Automotive Battery Monitoring

by Wireless Cell Sensors

Matthias Schneider, Sergej Ilgin, Niels Jegenhorst, Raik Kube, Simon Puttjer, K.-R. Riemschneider, Jurgen Vollmer

Hochschule fur Angewandte Wissenschaften Hamburg, Germany

karl-ragmar.riemschneider@haw-hamburg.de matthias.schneider@haw-hamburg.de

Abstract—Systems monitoring lead acid vehicle batteries(starter batteries and forklift batteries) currently only observethe whole battery at the outer clamps. Common Lithium batterymanagement systems measure each cell using complex wiredsolutions. This causes the serious galvanic isolation problems. Theresearch project BATSEN proposes the use of wireless batteriescell sensors. This allows the separated integration inside eachcell, avoids complex wire and connector systems, but above allthere is no galvanic isolation problem.Therefore wireless sensors shall monitor each battery cell bymeasuring voltage and temperature. A central battery controlunit will be used to combine the cell measurements with a currentmeasurement and estimates the State of Charge (SOC) and theState of Health (SOH) of the battery. As a first step wirelesssensors for lead acid batteries will be developed. In the futurethe sensors will also be applied in lithium batteries for electricor hybrid vehicles.

Index Terms—Automotive Battery Monitoring, Wireless Sen-sors, Electric Vehicles, Battery Sensors

I. INTRODUCTION

Vehicles with a combustion engine as well as electric driven

vehicles, electric or hybrid cars) use batteries with serial

connected cells. For a reliable function a monitoring device

observe the state of the batteries. For lead acid batteries this

is done by measuring voltage and current on the outer battery

clamps [19]. By this measurement only an averaged state

of the cells can be predicted. By manufacturing tolerances

and slightly different environment influences cells can develop

small charge differences. These differences grow by cyclic use

of the battery over time. The weaker cells will be more stressed

and degraded in capacity and health. So the battery durability

is limited by the weakest cell. Hence it is useful to monitor

each battery cell individual.

II. DISTRIBUTED FUNCTION

Wired cell measuring comes with some drawbacks like

the effort for a tough wire and connection solution and a

potential isolation [1], [12], [21]. A wireless access to the

cells has none of the problems and offers an optional sensor

integration directly into the cells of lead acid batteries [17].

This integration ensures a good temperature coupling to the

sensor and leaves the normal encapsulation of these cells

intact, as depicted in figure 1.

The sensors measure the temperature and voltage on each

cell. The battery electronic control unit (ECU), see picture

3 receives the measurement data, as shown on figure 2. The

Wireless

Cell Sensor

incl. Antennas

neg.

Grid

pos.

Grid

-+

Battery U

incl. Current Sensor

EC

Wireless Cell

Sensors

+

ECU-Antenna

on CarrierAutomotive

Boardnet

(Supply System)

Cell

Monitored

Battery

Automotive

Control-Net

Wireless

Communication

Network

into the

Battery CellsPole

Contacts

CAN/LIN

Fig. 1. Overview: cell integrated wireless monitoring

VT

VT

VT

VT

VT

VBatterycontrol

unit

Wireless Communication

Cell 1

Sensor 1

Cell 2

Sensor 2

Cell 3

Sensor 3

Cell 4

Sensor 4

Cell n

Sensor n

Load

Uplink

Downlink

Fig. 2. Distributed function of monitoring concept

wireless sensors installed on a forklift battery can be seen in

picture 4.

The cell values of voltage and temperature are combined

with the joint measurement of the current at the battery clamps.

The state of the battery is estimated with this information. The

communication between the sensors and the control unit adapts

978-1-4577-1772-7/12/$26.00 ©2012 IEEE

Fig. 3. Experimental electronic control unit

Fig. 4. Forklift battery with wireless cell sensors

to the different operating modes. Sensor data preprocessing is

applied to fulfil the operation mode requirements, see figure

5. The distributed operation of measuring cell sensors and the

control unit is one issue of the research project. Preliminary

work on forklifts [3], [9], [10], [14] has been made.

III. COMPOSITION OF THE CELL SENSORS

Depending on the battery type autonomous, semi-

autonomous (synchronization) and central controlled sensors

will be developed. These sensor classes are listed in spread-

sheet 6.

