Battery and Ultra-Capacitor Hybrid Energy StorageSystem and Power Management Scheme for

Solar-Powered Wireless Sensor NodesJordan Varley, Matthew Martino, Shahab Poshtkouhi, Olivier Trescases

University of Toronto, Department of Electrical and Computer EngineeringE-mail: shahab.poshtkouhi@utoronto.ca

Abstract— This paper presents a Wireless Sensor Node (WSN)architecture with solar power generation and a hybrid energystorage scheme. The WSN is composed of three key modules:Energy Harvesting, Energy Storage, and the Control/Processingunit. The harvesting module consists of a miniature 179 mWsolar array and MPPT hardware. A rechargeable 350 mAhLithium-Ion battery and an ultra-capacitor are used as the energystorage elements. The low-ESR ultra-capacitor efficiently suppliesthe load power, which can reach as high as 295 mW peak,while the battery provides high-density storage. These elementsare interfaced through a digitally controlled bi-directional dc-dc converter, which efficiently regulates the power-flow in theWSN. Multiple sensors and circuitry are implemented to measurepositional and environmental data, as well as receiving andtransmitting data via RF communication. A long-term test ofthe WSN is conducted to demonstrate the effective systemfunctionality.

I. INTRODUCTION

Wireless Sensor Nodes (WSN) are used in distributed meshnetworks to sense local environmental conditions, processdata and transmit information periodically. In some cases,WSNs may also require the ability to activate high-powerlocal actuators, such as valves, motors, lights or speakers.WSNs are usually built to extract energy from resources in thesurrounding environment, such as temperature gradients [1],wind [2], motion [3] and solar irradiation [4], [5]. Miniaturephotovoltaic (PV) panels in the 100-500 mW power range areused in many of these sensor nodes [6], [7]. General purposeWSNs are commercially available [8]. They have also beendeveloped for specific purposes such as radiation-level andpollution detection [9]. The harvested power depends on theenvironmental conditions, such as sun irradiation, cloud cover-age and shading, as well as ambient temperature. In this paper,a sub-200 mW generation scheme is demonstrated. Continuousautonomous operation of the WSN is key, especially at night,and in sub-optimal sunlight conditions.

So far, the majority of the research has been focused onthe energy harvesting methods, rather than efficient storageand load management [1], [3], [6], [7]. Efficient storageand distribution of the collected energy is crucial in orderto achieve a completely optimized design and ensure thelong-lasting operation of the autonomous WSN. It is thusdesired to exploit the favorable characteristics of differenttypes of energy storage elements [10]; ultra-capacitors, which

have a high power-density, and rechargeable batteries, whichfeature high energy-density. By integrating these two elements,the efficiency can be enhanced in the storage and deliverystages. In [11], a WSN architecture is presented utilizing abattery and ultra-capacitor, but only one of these units ischarged/discharged at a time.

The specific power, Puc,max, is calculated based on themaximum extractable power when the ultra-capacitor is fullycharged and is given by

Puc,max =V 2uc,max

4Ruc, (1)

where Vuc,max and Ruc represent the ultra-capacitor ratedvoltage and Equivalent Series Resistance (ESR) respectively.Due to their low ESR, ultra-capacitors are well suited to supplyhigh-power sensors and actuators. The 5 F ultra-capacitor usedin this work for example yields Puc,max = 240 W. Batteries,on the other hand, provide a high-density platform, especiallyat night when the solar power is not available.

G

T

CPLD Core

Sensors

PV Array

Boost Converter

with MPPT

Ultra-Capacitor

Battery

Bidirectional

Buck-Boost

Converter

Buck Converter

Buck Converter3.3V

1.8V

Micro-

Controller

RF

Transceiver

GPS

Receiver

CPLD Core

Micro-

Controller

Control/

Processing Unit

G

T PV Array

Boost Converter

with MPPT

Energy

Harvesting Unit

Vpv

Vuc Vbt

3 V – 4.2 V

3.6 V – 5 V0 V – 1.89 V

Storage Unit

Ultra-Capacitor

Battery

Bidirectional

Buck-Boost

Converter

VuVV c VbVV t

3 V – 4.2 V

3.6 V – 5 V

Storage Unit

IbtIuc

Fig. 1. High-level architecture of the proposed WSN.

