Battery Management for Sensing Resistors

Kory Schroeder, Director of Marketing & Product Engineering at Stackpole Electronics, Inc.


Battery management is a rapidly growing market segment for sensing resistors

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Figure 1. Battery Management Block Diagram

With end products ranging from laptop computers and other consumer electronics to industrial infrastructure UPS systems, solar power storage, and hybrid vehicles, battery management is a rapidly growing market segment for sensing resistors. Understanding the basic structure for battery management circuits and the resistor requirements for various end products enables designers to develop better systems with improved functionality and efficiency.

Battery Management Basic Layout

Typical battery management topologies and reference designs will start with a power manager and charger platform. This block will regulate the incoming power from sources such as USB chargers, AC to DC power transformers, and wireless charging stations.  This power is then distributed to either the circuitry itself or to the battery pack.  If unplugged, then power transfers to the charger / power management block from the battery pack instead.  The battery level is also monitored by the battery fuel gauge and protection block.  This block will provide the output levels for the battery that is seen by the user as well as providing interface protection.  Note there is not typically a direct path from the battery to the system circuitry itself for isolation purposes.

Tablet PC’s, Netbooks, Portable Media Players, and Battery Backup Systems

These types of commercial devices will usually employ 1 to 3 cell charging systems at 4.2V per cell and 4.5V to 17V input voltage.  These devices require efficiency in the 90% range and typically have charging systems of 1 to 5 amps.  The sense resistors for these devices focus more on low cost and small size, since resistance values typically range from 10 milliohm to 100 milliohms.  Higher resistance value designs would normally be used for smaller and more portable devices.  In many cases, there are sense resistors used to monitor both the power delivered to the system circuitry and the power delivered to or used by the battery.

Thick film current sense resistorswith resistance values from 3 milliohm to 1 ohm and tolerances down to 1% depending on size, offer low cost with the ability to support that kind of efficiency. Foil on ceramic current sense chip resistors, such as Stackpole’s CSRF Series, offer very low resistance values and low TCR in small chip sizes. This includes 0.25W rated 0402 size chips in values down to 3 milliohm and 0.5W rated 0603 size chips in values down to 2.5 milliohm. Foil on ceramic carrier technology is often chosen for devices requiring higher precision and efficiency.

Power Tools

Portable electric power tools require a wide range of current levels depending on the particular tool.  Drills for example will typically utilize a single 1 to 3W size chip resistor in the 1 to 3 milliohm range.  All metal devices (100% lead free) with high current handling, high power, with low resistance values and good TCR are ideal for power tool applications. This includes ultra-precision, high-current sense chip resistors that combine tight TCR and precision tolerances down to +/- 0.5%. Many offer AEC-Q200 qualification and provide low inductance values.

Raised element SMD current shunt resistors also offer low resistance values, high power ratings, and low TCR. With resistance values as low as 0.2 milliohm and power ratings upto 10W mean the raised element SMD current shunts can accurately sense currents up to 224 amps. With TCR values of 50 ppm, these shunt resistors are suitable for a wide range of values, which ensures accurate sensing in a wide range of environments and under high current loading often required in power tool applications.

Similar topologies are used for small electronic vehicles, and e-scooters as well as industrial backup power management.

Solar Power Inverter And Power Management

Photovoltaic systems of any significant size which utilize battery systems require very high efficiency.  Therefore, the choice is again high power all metal resistors of at least 3 watts and resistance values of 0.5 milliohm or lower. Because size is normally less of an issue, these designs can use large size parts with extremely low values and have space for the high-speed comparator and level shifter necessary for most power IC’s to handle the voltage output of such a low resistance value.

Surface mount metal element high current shunt with four terminals Kelvin sense resistors provide the most accurate sensing as it removes the effect of lead resistance in the sensing function. Stackpole’s HCSK Series has the added benefit of lower PCB temperatures and the ability to operate up to higher temperatures. Rated at 5W at an ambient temperature of 100C, the HCSK2725 utilizes unique materials and a raised element that provides tolerances as low as 1% and TCR of 50 ppm /ºC. The robust current shunt has excellent stability over life, high temperature exposure, moisture resistance, rapid change of temperature, and short time overload testing.

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Figure 2. The HCSK High Current Four Terminal Shunts For High Sensing Efficiency

The exploding markets for portable electronics, power banks, electrical vehicles, and alternative energy storage systems are creating demand for more current sensing resistors. Current sense resistors are available with a wide range of features and performance attributes to meet the specific application requirements.