cost, this method of charge termination is imprecise. A better method is to terminate charging by detecting voltage drops. The -ΔV method is most effective for NiCd batteries with charging rates of 0.5C or higher. - ΔV end-of-charge detection should be combined with button battery 2025 temperature measurement, as deteriorated and mismatched cells can reduce ΔV.

　　More accurate full charge detection can be achieved by detecting the temperature increase rate (dT/dt), which is a better charge detection method than the fixed temperature termination method. The charge termination method based on the dT/dt and -ΔV termination combination method has a longer life cycle and can avoid overcharging.

　　Fast charging improves charging efficiency. The efficiency is close to 1.1 (91%) at 1C, while the charging time of an unloaded button battery 2025 is a little more than 1 hour. When charging at 0.1C, the efficiency drops to 1.4 (71%), and the charging time is about 14 hours.

　　Because the charge acceptance of NiCd batteries is close to 100%, almost all the energy is absorbed during the first 70% charge, while the button battery 2025 remains slightly cold. Ultra-fast chargers take advantage of this feature to charge the button battery 2025 to 70% level in a few minutes, with the added current equal to several times the C rate without generating heat. After reaching the 70% level, the button battery 2025 continues to charge at a lower rate until the button battery 2025 is fully charged. Finally, add 0.02~0.1C trickle current to end the button battery 2025 charging.

　　NiMH button battery 2025 charging

　　Although NiMH chargers are similar to NiCd chargers, NiMH chargers use the dT/dt method, which is the best way to charge NiMH batteries. The end-of-charge voltage drop of NiMH batteries is relatively small, and for small charge rates (below 0.5C, depending on temperature) there can be no voltage drop at all.

　　New NiMH batteries develop unreliable peaks early in the charge cycle, which causes the charger to end the charge prematurely. In addition, using -△V to detect the end of charge can protect against overcharging, which itself limits the number of charges/discharges before the button battery 2025 fails. It does not seem to be available under all conditions (new or old, hot or cold, fully or partially discharged) The -dV/dt algorithm can make charging NiMH batteries more efficient. For this reason, NiMH batteries cannot be charged with a NiCd charger unless it is terminated using the dT/dt method. Because NiMH batteries cannot absorb overcharge, the trickle charge must be smaller than NiCd (around 0.05C).

　　Slow-charging NiMH cells is more difficult because the voltage and temperature distribution associated with C-rates in the 0.1C-0.3C range do not provide an accurate enough indication of full charge status. Therefore, slow chargers must rely on timers to indicate when the charging cycle should end. So, in order to fully charge a NiMH button battery 2025, a fast charge close to 1C (or C rate as specified by the button battery 2025 manufacturer) should be applied while monitoring voltage (ΔV=0) and temperature (dT/dt) to determine when charging should Finish.

　　Charging lithium-ion and lithium-polymer batteries

　　In fact, nickel-based button battery 2025 chargers are current-limiting, while lithium-ion button battery 2025 chargers limit voltage and current. The charging voltage of the first generation lithium-ion button battery 2025 is limited to 4.10V/button battery 2025. Higher voltage means greater capacity, and the 4.20V button battery 2025 voltage is achieved by adding chemical additives. Modern lithium-ion batteries are generally charged to 4.20V (tolerance ±0.05V/button battery 2025).

　　Full charge is achieved after the charging terminal voltage reaches the voltage threshold and the charging current drops to 0.03C (close to 3%Ich, see Figure 6). Most chargers take about 3 hours to reach full charge, while some linear chargers claim to charge Li+ batteries in about an hour. This charger usually terminates charging when the button battery 2025 terminal voltage reaches 4.2V. However, this provision only charges the button battery 2025 to 70% of its capacity.

　　A larger charging current cannot shorten the charging time too much. A larger charging current can reach the voltage peak faster, but float charging takes longer. As a rule of thumb, float charging is twice the initial charging time.

　　Lithium-Ion button battery 2025 Safety Measures

　　Because overcharging (or overdischarging) lithium-ion batteries can cause button battery 2025 explosions and personal injury, safety is a major concern when using these batteries. Therefore, commercial Li-ion button battery 2025 packs contain protection circuits like the DS2720 (Figure 7). The DS2720 provides all the button battery 2025 protection functions required for rechargeable Li+ button battery 2025 applications: protection of the button battery 2025 during charging, protection circuitry to prevent excessive current flow, and Limit button battery 2025 drain and level to maximize button battery 2025 life.

