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Current Sensing Considerations in Li-ion Battery Management

BATTERY MANAGEMENT OVERVIEW

Battery management encompasses two main functions: 1. Charging adapter, and 2. Battery fuel gauging and protection on the battery side. The block diagram in Figure 1 depicts the overall system. In each of these systems current sensing plays an essential role in extracting maximum performance, life, and safety from the battery.

Figure 1

Generic battery management system with charge adapter and battery management.

Generic battery management system with charge adapter and battery management.

BATTERY MANAGEMENT-DEVICE SIDE CIRCUITRY

Accurate Fuel Gauging and System Management (All Images courtesy of ON SEMI)

For fuel gauging Lithium-ion batteries, charge-integration over time is a frequently used method for coulomb counting and determining SoC (State-of-Charge). In other battery technologies, open-circuit voltage has been used as fuel gauging, but Lithium-ion batteries display a much more constant voltage over the discharge cycle which does not provide reliable information on State-of-Charge, as shown in Figure 2.

Figure 2

Generalized discharge curve for Li-ion battery shows that determining charge by voltage is less than ideal for accuracy. Sensing current makes Coulomb counting a more accurate alternative to state-of-charge measurement.

Generalized discharge curve for Li-ion battery shows that determining charge by voltage is less than ideal for accuracy. Sensing current makes Coulomb counting a more accurate alternative to state-of-charge measurement.

Coulomb counting, assuming starting from a known state, integrates the discharge current to count coulombs removed from the battery and charge current to count coulombs restored. It should readily be evident that this is a process that benefits from the best possible accuracy in current sensing. It is not the intent of this article to dwell on the myriad details of coulomb counting, but mainly to focus on the role current sensing plays in battery management. (See Unknown Document 568478

Another aspect of Lithium-ion batteries is the importance of battery management. Lithium-ion batteries impose strict operating limits on voltage, current, and temperature to avoid undesirable consequences such as reduced battery life, internal short circuits, or battery damage.

Battery Management System Example

Figure 3 is a block diagram of circuitry in a typical Li-ion battery pack. It shows an example of a safety protection circuit for the Li-ion cells and a gas gauge (capacity measuring device). The safety circuitry includes a Li-ion protector that controls back-to-back FET switches. These switches can be opened to protect the pack against fault conditions such as overvoltage, undervoltage, and overcurrent. The diagram also includes a temperature sensitive three-terminal fuse that will open due to prolonged overcurrent or overtemperature, or it can be forced to open by redundant protection circuitry in case there is a fault where the primary protection circuitry fails to respond. Opening this fuse is a last resort, as it will render the pack permanently disabled.

The gas-gauge circuitry measures the charge and discharge current by measuring the current with a shunt resistor and a current sense amplifier. The current measurement is integrated to determine the change in coulometric capacity. In addition, the gauge measures temperature and voltage, evaluates gas-gauging algorithms to determine the available capacity in the battery, and computes time-to-empty and other values required by the host. The available capacity as well as other measurements and computational results are made available and transmitted to the system control.

Figure 3

Block diagram of device side circuitry in a typical Li-ion battery pack.

Block diagram of device side circuitry in a typical Li-ion battery pack.

SPECIFIC CONSIDERATIONS IN CURRENT SENSING

Selecting the Resistor (See Figures 4 and 5)

Selecting the resistor value, precision, and physical size is dependent upon the current expected to be measured. The larger the resistor value, the more accurate the measurement possible, but a large resistor value also results in greater current loss, reducing efficiency. For low power battery-powered devices that must have low losses, resistors that are about a millimeter in length with values of hundredths or thousandths of an ohm are often used.

Although a resistor is often thought of as a simple two-terminal device, for accurate measurements four-terminal current-sense resistors are used with two terminals at each end of the resistor This provides two terminals for the current path of the application circuit, and a pair for the voltage detection path of the sense amplifier. This four-terminal setup, also known as Kelvin Sensing, insures an accurate connection for sensing, and higher accuracy translates to more accurate coulomb counting.

Accurate resistor installation and connection is essential to best accuracy. There are two general categories of current shunts, standard shunts and Kelvin connection shunts. Standard shunts are the most economical but careful attention must be paid to the connection methods. Figure 4 details the general case for correct and incorrect connections, but when possible check with the shunt manufacturers literature or consult with the manufacturer for ideal connections.

Figure 4

While this depicts general recommendations for non-Kelvin shunt connections always check manufacturer's literature, if available, for any specific recommendations

While this depicts general recommendations for non-Kelvin shunt connections always check manufacturer's literature, if available, for any specific recommendations

The Kelvin connection shunts make getting accuracy easy from the standpoint of connections. Dedicated connection pins make this possible and Figure 5 depicts a picture of both standard and Kelvin connection shunts as well as a connection diagram.

Figure 5

Shown here are both standard and Kelvin connection shunt (photo courtesy of SEI Stackpole) with a connection schematic.

Shown here are both standard and Kelvin connection shunt (photo courtesy of SEI Stackpole) with a connection schematic.

