Rechargeable battery technology and charging methods

Rechargeable battery technology and charging methods

Batteries have never been used as widely as they are today. Batteries are becoming smaller, lighter, and can hold more energy per unit volume. The main driving force behind battery development is the development of portable devices (mobile phones, laptops, camcorders, MP3 players).

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

Battery Definition

Batteries are called energy storage systems, which also include freewheeling and clock sources. From the perspective of modern technology, batteries are usually portable devices that generate electrical energy through self-storage chemical systems.

Disposable batteries (called non-rechargeable or primary batteries) generate electrical energy from a chemical reaction in a constantly changing battery. Discharging a disposable battery causes a permanent and irreversible change in the battery's chemical composition. Rechargeable batteries, on the other hand, are called secondary batteries, which are charged by a charger and discharged in use. Thus, secondary batteries can generate energy and store energy multiple times.

The charge or discharge current (amperes) is usually expressed as a multiple of the rated capacity (called the C rate). For example, a battery rated at 1 ampere per hour (1Ah) has a C/10 discharge current of 1Ah/10 = 100mA. The rated capacity (Ah or mAh) of a battery is the amount of electricity it can store (produce) when fully charged under specific conditions. Therefore, the total energy of a battery is the capacity times the voltage, which is measured in watt-hours.

Figure 1 Semi-constant current charging (mainly used in applications such as electric shavers, digital cordless phones and toys)

Figure 2 Timer-controlled charging (mainly used in notebook computers, data terminals, wireless devices and cellular phones)

Figure 3 - △V cutoff current (used in notebook computers, data terminals, camcorders, wireless devices, and cellular phones)

Figure 4 - dT/dt termination charging (applied in power tools and electrical tools)

Battery performance test

The battery's chemistry and design together limit the current that the battery can deliver. If there were no practical factors limiting performance, a battery could produce an infinite amount of current. The main factors limiting battery performance are the reaction rates of the chemistry, the battery design, and the area where the reaction occurs. Some batteries have the ability to produce large currents. For example, a nickel-cadmium battery can produce currents large enough to melt metal and start fires. Other batteries can only produce small currents.

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

No battery can store energy forever. Inevitably, the battery's chemical reaction capacity gradually deteriorates, resulting in a decrease in the battery's ability to store charge. The ratio of a battery's capacity to its weight (or size) is called the battery's storage density. In a battery of a given size and weight, a high storage density means more energy can be stored.

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

Battery Charging

A new rechargeable cell or battery pack (several cells in a pack) is not guaranteed to be fully charged. In fact they are likely to be almost discharged. Therefore, the first thing to do is to charge the cell/battery according to the manufacturer's guidelines for the chemistry.

Each charging operation applies voltage and current sequentially according to the battery chemistry. Therefore, the charger and charging algorithm meet the different requirements of battery chemistry. The terms often encountered in battery charging are: constant current (CC) for NiCl and NiMH batteries and constant current/constant voltage (CC/CV) for Li-ion and Li-polymer batteries (see Figure 1-6).

NiCd battery charging

Charge NiCd batteries with constant current (0.05C-1C). Some low-cost chargers terminate charging by absolute temperature. Although simple and low cost, this method of charge termination is inaccurate. A better method is to terminate charging by detecting voltage drop. The -△V method is most effective for NiCd batteries charged at 0.5C or higher rates. -△V charge termination detection should be combined with battery temperature measurement because deteriorated batteries and mismatched batteries can reduce △V.

A 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 combination of dT/dt and -△V termination has a longer life cycle and can avoid overcharging.

Fast charging can improve charging efficiency. At 1C, the efficiency is close to 1.1 (91%), and the charging time of the no-load battery is a little over 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% of charging, while the battery remains slightly cool. The ultra-fast charger takes advantage of this feature and charges the battery to the 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 battery continues to charge at a lower rate until the battery is fully charged. Finally, add 0.02~0.1C trickle current to end the battery charging.

NiMH battery charging

Although NiMH chargers are similar to NiCd chargers, NiMH chargers use the dT/dt method, which is the best method for charging NiMH batteries. NiMH batteries have a smaller voltage drop at the end of charge, and no voltage drop at all for small charge rates (less than 0.5C, depending on temperature).

New NiMH batteries have unreliable spikes too early in the charge cycle, which can cause the charger to terminate the charge prematurely. In addition, using -△V to detect the end of charge can protect against overcharge, which itself limits the number of charge/discharges before the battery fails. There seems to be no -dV/dt algorithm available that can charge NiMH batteries more efficiently under all conditions (new or old, hot or cold, fully or partially discharged). For this reason, NiMH batteries cannot be charged with a NiCd charger unless it uses the dT/dt method to terminate the charge. Because NiMH batteries cannot absorb overcharge, the trickle charge must be less than NiCd (about 0.05C).