　　The DS2720IC uses external switching devices, such as low-cost N-channel power MOSFETs, to control the path of charge and discharge currents. The IC's internal 9V charge pump provides high-side drive for the external n-channel MOSFET, which provides lower on-resistance than the same functional FET in a common low-side protection circuit. FET on-resistance decreases as button battery 2025 discharges (see Figure 8)

　　The DS2720 can control external FETs from the data interface or dedicated inputs, thus eliminating additional power switch control in rechargeable Li+ button battery 2025 systems. Through its 1-Wire interface, the DS2720 provides host system read/write access to status and control registers, instrument registers, and general data storage. A factory-programmed 64-bit unique address allows the host system to address each device individually.

　　DS2720 provides two user memories for button battery 2025 information storage, EEpROM and lock table EEpROM. EEpROM is true non-volatile (NV) memory and its contents (vital button battery 2025 data) remain unaffected by severe button battery 2025 drain, sudden short circuit or ESD shock. When locked, the lock table EEpROM becomes read-only memory (ROM), which provides additional security for retaining button battery 2025 data.

　　protected mode

　　Overvoltage: If the button battery 2025 voltage detected at VDD exceeds the overvoltage threshold Vov for longer than the overvoltage delay TOVD, the DS2720 turns off the external charging FET and sets the OV flag in the protection register. During the overvoltage period, the discharge path remains open. When the button battery 2025 voltage drops below the charge enable threshold voltage VCE or discharge causes VDD-VpLS>VOC, the charging FET is re-enabled (unless blocked by other protection conditions).

　　Undervoltage: If the button battery 2025 voltage detected at VDD is lower than the undervoltage threshold VUV for longer than the undervoltage delay TUVD, the DS2720 turns off the charging and discharging FETs and sets the UV flag in the protection register to enter sleep mode. After the button battery 2025 voltage rises above VUV and the charger is connected, the IC turns on the charge and discharge FETs.

　　Short circuit: During the TSCD cycle, if the button battery 2025 voltage detected at VDD is lower than the consumption threshold voltage VSC, the DS2720 turns off the charge and discharge FETs and sets the DOC flag in the protection register. The current path through the charge and discharge FET will not be re-established until the voltage on pLS rises above VDD-VOC. The DS2720 provides a test current flowing through the internal resistor RTST (from VDD to pLS, pulling up pLS when VDD rises above VSC). This test current allows the DS2720 to detect excursions in low-impedance loads. In addition, the charging path can be restored by RTST from pLS to VDD.

　　Overcurrent: If the voltage applied to the protection FET (VDD-VpLS) is greater than VOC for longer than TOCD, the DS2720 turns off the external charge and discharge TET and sets the protection register DOC flag. The current path will not be re-established until the voltage on pLS rises above VDD-VOC. The DS2720 provides a test current through internal resistor TRST (from VDD to pLS) to detect excursions in non-conforming low-impedance loads.

　　Overtemperature: If the DS2720 temperature exceeds TMAX, the external charge and discharge FETs are immediately shut down. The FET will not turn on until the following two conditions are met: the button battery 2025 temperature drops below TMAX and the host resets the OT bit.

　　Charging temperature

　　Charging should be done at room temperature as much as possible. Nickel-based batteries should be quickly charged between 10℃~30℃. Below 5℃ (41oF) and above 45℃ (113oF), the charge acceptance of nickel-based batteries decreases sharply. Lithium-ion batteries exhibit good charging performance over the entire temperature range, but below 5°C (41oF), the charge rate is less than 1C.

　　Conclusion

　　NiMH chargers can accommodate NiCd batteries, but not the other way around. Chargers designed for NiCd batteries will overcharge NiMH batteries. Fast charging enhances the life and performance of nickel-based batteries because it reduces the memory effect caused by internal crystallization. Nickel and lithium-based batteries require different charging algorithms. Li+ batteries require protection circuitry to monitor and protect against overcurrent, short circuit, over- and under-voltage, and over-temperature.

　　Note: When the button battery 2025 is not used frequently, remove the button battery 2025 from the charger and fully charge the button battery 2025 before use.

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