The architecture of the proposed WSN is shown in Fig. 1.The boost dc-dc converter interfaces with the PV array toextract solar energy by performing Maximum Power PointTracking (MPPT). The generated energy can be depositeddirectly in the ultra-capacitor, or transferred to the batterythrough a bi-directional non-inverting buck-boost converter.This converter is used to regulate the power-flow and State-of-Charge (SOC) in the battery and ultra-capacitor. The storedenergy is used to run the control/processing unit, as well

as multiple sensors such as humidity, temperature, and lightintensity detectors. A GPS receiver is also included, such thatdifferent units can be tracked. This is useful in the networksconsisting of multiple WSNs scattered around a relatively largearea, or in mobile WSN applications.

This paper is organized as follows. Section II discusses thesystem, architecture, the proposed power management schemeand the WSN hardware implementation. In Section III, theefficient operation of the system is demonstrated by wirelesslymonitoring the power-flow and system voltages during a nine-day continuous test. Finally, Section IV concludes the paper.

II. WSN DESIGN AND IMPLEMENTATION

A. PV Array

The WSN is powered by ten miniature 1.89 V mono-crystalline PV panels [12]. Each panel consists of four parallelstrings with three cells in each string, and has a nominalmaximum power of 17.9 mW. The PV panel characteristicsare given in Table I. The panels are parallel-connected due tothe inherent robustness to shading and mismatch, compared tothe series connection [13]. The output of the solar array is fedto a boost converter stage, which performs MPPT by regulatingthe PV voltage, Vpv , to near its Maximum Power Point (MPP)voltage, Vmp. Fractional Open-Circuit Voltage (FOCV) ischosen as the MPPT method, where a constant fraction ofthe open-circuit voltage, Voc, is set as the reference for thePV voltage regulation loop [6]. This method is known forits simplicity and low-cost implementation, compared to otherconventional MPPT methods, such as perturb-and-observe, andincremental conductance methods [14].

TABLE IMONO-CRYSTALLINE PV PANEL CHARACTERISTICS [12]

Parameter Value Unit

Nominal Short Circuit Current, Iscn 15 mANominal Open Circuit Voltage, Vocn 1.89 VCurrent at Nominal Max. Power, Impn 12 mAVoltage at Nominal Max. Power, Vmpn 1.5 VNominal Max. Power, Pmaxn 17.9 mW# of Series Cells in Each String 3# of Parallel Strings 4

An additional open-circuited solar panel is included to actas the pilot cell for online Voc measurements. This voltage isperiodically sampled and passed to a Complex ProgrammableLogic Device (CPLD). The ratio K = Vmp

Vocis empirically

determined and pre-loaded in the CPLD.

B. Boost Converter

The solar energy is transferred from the PV array to the stor-age elements using a dc-dc boost converter. In order to achieveMPPT, Vpv must be regulated to Vmp. This is uncommon fortraditional Switched Mode Power Supplies (SMPS) controlschemes, as the input voltage needs to be regulated in this

case. A recently released integrated boost converter IC, whichis designed specifically for energy harvesting applications, wasselected for this work [15].

bq25504 Integrated Boost

Converter

PV Array

LBST

VBAT_OK

Cin

Ultra-

Capacitor

LBST

8 bitFrom

CPLD

Vpv,ref

VREF_SAMPVpv Vuc

To

CPLD

3.3 VDigital

Potentiometer

Fig. 2. The MPPT stage with the integrated boost converter IC.

The MPPT stage is shown in Fig. 2. The integrated boostconverter utilizes Pulse Frequency Modulation (PFM) forefficient regulation of the PV voltage. A digital potentiometercontrolled by the CPLD is used to feed the IC with the de-sired reference voltage. Ultra-capacitor voltage, Vuc, remainslimited to its rated maximum of 5 V by disabling the boostconverter in case of over-charge.

C. Control and Processing Unit

Environmental sensors are employed to measure moisture,relative humidity, ambient temperature and light intensity, aswell as a GPS and an RF transceiver (XBEE). The GPSreceiver is activated for one minute every hour during thedaytime to receive satellite signals, while the RF transceiverneeds to be activated for a few seconds each hour. The sensorycircuits and communication devices are controlled by a low-power micro-controller. The micro-controller operates in thelow-power sleep state for the majority of the time, and wakesup to make sensor readings four times per hour. It stores thedata internally until the next radio transmission occurs.

