STMicroelectronics UM3424 Battery Management System Evaluation Board

Getting started with the battery management system module based on  L99BM114 and L99BM1T

Introduction

• The STEVAL-L99BM114TX is a battery management system (BMS) evaluation board that manages from 4 to 14 battery cells.
• The main advantage of this evaluation board is ensuring isolated connection to an external MCU, thanks to the embedded transceiver.
• The board is based on the L99BM114 Li-ion battery monitoring and protection chip for high-reliability applications and the L99BM1T general purpose SPI to isolated SPI bidirectional transceiver.
• The main activity of the L99BM114 is monitoring the cells and battery node status through stack voltage measurement, cell voltage measurement, temperature measurement, and coulomb counting.
• Measurement and diagnostic tasks can be executed either on demand or at set intervals.
• Measurement data are available for an external microcontroller to perform charge balancing and to compute the state of charge (SOC) and the state of health (SOH).
• The L99BM1T general purpose SPI to isolated SPI bidirectional transceiver can transfer communication data incoming from a traditional 4-wire based SPI interface to a 2-wire isolated interface (and vice versa). In our board, the transceiver is configured as a slave.
• Figure 1. STEVAL-BMS114TX evaluation board

Notice:
For dedicated assistance, submit a request through our online support portal at www.st.com/support.

BMS evaluation board overview

Features

• Hosts the L99BM114 multicell battery monitoring and balancing IC
• Hosts the L99BM1T general purpose SPI to isolated SPI bidirectional transceiver
• Voltage monitoring of every single cell and of the entire battery node
• Current sensing of the entire battery node
• 5 GPIOs to connect temperature sensors as NTCs
• CN1 connector that allows establishing communication with an MCU board via SPI
• CN2 connector that interfaces directly to an MCU board for control and diagnostic functions
• Passive balancing
• Compact size: 100 x 76 mm

Main components

1. ISOH port to connect the board to an STEVAL-BMS114 in a daisy chain
2. Connector for MCU ADCs dedicated to the NTC sensors reading
3. L99BM1T general-purpose SPI to isolated SPI transceiver
4. CN2 connector for diagnostic functions
5. CN1 connector to communicate with an MCU board via SPI
6. Balancing resistors
7. Connector for the battery pack
8. Hot plug protection
9. GPIOs for external NTC connection handled by L99BM114
10. L99BM114 multicell battery monitoring and balancing IC

Table 1. STEVAL-L99BM114TX connector details

Embedded devices

L99BM114

• The L99BM114 is intended for operation in systems using lithium battery packs. The IC embeds all the features needed to perform battery management. A single device can monitor from 4 up to 14 cells.
• The device can be supplied with the same battery it monitors, and its main activity consists of monitoring cells and battery pack status through stack voltage measurement, cell voltage measurement, temperature measurement, and coulomb counting. Measurement and diagnostic tasks can be executed either on demand or periodically, with a programmable cycle interval.
• Measurement data is available for an external microcontroller to perform charge balancing and to compute the state of health (SOH) and state of charge (SOC).
• The IC works in normal mode performing measurement conversions, diagnostics, and communication. The device can also be put into a cyclic wakeup state in order to reduce the current consumption from the battery.
• Passive cell balancing can be performed either via internal discharge path or via external MOSFETs. The controller can either manually control the balancing drivers or start a balancing task with a fixed duration. In the second
• case, the balancing may be programmed to continue also when the IC enters a low power mode called silent balancing, to avoid unnecessary current absorption from the battery pack.
• Thanks to the GPIOs, the device also offers the possibility to operate a distributed cell temperature sensing via external NTCs resistances.
• The external microcontroller can communicate with L99BM114 via SPI protocol. The physical layer can either be a traditional 4-wire based SPI or 2-wire transformer/capacitive based isolated interface through a dedicated isolated transceiver device.
• The L99BM114 performs automatic validation of any failure involving the cells or the whole battery pack. The device can detect the loss of the connection to a cell or GPIO terminal. Moreover, it features a hardware self-check (HWSC) that verifies the correct functionality of the internal analog comparators and the ADCs. All these checks are automatically performed in case a failure involving both cells or when the battery pack is detected. The current sensing interface used for coulomb counting is also capable of detecting failures such as open wires and overcurrent in sleep mode. The cell balancing terminals can detect any short/open fault and the internal power MOS are protected against overcurrent.