Actual test sensors are equipped with an MSP430 micro-

controller to control the measurement and control the RF-

IC. These family of controllers consume very low power and

contain an internal RC-Oscillator. The measurements are per-

formed with the internal ADC and an integrated temperature

High Current

Charge

No

Operation

Low Current

Discharge

High Current

Discharge

Low Current

Discharge

Measurement Rate Transmission Rate>

Measurement Rate Transmission Rate<

Measurement and Transmission Rate low

Fig. 5. Requirements on the operating modes of the sensors

Sensor class Class 1 Class 2 Class 3

Communication

between sensor

and battery

control unit

Uplink only Uplink and

downlink with

broadcast-wake-

up

Uplink and

downlink with

multicast and

addressed

commands

Receiver sensor No receiver Passive receiver Active receiver

Switch between

operating modes

autonomous

decision

semi-

autonomous

decision

central controlled

Measure and

network

organisation

No

synchronisation

Simple central

synchronisation

Complex.

bidirectional

synchronisation

Cell balancing Autonomous

realization

difficult

Realization

possible

Central controlled

Table 6. Overview of the designated sensor classes

Fig. 7. Starter battery test prototype with class 1 sensors

diode. For the supply a DC-DC converter is used with an

input voltage range from 0.5 V to 5 V to enable operation from

different battery cell types and even from nearly discharged

cells. The wireless connection is provided by an RF-IC for

433 MHz ISM band. An OOK modulation with a proprietary

protocol is used for the uplink communication. This RF-IC

[20] has no need for an external Chrystal-Oscillator. It is

intended to integrate all the sensor hardware in a single IC

in the future of the project.

Fig. 8. Class 1 Cell Sensor [6].

A. Cell sensors without downlink (class 1)

A possible field of application of this sensor class is

the starter battery, a test arrangement is shown in figure 7.

In production starter batteries have very low manufacturing

costs. Therefore class 1, depicted in figure 8 sensors have no

downlink receiver to limit the costs. Because of the missing

receiver synchronization it is not possible. Furthermore the

sensors can not sense if the channel is currently in use. So,

collisions can occur when two or more sensors try to send

their frame data. To reduce the probability for collisions a low

data rate combined with pseudo random channel access are

implemented in the sensors communication protocol. The data

frame rate was chosen with respect to the expected collision

probability. This relation was investigated in a master thesis

[11]. If the cell voltage changes quickly, additional voltage

measurements are needed, e.g. during engine startup. To ensure

a sufficient sampling of the voltage, the measurement rate is

increased by the sensor. Therefore the measurement rate can

exceed the transmission rate, see figure 5. Then the data are

stored with a local time stamp in a waiting queue in the sensor

and transmitted as soon as possible. If the voltage dynamic

decreases the measure rate decreases as well and the sensor

transmit the data stored before. In the battery control unit the

correct measurement history is then calculated as shown in

figure 9. Because of the diverging RC-Oscillators frequencies

(error 1-5 %) the sensor time stamps must be corrected by the

control unit [15].

B. Cell sensors with passive downlink receiver (class 2)

In a just finished master thesis [8] a test system for this

sensor class was developed, see figure 10. The battery control

unit is extended by an RFID-Reader IC for 13,56 MHz with

the respective PCB circuit. This reader is the transmitter for

the downlink to the sensors. The corresponding transponder

front end circuit on the sensors consists of discrete components

[4]. Main advantage is the passive operation with very low

power consumption and the possibility to enable the DC-DC

conversion by the downlink receiver, see circuit overview on

figure 11. So the entire sensor, not only the microcontroller,

can remain in a low power sleep mode until the battery control

0 5 10 15 20 25 30 35 40 45 501.5

2

2.5

Ce

ll vo

lta

ge

in

V

voltage history of a cellmeasure points (Inqueue)

0 5 10 15 20 25 30 35 40 45 500

5

10

15

20

me

asu

rem

en

ts in

qu

eu

e

Queue-filling in sensor

0 5 10 15 20 25 30 35 40 45 501.5

2

2.5

Time in s

ce

ll vo

lta

ge

in

V

cell voltage history estimated by control devicereceived measurements

Fig. 9. Cell voltage on high current event, usual charge operation andquiesence operation on start up and stop of a diesel engine in a MercedesVito 2,4D. Top: Sensor measurement. Center: Queue-filling in sensor. Bottom:reconstructed measurement history.

Fig. 10. Battery control unit and test sensor class 2 with:(1) UHF-Uplink-antenna and transmitter on the bottom layer of the sensorpcb(2) HF-Downlink-antenna on the top layer of the sensor (test construction)(3) Battery control unit with lcd display(4) Transmitter for HF downlink of the control unit(5) Transmitter antenna for HF downlink(6) UHF-Receiver with receiver antenna on control unit

unit sends a wake-up. Furthermore the downlink is used to

organize the channel access of each sensor. So, collisions

are avoided and the package rate is significantly increased

compared to class 1 sensors.