The sensors are shut down in sleep state, by turning offa switch in their power path, in the cases where dedicatedshut-down pins are not available.

The digital controller for the boost and buck-boost dc-dc converters are implemented in the CPLD. The internalclock frequency is set to 256 kHz, generated by a hybridcounter/delay-cell scheme in the CPLD, to reduce the powerconsumption and limit the cost [16]. Key voltages and currentsare sampled using four ADCs. With this information fed tothe power management module, the power-flow control of theWSN is implemented as depicted in Fig. 3. Two modes ofoperation are defined based on Voc, which is used to gauge ifsunlight is present:

1) If Voc > 1 V, the incident PV power is likely at leasta few mW. In this mode, the power-flow decisionsare made based on the ultra-capacitor and battery volt-ages, Vuc and Vbt respectively, which also representtheir SOCs. In this mode, Vuc is targeted to be in

3.6 4.1 4.3 4.5 4.7 5

Vuc (V)Voc > 1 V

Minimum ultra-

capacitor voltage

(1) Transfer charge from battery to ultra-capacitor

(2) Transfer charge from ultra-capacitor to battery

(1)

(1)

(2)

(2)Maximum ultra-

capacitor voltage Voc < 1 V

Desired ultra-capacitor operating range

Fig. 3. WSN power management different operating modes.

the 4.1-4.3 V range. This provides sufficient headroomfor Vuc to surge in case the input power rapidly in-creases. If Vuc falls outside of this range, the batteryis charged/discharged through the buck-boost converterto bring it back.

2) If Voc < 1 V, the incident PV power is unlikely toovercome the overhead power of the dc-dc convertersand power management circuitry. In this mode, Vuc istargeted to operate in the 4.5-4.7 V range. A reducedvoltage headroom is needed in this case, as the potentialinstantaneous incident power is expected to be less thanin mode I. As in the previous mode, the battery is usedto keep Vuc in this range, by transferring charge to/fromthe ultra-capacitor when necessary.

In all modes, Vbt and Vuc are monitored and thecharge/discharge processes are deactivated if over- or under-voltage conditions are detected. The charge/discharge currentof the battery is regulated by the interfacing dc-dc converter,as discussed in the next subsection.

D. Energy Storage and Delivery Module

The energy storage module consists of a 5 F ultra-capacitor, a 350 mAh rechargeable Lithium-Ion battery and acustom digitally-controlled bi-directional non-inverting buck-boost converter. The energy storage and delivery module isresponsible for:

1) Storing the extracted energy from the solar boost con-verter into either of the two on-board energy storageelements.

2) Transferring energy from the solar boost converter or theonboard energy storage elements to the load on regulated3.3 V and 1.8 V rails.

3) Sampling Vbt, battery current, Ibt, and Vuc to controlthe power flow and monitor energy usage.

A bi-directional dc-dc converter transfers energy betweenthe battery and ultra-capacitor. Due to the overlapping of theLithium-Ion battery and ultra-capacitor voltage ranges (3-4.2V and 3.6-5 V, respectively), a four-switch non-inverting buck-boost architecture was selected, as shown in Fig. 4(a). Themain specifications of the bi-directional buck-boost converterare listed in Table II. The converter regulates Ibt, based onthe operating conditions. The converter mode is referenced tothe battery, meaning that the converter operates in boost modewhen Vbt < Vuc. Three converter modes are possible with thisarchitecture: boost, buck, and buck-boost. The operating mode

of the converter is selected based on the instantaneous valuesof Vbt and Vuc, as shown in Table III.

C

C1Vbt

c1(t)

c2(t)

c3(t)

c4(t)

LC2

+

-

Vuc

+

-

S1

S2

S3

S4

Vx1 Vx2

IL

(a)

t1 t2 toff

Iave

Ipk

IL

t

m1 m2

s

sf

T1

=

(b)

Fig. 4. (a) Four-switch non-inverting buck-boost converter topology, and (b)Inductor current waveform for PFM with fixed peak current.

