Texas Instruments | Using Thermistors to Enhance Thermal Protection for Battery Management Systems | Application notes | Texas Instruments Using Thermistors to Enhance Thermal Protection for Battery Management Systems Application notes

Texas Instruments Using Thermistors to Enhance Thermal Protection for Battery Management Systems Application notes
Using Thermistors to Enhance Thermal Protection for
Battery Management Systems
Mina Shawky, Temperature and Humidity Sensing
Introduction
A Battery Management System (BMS) is widely used
in automotive, industrial, and personal electronics
sectors for battery cell management. Typically, a BMS
is used to monitor battery cells by relaying information
to the microcontroller (MCU) or microprocessor (MPU)
to optimize system performance and increase
longevity of the cells. In some instances, the BMS can
take actions locally and doesn't need communication
with the MCU/MPU to execute tasks.
A BMS is widely used to protect the batteries from
functioning outside their temperature, voltage, and
current operating range. Furthermore, it monitors the
state of charge (SOC), state of health (SOH), and
state of power (SOP). Depending on these conditions,
a BMS can take action to protect the system by
shutting down, implementing cell balancing, or feeding
into the cooling control system.
Battery chemistry is temperature-dependent, and
operation outside its thermal range could lead to a
reduction in battery life and performance over its life.
Different battery technologies have unique charging
and discharging characteristics that are affected by
temperature, shown in Table 1. The discharge
temperature range is typically wider than the charge
temperature range. Charging the cells too quickly may
lead to a reduced life and venting. Furthermore, long
exposure to high temperatures could lead to cell
degradation and can result in thermal runway and
explosion. On the other hand, low temperatures
increase the resistance of the battery causing the
charge capacity to drop significantly.
Thermistors are passive components that respond to a
change in temperature by changing their resistance.
NTCs are made from ceramic while PTCs can be
made from silicon, metal, polymer, or ceramic.
Thermistors have been widely used in BMS due to
their versatility, low cost, and straightforward
implementation. A voltage divider is commonly used to
bias the thermistor. The voltage read across the
thermistor is then converted to a temperature reading
by the MCU/MPU to actively monitor the battery cells.
It can also be used as a switch to turn on a cooling
system or shut down the system.
Thermistor Comparison for BMS
When designing a thermistor-monitoring circuit for a
BMS, the designer must account for thermistor
linearity and tolerance, bias resistor temperature drift,
and Analog to Digital Converter (ADC) resolution
errors. These errors can greatly influence the
thermistor output and compromise the safety of the
system.
NTC thermistors are inherently nonlinear, as shown in
Figure 1, which makes it difficult to achieve high
accuracy over the whole temperature range.
Table 1. Common Charge and Discharge
Temperature Limits for Various Batteries
BATTERY TYPE
CHARGE
TEMPERATURE
DISCHARGE
TEMPERATURE
Lead Acid
–20°C to 50°C
–20°C to 50°C
NiCd, NiMH
0°C to 45°C
–20°C to 65°C
Li-ion
0°C to 45°C
–20°C to 60°C
Figure 1. Resistance vs Temperature of PTC vs
NTC
Thermistor Overview
Thermal monitoring allows the BMS to make informed
decisions and take the proper action to protect the
battery cells. In this tech note, a silicon-based positive
temperature coefficient (PTC) thermistor is compared
to a negative temperature coefficient (NTC) thermistor.
SNIA032 – August 2019
Submit Documentation Feedback
At high temperatures, the ratio of volts per degree
diminishes, and the resolution is coarse due to noise
and errors. Such effects of non-linearity are often
managed through the use of hardware and softwarebased linearization methods.
Using Thermistors to Enhance Thermal Protection for Battery Management
Systems Mina Shawky, Temperature and Humidity Sensing
Copyright © 2019, Texas Instruments Incorporated
1
www.ti.com
Software calibration will increase power consumption
and the required processing power of MCU/MPU.
Hardware calibration can affect the sensitivity of the
voltage reading and add to the system cost. Unlike
NTCs, PTCs have a positive linear shift in resistance
with an increase in temperature.
If an NTC is disconnected from the system due to poor
layout or other mechanical stresses, the
microcontroller (MCU) or microprocessor (MPU) will
read a low temperature due to the open circuit (high
resistance). On the other hand, if a PTC is
disconnected from the system, the MCU/MPU will read
a high temperature and take the necessary actions for
protection, increasing the overall reliability and safety
of the module. Furthermore, software calibration for
NTCs requires an extensive lookup table (LUT) or a
high order Polyfit to accurately convert the system
ADC output values to a temperature reading.
Figure 2. LUT Comparison Between PTC vs NTC
In contrast, silicon-based PTCs have a much smaller
lookup table and save memory resources used for the
MCU/MPU. Figure 2 shows the trade-off between
linearization error, due to the resolution of the ADC,
and LUT memory. By using a PTC, calibration errors
and the overall system cost are minimized.
Figure 3 shows a system level implementation of using
the TMP61 to monitor the temperature of battery cells
in a BMS.
Thermistor Circuit Design Considerations
In some instances, both thermal monitoring and
protection are required. The specific implementation of
the thermal monitoring circuit depends on application
requirements, such as accuracy, size (footprint),
power, and cost.
A typical thermistor-conditioning circuit is shown in
Figure 4. Thermal management can be achieved by
actively monitoring the battery cells using an ADC, or
by using the output of the thermistor to compare it to a
reference voltage for overtemperature (OT) or
undertemperature (UT) protection.
Figure 4. Example Discrete Implementation of a
Temperature Monitor and Switch
Device Recommendations
The TMP61 is a silicon-based PTC thermistor
designed for temperature measurement, protection,
compensation, and control systems. The TMP61 has a
tolerance of ±1% between –0°C to 70°C, and a wide
operating range of –65°C to 150°C. Compared to
traditional NTCs, the TMP61 offers enhanced linearity
and consistent sensitivity across the full temperature
range. To learn more about the TMP61, batteries, and
temperature protection, refer to the reference material
in Table 2.
Table 2. Related Documentation
Figure 3. BMS Overview
2
COLLATERAL
DESCRIPTION
Data Sheet
TMP61 Silicon-Based Linear Thermistors
Application
Report
Methods to Reduce Thermistor Linearization Error,
Memory, and Power Requirements Over Wide operating
Temperature Ranges
Tech Note
How to Protect Battery Power Management Systems
From Thermal Damage
Tech Note
Methods to Calibrate Temperature Monitoring Systems
Web Link
BU-410: Charging at High and Low Temperatures
Using Thermistors to Enhance Thermal Protection for Battery Management
Systems Mina Shawky, Temperature and Humidity Sensing
Copyright © 2019, Texas Instruments Incorporated
SNIA032 – August 2019
Submit Documentation Feedback
IMPORTANT NOTICE AND DISCLAIMER
TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE
DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS”
AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY
IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD
PARTY INTELLECTUAL PROPERTY RIGHTS.
These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate
TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable
standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you
permission to use these resources only for development of an application that uses the TI products described in the resource. Other
reproduction and display of these resources is prohibited. No license is granted to any other TI intellectual property right or to any third
party intellectual property right. TI disclaims responsibility for, and you will fully indemnify TI and its representatives against, any claims,
damages, costs, losses, and liabilities arising out of your use of these resources.
TI’s products are provided subject to TI’s Terms of Sale (www.ti.com/legal/termsofsale.html) or other applicable terms available either on
ti.com or provided in conjunction with such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable
warranties or warranty disclaimers for TI products.
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2019, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Related manuals

Download PDF

advertising