C. Cell sensors with active downlink receiver (class 3)

The class 3 sensors will contain an UHF downlink. A

centralized measurement and transmission control strategy will

be implemented. The battery electronic control unit sends ad-

dressed commands and multicast/broadcast commands to the

sensors. Class 3 sensors will have a symmetric bidirectional

communication hardware. These sensors work under similar

conditions as IEEE 802.15.4. This implies protocol restrictions

HFDownlink

UHFUplink

Voltagelimiter

Voltagedoubler

Trans-mitter

Detect &hold

circuit

Step-Up-Down

Converter

EN

MCU

Load-modulator(optional)

cell 1,2,…,n

cell 1,2,…,n

Pass

ivAk

tiv

ResonatorUplink

(optional)

Lowpass &demod-ulator

Cell sensor 1 Cell sensor 2 Cell sensor n

Fig. 11. Circuit function overview of class 2 sensors

Fig. 12. Cell sensor with balancing effector

to the measurement control timing and is subject of investi-

gations. Because of the expected higher costs it has to be

investigated if these sensors fulfil the cost requirements.

D. Cell Balancing

Besides the monitoring of the cell states, the sensors can

also be used to balance the charge of the cells. Damage of

the weaker cells caused by deep discharge and overload can

be reduced by balancing. Our sensors will be equipped with

small switchable shunt loads, see figure 12.

Part of the long term battery maintance strategy, is to switch

these shunt loads to discharge the stronger battery cells to

the level of the weakest cell. The following cycle start with

better matched cell conditions. Similar features are included

in modern lithium battery monitoring ICs already [12]. These

ICs use wired connections to the cells and have the mentioned

drawbacks compared with our wireless solution.

B-Level Functions

A-Level Functions

C -Level Functions C-Level Functions...

Co

mm

an

ds

Me

ssa

ge

s

Co

mm

an

ds

Me

ssa

ge

s

Co

mm

an

ds

Me

ssa

ge

s

Me

ssa

ge

s

Co

mm

an

ds

Me

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ge

s

Fig. 13. Layer structure Battery Monitoring and Control Language BMCL

IV. SYSTEM INTEGRATION

A. Flexible support for battery models and battery monitoring

Support of different battery types for the listed vehicle

classes must be implementable in the same sensor system.

Therefore different battery models and monitoring algorithms

must be supported by hardware as well as by the software.

To provide a structured description of the functions, a Battery

Monitoring and Control Language (BMCL) is under devel-

opment. This language will contain of functions, messages

and commands divided into three abstraction layers. A-Level:

application level, B-Level: battery monitoring level and C-

Level: cell monitoring level. The command language abstrac-

tion concept combined with a broad range of allowed cell

voltages is needed to enable the use of our sensors for lead

acid and lithium batteries.

B. Sensor calibration

Because of the flat discharge characteristics of modern

lithium technologies (e.g. lithium titanate cells) a precise volt-

age measurement in the range of a few millivolt is needed. Ad-

ditionally automotive temperature requirements (AEC Q100

Grade 3) from -40 ◦ to 85 ◦ have to be fulfilled. Even if

the batteries themselves do not cover this range, the sensor

must measure temperature with a precision of one degree.

The microcontroller chip on the sensor contains an integrated

diode on chip for temperature estimation. This temperature

diode provides uncalibrated a precision of several degree only.

Important is to calibrate the cells sensors using a regression

methode. Compensation is done by preprocessing in the sensor

controller, see figure 14 [6].

C. Relation to actual developments in the field of battery

management

Several companies have developed solutions for battery

monitoring [1], [12], [21]. An electronic battery clamp for the

monitoring of starter batteries have been introduced in several

Fig. 14. Improved crystal-less class 1 sensors in parallel connection forcalibration

Fig. 15. Experimental operation of sensors on a starter battery, referencemeasurement performed with wired cell access

automobils. A combination of our approach with such existing

solutions is imaginable in the future.

V. CONCLUSION

The research project develops concepts for wireless cell

sensors and investigates cost effective solutions for different

fields of applications. Main issues are the microelectronic

implementation and the testing with different battery appli-

cations.

Acknowledgements

The research project is granted by the BMBF (FKZ

17001X10) and is supported by industrial partners: Volk-

swagen AG, Bertrandt AG, Still GmbH, OMT GmbH, Fey

Electronic GmbH and Coilcraft Ltd.

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