Slow charging of NiMH batteries is more difficult, this is with the 0.1C-0.3C range of C-rate related voltage and temperature distribution can not provide a sufficiently accurate indication of the fully charged state. Therefore, the slow charger must rely on a timer to indicate when the charging cycle should end. So, in order to fully charge NiMH batteries, should be applied close to 1C (or according to the battery manufacturer's calibrated C rate) fast charge, while monitoring the voltage (△V = 0) and temperature (dT / dt) to determine when the charge should end.

Li-ion and Li-polymer battery charging

Actually, the charger for nickel-based batteries is current-limited, while the charger for lithium-ion batteries is voltage- and current-limited. The charging voltage of the first generation of lithium-ion batteries was limited to 4.10V/cell. Higher voltage means greater capacity, and the 4.20V cell voltage was achieved by adding chemical additives. Modern lithium-ion batteries are generally charged to 4.20V (tolerance ±0.05V/cell).

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 reach full charge in about 3 hours, while some linear chargers claim to charge Li+ batteries in about an hour. Such chargers usually terminate charging when the battery terminal voltage reaches 4.2V. However, this specification only charges the battery to 70% of its capacity.

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

Lithium-ion battery safety measures

Because overcharging (or overdischarging) lithium-ion batteries can cause battery explosion and personal injury, safety is a major concern when using these batteries. Therefore, commercial lithium-ion battery packs contain protection circuits such as the DS2720 (Figure 7). The DS2720 provides all the battery protection functions required for rechargeable Li+ battery applications: protecting the battery during charging, preventing excessive current from flowing through the protection circuit, and limiting battery depletion to a level that maximizes battery life.

The DS2720IC uses external switching devices (such as low-cost N-channel power MOSFETs) to control the path of the 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 FET with the same function in a common low-side protection circuit. The FET on-resistance decreases as the battery discharges (see Figure 8).

Figure 5 Trickle charging (mainly used in emergency lights, guide lights, storage devices, and backup equipment)

Figure 6 Constant current, constant voltage charging (for cell phones, wireless devices, and notebook PCs)

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

The DS2720 provides two user memories for battery information storage, EEPROM and lock table EEPROM. EEPROM is a true nonvolatile (NV) memory whose contents (important battery data) remain unaffected by severe battery consumption, sudden short circuits, or ESD shocks. When locked, the lock table EEPROM becomes read-only memory (ROM), which provides an additional safety measure for retaining battery data.

Protected Mode

Overvoltage: If the battery voltage detected at VDD exceeds the overvoltage threshold Vov for a time greater than the overvoltage delay TOVD, the DS2720 turns off the external charging FET and sets the OV flag in the protection register. During overvoltage, the discharge path remains open, and when the battery voltage drops below the charge enable threshold voltage VCE or the discharge causes VDD-VPLS>VOC, the charging FET is re-enabled (unless locked by another protection condition).

Undervoltage: If the battery voltage detected by VDD is lower than the undervoltage threshold VUV for a time greater than the undervoltage delay TUVD, the DS2720 turns off the charge and discharge FETs and sets the protection register UV flag to enter sleep mode. After the battery 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 battery 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 FETs will not be reestablished until the voltage on PLS rises to greater than VDD-VOC. The DS2720 provides a test current flowing through the internal resistor RTST (from VDD to PLS, when VDD rises to greater than VSC, PLS is pulled up). This test current enables the DS2720 to detect excursions in low impedance loads. In addition, the charging path can be restored through RTST from PLS to VDD.

Overcurrent: If the voltage applied to the protection FET (VDD-VPLS) is greater than VOC for more 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 reestablished until the voltage on PLS rises to greater than VDD-VOC. The DS2720 provides a test current through the internal resistor TRST (from VDD to PLS) to detect the excursion of unqualified low-impedance loads.

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

Charging temperature

Try to charge at room temperature. Nickel-based batteries should be fast-charged between 10°C and 30°C. Below 5°C (41°F) and above 45°C (113°F), the charging acceptance of nickel-based batteries drops sharply. Lithium-ion batteries show good charging performance over the entire temperature range, but below 5°C (41°F), the charging rate is less than 1C.

Conclusion

NiMH chargers can accommodate NiCd batteries, but not vice versa. A charger 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 circuits to monitor and protect against overcurrent, short circuit, over- and undervoltage, and overtemperature.

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

Figure 7 DS2720 lithium battery protection circuit

Figure 8. The DS2720 (high-side) mode controlled protection FET resistance is lower than the conventional low-side mode FET operating value. The FET resistance controlled by the DS2720 decreases with the battery voltage.