Lithium-ion battery management system design
This article discusses the power lithium battery management system is designed to achieve the power of lithium battery overcharge protection, over discharge protection, overcurrent protection and equalization charging functions.
Introduction
The extensive application of lithium-ion batteries for more than ten years, but early days for mobile phones, laptop computers, cameras, DVD and a series of small mobile electronics, such occasions are often single-string to use, low load current, high safety factor. The last two years, lithium-ion batteries for its lightweight, high energy density, no pollution, has started in electric bicycles, power tools and power toys, rapid application areas, and gradually applied to the field of hybrid and electric vehicles. However, the safety of lithium-ion battery power is still one of the most concern now, so their conservation is very important. In addition to lithium-ion batteries to ensure continuous improvement of their own security, must be of the battery management system, the battery and its application to balanced development. Lithium-ion battery protection including over charge protection, over discharge protection, over current and short circuit protection.
1, the function of protection circuit
1.1 Overcharge protection
Of lithium-ion batteries, which charge the highest single energy-saving core voltage must not exceed the specified value, or the battery electrolyte will be decomposed, making the temperature rise and produce gas, reducing the service life of batteries, serious or even cause an explosion Therefore, we must guarantee protection circuit can not be over-charging, the battery must be in each section to monitor the battery voltage, when the batteries when the voltage exceeds a set value, that is activated over charge protection, protection circuit cut off by the charge loop, stop charging. Voltage in the batteries and return to allow the lifting of overcharge voltage lock mode to stop protection. Different materials, lithium-ion battery voltage and the release of its protection voltage values have different requirements.
In addition, attention must also be produced by malfunction due to noise, in order to prevent the miscarriage of justice and misuse, but also set the overcharge protection delay, and delay time can not be shorter than the duration of the noise. When the voltage overcharge detection voltage continuously over more than a certain time to trigger overcharge protection.
1.2 Over-discharge protection
Excessive discharge of lithium-ion battery will shorten its life, and damage the battery is often irreversible. In order to prevent over-discharge of lithium-ion battery status, when the lithium-ion battery voltage is lower than the over-discharge voltage detection point, over-discharge protection is activated, suspended discharge, and will keep the battery low quiescent current standby mode, parameter setting off similar sufficient protection.
1.3 Over current / short circuit protection
The largest lithium-ion battery discharge current is limited and excessive discharge current lithium batteries also can cause irreversible damage, affecting their life.
Short-circuit current protection to protect this function is an extension, if the short-circuit due to external causes such as high-current discharge to immediately stop the discharge, or on the lithium battery itself and the external devices may cause serious damage.
Over-current protection delay time is normally at least a few hundred microseconds to milliseconds, while the short-circuit protection delay time is microseconds, almost instantly cut short the circuit to avoid the huge short-circuit the battery injury.
To power tools, the protection of current value and set the delay time and power tools must also be combined as the parameters, otherwise it will affect the tool's output torque and motor life.
1.4 cell balancing
Power lithium-ion batteries are generally required to Ji Chuan, dozens or even hundreds of strings above the string, because the battery in the production process, from start to become a finished film to go through many procedures, even after rigorous testing procedures, so that each power voltage, resistance, capacity consistent, but the use of a period of time, the battery internal resistance, voltage, capacity and other parameters fluctuate, forming an inconsistent state, it will have this or that difference. This difference reflected in full or discharged battery when the voltage between the battery cells in series is not the same. This situation led to the process of charging the battery pack, the battery voltage is too high trigger early cell battery overcharge protection, and in the process of discharging the battery cell voltage is too low result in battery discharge protection, so that the overall battery capacity decreased significantly, reflected by the battery capacity of the battery pack in the worst performance of the capacity of the battery cell, and use are prone to the phenomenon of overcharge and over discharge, and are not easily found, leading to early failure. Therefore requested to complete the battery cell protection circuit for the balanced operation for high voltage batteries from taking excess current, excess consumption of electricity, to achieve cell balancing to maximize the effectiveness of lithium battery power, extending battery life and increase security. The balance of the current methods commonly used storage balance and resistance balance.
Equilibrium is the use of battery energy storage inductor or capacitor energy storage components such as charge and discharge through the relay or switching device without energy storage components to achieve balance between the battery switch to the energy transfer between cells. This method of balancing the general control network charge complex, high-security management requirements, in the use of energy storage devices should be careful to take a good charge and discharge time, its biggest advantage is that charge and discharge (work) in use, can balance the unit cell function, and does not consume power lithium-ion battery pack.