Offset Voltage

All too often when talk turns to offset voltage, it is discussed in terms of accuracy, which is essentially true but misses an important aspect relative to current sensing. Offset voltage determines the minimum practical shunt voltage drop that can be measured accurately. A lower minimum voltage drop allows for the minimum full-scale voltage drop. Reducing full-scale voltage drop has two important benefits:

1. A lower full-scale shunt drop reduces power losses, and this contributes to longer battery life.

2. A lower shunt voltage drop means less power is dissipated in the shunt which often enables the use of smaller and more economical shunt resistors, as well as less temperature rise.

Common-Mode Range and Battery Voltage Considerations

There are two general categories of current sensing amplifiers, those with common-mode voltage ranges that extend to, or below, ground, the remaining category being amplifiers which can only measure reliably above some minimum common mode voltage which will always be specified.

Many applications will also power the current sensing amplifier from the same battery it is monitoring, in which case sensing all the way down to zero is unnecessary and not possible. Nonetheless it is desirable to monitor to the lowest level possible.

Some systems may power the current sense amplifier on a lower power separate supply, yet derived from the battery in which case the requirement for the current sense amplifier is likely to include the ability to operate at a low power supply voltage, and this will merit the use of an amplifier suited to operate at a low supply voltages below 3 volts.

Unidirectional and Bi-Directional Current Sensing

Battery management requires bi-directional current sensing to monitor both charge and discharge current. It is important to check this specification on current sense amplifiers since some of them are intended for unidirectional use only. It is also useful to note that a bi-directional current sense amplifier can always be used for unidirectional applications, but not vice-versa.

A detail regarding bi-directional current sensing is that the amplifier will have a reference pin to bias the quiescent output voltage of the amplifier. This permits output voltage swings above and below the reference level to represent charge vs. discharge conditions.

Figure 6 shows an example of a circuit where the current sense amplifier is powered directly from the battery. At full charge the battery is nominally 4.2 volts. The NCS213R current sense amplifier shown operates as low as 2.2 volts, a level below which the system is likely to be functional. With regard to common-mode voltage range, the NCS213R is rated down to -0.3 volt common-mode range, and while the circuit does not require common-mode to zero, it makes this amplifier a simple choice.

This circuit is configured to drive an ADC with a 0 to 2.5 volt input, such that 1.25 volts represents the center value of the amplifier output, thus the 1.25 volt bias to the amplifier reference pin. This reference bias needs to come from a low impedance to preserve high common-mode rejection in the amplifier. If a simple resistor divider is used to generate this reference it should be buffered by a unity gain amplifier.

Figure 6

Example battery current sensing circuit with bi-directional current sense amplifier. Using an ON Semiconductor NCS214R provides 60 μV maximum offset and 1% maximum gain error over temperature

Example battery current sensing circuit with bi-directional current sense amplifier. Using an ON Semiconductor NCS214R provides 60 μV maximum offset and 1% maximum gain error over temperature

The circuit shows the ON Semiconductor NCS21xR family, of which the NCS214R provides a gain of 100 with 60 μV maximum offset, allowing for a +/-1.25 volt output swing for a +/-12.5mV shunt drop.

BATTERY AND SYSTEM PROTECTION

Current sensing is also used to monitor and protect batteries. Current measurements are used to protect the battery against abuse and ensure its safe use by providing for emergency shut-down in over-current conditions. While protection and safety functions may not require a high level of current measurement accuracy, the discrete current sensing readily provides ideal signals for protective circuits.

Speed is an important consideration in protection and this capability is specified by parameters such as the bandwidth and slew rate of the current sense amplifier. The NCS214R previously mentioned provides a bandwidth of 60 kHz and a 1 V/μs slew rate, insuring fast response to over current transients.

CURRENT SENSING AND TEMPERATURE CONTROL

Current sensing can play a role in temperature control strategies. Temperature sensor based thermal controls require a rise in temperature to detect and invoke thermal controls. Such a control strategy makes more sense where it is desired to stabilize a system at an elevated temperature.

Smart thermal controls should respond to the thermal load of the system which is represented by power taken from or fed to the battery. Current sensing provides the signal that makes proactive thermal control possible.

Sensing the current, which is representative of power, provides “leading edge” information regarding an impending temperature event that is predictable. You can use this to invoke temperature control strategies even before the temperature rises, and your temperature sensors now serve the role of protecting against major temperature faults or cooling strategy failures. This is a strategy that can keep your system as cool as possible, contrasted with temperature sensing based methods that tend to keep the system at an elevated control temperature.

MANAGEMENT OUTSIDE THE BATTERY-ADAPTER SIDE CIRCUITRY

Figure 7 shows a block diagram of the AC adapter portion of the complete battery charging system. The adapter converts AC to DC and provides regulation and protection while the circuitry, on the device side, in Figure 3 controls the details of charging. The current signal in the adapter is connected to both regulation and protection circuits in the adapter.

Current sensing is an essential part of both charge and discharge management for Li-ion batteries and while there are fully integrated solutions available, achieving the best possible battery life and safety can dictate the use of discrete circuitry to provide the required accuracy. In many cases the exact algorithms are proprietary and highly confidential and require the use of discrete approaches in areas such as current sensing to achieve their performance objectives.

Figure 7

Adapter side circuitry for AC (or other sources) conditioning and protection requires current sensing.

Adapter side circuitry for AC (or other sources) conditioning and protection requires current sensing.

Reference

Keep Cool by Sensing Current to Control Fans

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