L99BM1T

• L99BM1T is a general purpose SPI to isolated SPI transceiver intended to create a communication bridge between devices located into different voltage domains.
• L99BM1T is able to transfer communication data incoming from a classical 4-wire based SPI interface to a 2-wire isolated interface (and viceversa).
• The transceiver supports both transformer and capacitive isolation, since the isolated signal generated according to a proprietary protocol is suitable to be transmitted over both decoupling circuitries.
• The device can be configured either as Slave or as Master of the SPI bus and supports any protocol made of SPI frames 8 to 64 bit long. The transceiver manages the transfer of the information without performing any protocol check. SPI peripheral can work up to 10 MHz when configured as Slave. SPI clock frequency can be programmed among (250 kHz; 1 MHz; 4 MHz; 8 MHz) when configured as Master.
• Isolated SPI peripheral features two different operating modes: slow @333 kbps and fast @2.66 Mbps.
• The asynchronicity between the two sides is internally managed, allowing all possible configuration frequencies on both peripherals to be used in application.
• L99BM1T features an internal queue of 3 slots for the frames received on the SPI port and a queue of 20 slots for the ones received on the isolated SPI side. This allows buffering and decoupling the two different clock domains.
  The device is natively compatible with L99BM114 isolated SPI, allowing its usage in the BMS applications.
• L99BM1T is compatible with both 3.3 V and 5 V internal logics.

Voltage operating range

• In our BMS evaluation boards, the maximum voltage range for each cell is 4.2 V.
• The power supply range is from 9.6 V to a maximum of 64 V.
• Linear regulators
• The STEVAL-BMS114 features several linear voltage regulators, which are switched on according to a specific sequence at power-up (see Figure 1).

VREG

• This linear regulator exploits an external MOS to decrease the power dissipation inside the L99BM114.
• It acts as a pre-regulator, supplying all other internal regulators (VANA, VCOM, VTREF, and VDIG). It is switched off in low power modes (sleep, silent balancing, off phase of the cyclic wakeup).

VANA

• This low drop regulator supplies all the INTERNAL ADC, comparators, monitors, main bandgap, current generator, and other analogic blocks.

VCOM

• The isolated communication receiver/transmitter and the GPIO output buffers are supplied by this low drop regulator.

VTREF

• This low drop regulator is used to supply external components such as NTCs for temperature sensing.
• The recommended application circuit in NTC analog front end guarantees that each NTC channel sinks no more than 500 µA.
• VTREF regulator is disabled by default. Its operation can be controlled via SPI.
• In absolute measurements, there is no reference value, while the ratiometric measurement is based on reference value defined by the VTREF regulator. If the VTREF goes low in case of an error, the VTREF varies to compensate this error.
• All of the above regulators have dedicated UV/OV diagnostics.

BMS topologies

• The BMS boards can work in two different daisy chain topologies: single access and dual access ring.

Single access configuration

• In a single access daisy chain configuration, a series of BMS is connected to an MCU board through a single transceiver connected to the STEVAL-BMS114 isolated ISOL port. The BMS are connected to each other through the isolated ISOH port.
• The MCU communicates with the STEVAL-BMS1T hosted L99BM1T transceiver through the SPI protocol. The transceiver converts these signals into ISO SPI signals to communicate with the BMS.
• Figure 3. Single access BMS diagram
  

Dual access ring configuration

A dual access ring configuration is implemented by adding another transceiver that makes the communication bidirectional. The secondary transceiver is used as a backup in case the primary transceiver fails. Data moves in opposite directions around the rings, and each ring remains independent of the other unless the primary ring fails. The two rings are connected to continue the flow of data traffic.
Figure 4. Dual access ring BMS diagram

Cell current measurement

• The current flowing into the external shunt resistance RSENSE is measured through a differential amplifier stage (connected between ISENSEP/ISENSEM pins) feeding a 18-bit ADC.
• The current conversion chain can be enabled through the CoulombCounter_en bit and runs in background to perform the Coulomb Counting Routine.
• Moreover, L99BM114 also allows you to synchronize the Voltage Conversion Routine and the Coulomb Counting Routine for a precise State Of Charge estimation. Every time an on-demand voltage conversion is requested by setting SOC = 1, the actual conversion start is delayed until the first useful current conversion takes place. This might result in a maximum delay of TCYCLEADC_CUR, which must be taken into account by user software only in the case that current ADC is enabled.

Fault condition in daisy chains

The fault LED on the STEVAL-BMS1T is related to the state of all the BMS nodes in the daisy chain. If an undervoltage, overvoltage, overcurrent, or overtemperature occurs on any cell of a BMS, a fault condition is detected. To solve this condition, diagnosis via software code must be activated.