Resistance is generally balanced by resistance network controller on the battery pack off balance diversion, this method can be balanced multi-cell control simple. But the balancing process in the election if the resistance is too large, the equilibrium current is too small, had little effect; If the resistance is chosen too small, the resistance power of large energy loss system big, balanced, low efficiency, high system thermal management requirements the need for temperature measurement and control.
Resistance is the principle of equilibrium in the battery charging process, when a fast battery charging, the battery voltage is higher than the other, the system balanced by controlling the switch conduction resistance of the shunt and reduce the battery charging rate, in order to achieve sections the purpose of equalizing charge the battery.
2 Implementation of protection
For the protection of lithium-ion batteries have two main methods: microprocessor control and integrated circuit chip.
2.1 IC control
The current lithium-ion battery can chip a lot of protection, foreign countries, Taiwan and mainland China have a variety of chips to choose from, the current Japanese Ricoh Seiko adopted by more programs, programs mature, the external circuit is simple, but the price is more expensive.
IC implementation of various protection features almost the same, the protection mode and the external lines are also similar, in practice may need to select a different IC. Select IC's time to consider multiple, different types of IC overcharge protection voltage is different, there is also 4.35V 4.25V, there IC's own power, whether the peripheral circuit is simple enough, the various parameters of the protection IC is meet the requirements, the volume is small enough to be considerate.
Protection board in addition to other protection function, low power consumption are also important parameters. To prevent over-discharge protection IC to detect the battery voltage, over discharge detection voltage once they reach the following, you must turn off the power MOSFET and the cut-off discharge. But this time there are natural discharge the battery itself and the protection of IC current consumption exist, so need to protect the IC to minimize current consumption, in the protection of state, the quiescent current consumption must be small 0.1uA.
Another power lithium-ion rechargeable battery pack for work or the instant when the pressure will be generated, thereby protecting IC shall meet the requirements of high pressure.
Figure 1 is a Seiko S-8254A 4-string for the protection of IC application schematic. S-8254 series built-in high accuracy voltage detection circuit and delay circuit is used for 3 or 4-cell lithium-ion or lithium polymer rechargeable battery protection IC. Switching through the SEL pin, can be used to protect three or four series cells.
Figure 1 S-8254A 4 serial protection schematic
Of course, the current battery protection IC to protect the four most common lithium-ion batteries, but many applications require more than five working lithium-ion batteries in series, such as power tools, electric bicycles and UPS, at this time how to solve it? Shown in Figure 2, the circuit can 20A/24V output power, to Seiko S-8254AAV control chip as an application example, it uses two chips in series with the protection, protection of lithium-ion battery pack 8-string, too discharge protection voltage of 2.70V +/- 0.080V, overcharge protection voltage is 4.250V +/- 0.025V.
Figure 28 Series power lithium-ion battery protection circuit
The circuit balance control with R5408 chip, a high voltage measurement accuracy, balance current up to 1A.
2.2 MCU control
Protection of some of the existing integrated circuit chips mainly for the following 4 cell battery power-saving protection, and for four or more batteries than a single-level protection can be used in series or in-chip multi-level protection IC of several series way. But the use of multiple protection chip series way more than 4 cell battery power-saving protection circuit poor scalability. Meanwhile, the integrated protection of the chip is often only for one or more of the characteristics of a class of cells, the lack of flexibility, cost often higher. This, combined with lithium-ion battery charge and discharge characteristics of lithium-ion battery power on many occasions protection circuit, using the MCU (microprocessor) as the core design.
Control various functions of the microprocessor as the core, in addition to lithium-ion battery pack provides over charge, over discharge, over-current protection, effectively within the lithium ion battery lithium single charge and discharge provide dynamic equilibrium, the temperature protection, short circuit protection, and can provide, such as capacity forecasting, communication, identification and other functions.
3 anti-interference measures of hardware
Lithium-ion battery management system as part of an application system, and are subject to various electromagnetic interference, the actual work environment is relatively poor, it was necessary hardware and wiring on the PCB board to take certain anti-interference measures.
4 Other Requirements
Because the battery is primarily used to supply the main application project, thus requiring only a very low power consumption BMS.
5 Conclusion
Lithium-ion battery power monitoring is a relatively new subject, its management system will be integrated monitoring protection technology design, with the rest of the battery pack, charge, discharge, management, and automatic maintenance and other basic functions, to achieve practical and reliable requirements.
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