The overcurrent detection is linked to a threshold defined in the application, not in the software driver. The threshold must be modified according to the load.
For further details, refer to the L99BM114 datasheet.

Cell balancing

In the L99BM114, the Sx and Bx_x-1 pins are used to balance the charge of the cells by discharging the ones with a higher SOC. Balancing can be performed either with external resistors or internal MOSFETs.
Cell balance drivers are powered by VBAT stack voltage. Hence, balancing is theoretically possible even at low cell voltages, except for cell 14. In case VCELL14 < VCELL14_BAL_MIN, the corresponding balancing circuitry does not operate properly, and false overcurrent detection may occur.

Passive cell balancing with internal MOSFETs
The board is designed using internal MOSFETs.

The on-chip MOSFETs are switched on to sink a current from the cell, thus dissipating charge on RDIS. The affordable balancing current is restricted by the thermal relief on the current source circuits.
The maximum balance current on each cell is 200 mA. All cells can be balanced simultaneously, if the junction temperature does not exceed the maximum operating defined in the datasheet. To prevent thermal overstress, the die temperature diagnostic and overtemperature protections are implemented.

STEVAL-BMS114TX schematic diagrams

STEVAL-BMS114TX bill of materials

Table 2. STEVAL-BMS114TX bill of materials

Board versions

Table 3. STEVAL-BMS114TX versions

This code identifies the STEVAL-BMS114TX evaluation board first version

Regulatory compliance information

• Notice for US Federal Communication Commission (FCC)
• For evaluation only; not FCC approved for resale

FCC NOTICE – This kit is designed to allow:

1. Product developers to evaluate electronic components, circuitry, or software associated with the kit to determine whether to incorporate such items in a finished product and
2. Software developers to write software applications for use with the end product.

This kit is not a finished product and when assembled may not be resold or otherwise marketed unless all required FCC equipment authorizations are first obtained. Operation is subject to the condition that this product not cause harmful interference to licensed radio stations and that this product accept harmful interference. Unless the assembled kit is designed to operate under part 15, part 18 or part 95 of this chapter, the operator of the kit must operate under the authority of an FCC license holder or must secure an experimental authorization under part 5 of this chapter 3.1.2.

• Notice for Innovation, Science and Economic Development Canada (ISED)
• For evaluation purposes only. This kit generates, uses, and can radiate radio frequency energy and has not been tested for compliance with the limits of computing devices pursuant to Industry Canada (IC) rules..
• Notice for the European Union
• This device is in conformity with the essential requirements of the Directive 2014/30/EU (EMC) and of the Directive 2011/65/EU (RoHS II), including subsequent revisions and additions, as well as amended by the Delegated
• Directive 2015/863/EU (RoHS III). Compliance to EMC standards in Class A (industrial intended use).
• Notice for the United Kingdom
• This device is in compliance with the UK Electromagnetic Compatibility Regulations 2016 (UK S.I. 2016 No. 1091) and with the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment
• Regulations 2012 (UK S.I. 2012 No. 3032). Compliance to EMC standards in Class A (industrial intended use)

Revision history

Table 4. Document revision history

IMPORTANT NOTICE – READ CAREFULLY

• STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, enhancements, modifications, and improvements to ST
• products and/or to this document at any time without notice. Purchasers should obtain the latest relevant information on ST products before placing orders. STproducts are sold pursuant to ST’s terms and conditions of sale in place at the time of order acknowledgment.
• Purchasers are solely responsible for the choice, selection, and use of ST products and ST assumes no liability for application assistance or the design of purchasers’ products.
• No license, express or implied, to any intellectual property right is granted by ST herein.
• Resale of ST products with provisions different from the information set forth herein shall void any warranty granted by ST for such product.
• ST and the ST logo are trademarks of ST. For additional information about ST trademarks, refer to www.st.com/trademarks. All other product or service names
  are the property of their respective owners.
• Information in this document supersedes and replaces information previously supplied in any prior versions of this document.
• © 2025 STMicroelectronics – All rights reserved

Documents / Resources

STMicroelectronics UM3424 Battery Management System Evaluation Board [pdf] User Manual L99BM114, L99BM1T, UM3424 Battery Management System Evaluation Board, UM3424, Battery Management System Evaluation Board, Management System Evaluation Board, System Evaluation Board, Evaluation Board, Board

References

• User Manual