TABLE IIBIDIRECTIONAL BUCK-BOOST CONVERTER SPECIFICATIONS

Parameter Value Unit

Inductor, L 470 µHBuck-Boost Inductor’s 660 mΩ

Series Resistance, RL

S2 and S4 280 mΩ

On Resistance, Ron,ls

S1 and S3 450 mΩ

On Resistance, Ron,hs

Input and Output Capacitance, 10 µFC1 and C2

Ultra-Capacitor ESR, Ruc 26 mΩ

Battery ESR, Rbt 330 mΩ

Since the buck-boost converter operates below 200 mW,PFM operation is targeted [17]. The implemented PFM methodis based on a fixed peak inductor current, Ipk, and average

TABLE IIIBUCK-BOOST CONVERTER OPERATION MODES

Converter Mode Condition

Boost Vbt ≤ Vuc - 0.5 V

Buck Vbt ≥ Vuc + 0.5 V

Buck-Boost | Vbt - Vuc |< 0.5 V

inductor current, Iave, regulation, dictated by Vbt and Vuc

as shown in Fig. 4(b). Ipk is chosen close to the nominalsaturation current rating of the inductor, L, to allow for theoptimal usage of the magnetization curve’s linear region. Thedigital controller architecture is shown in Fig. 5. The sampledVbt and Vuc are fed into a Look-Up Table (LUT) which isprogrammed into the non-volatile CPLD. This LUT outputspre-calculated times t1 and t2, in order to ramp up the inductorto the Ipk = 100 mA peak and then discharge it back to 0 mA.The pre-calculated times, t1 and t2, respectively, allow for aprecise amount of charge to be transferred in a cycle, and withthe use of a third timing parameter, toff , Iave can be regulatedto

Iave =Ipk(t1 + t2)

2Ts, (2)

where Ts is the switching period

Ts = t1 + t2 + toff . (3)

The inductor rising (falling) slope in buck (boost) modeis directly proportional to (Vbt − Vuc). This results in anunreasonable amount of time to energize the inductor ifVbt ≈ Vuc. As a result, the switching frequency, fs = 1

Ts,

is decreased, and potentially pushed into the audible band.The buck-boost topology alleviates this concern, yet introducesadditional switching losses in the converter.Iave is sensed through a sense resistor and amplified by a

low-power op-amp. A hysteresis controller is used to adjustthe idle time, toff , to achieve the desired average currentin the inductor, Iave,ref . Iave,ref , is calculated based on thedesired instantaneous battery current, Ibt,ref , which can benegative or positive, based on the power-flow direction. Whenthe converter is in boost mode, the average battery current, Ibt,is equal to Iave, while these values are generally different forthe buck-boost and buck modes. In these modes, an adjustmentfactor, Kadj , which depends on the operating conditions, isapplied to Ibt,ref to produce Iave,ref :

Kadj =

t1+t22t1

, if Ipk > 0t1+t22t2

, if Ipk < 0.(4)

Energy is transferred to the load by means of two off-the-shelf buck converters with regulated outputs of 3.3 V and 1.8V. The 1.8 V supply drives the CPLD core, while the 3.3 Vsupply drives the CPLD I/Os, the micro-controller, the sensorsand the RF transceiver. The buck converters incorporate Pulse

Hysteresis

Controller

Ibt,ref

Iave,ref toff Gate Signal

Generator

c1c2c3c4

Iave

-

+

VucVbt

Decision

UnitOperation

Mode

Power Flow

Direction

LUTt1,t2

Kadj

Fig. 5. Digital average current mode controller scheme for the buck-boostdc-dc converter.

Width Modulation (PWM) for heavy-load operation, and PFMfor light-load mode (below 50 mA). However, the buck con-verters spend the majority of time in PFM, as the load powersconsume less than 5 mW in the sleep state. The converters onlychange mode when bursts of power are needed, for instancewhen the RF transceiver is activated.

III. EXPERIMENTAL RESULTS AND ANALYSIS

The WSN prototype is shown in Fig. 6. The specificationsfor different modules are listed in Table IV. The efficiencymeasurements were done in the lab environment, while thelong-term test was run outdoors in the presence of naturallight.

18.8 cm

12.5 cm

(a)

Fig. 6. Developed WSN prototype.

The converter efficiency for the charged battery (Vbt = 4.1V) and different Vuc values is shown in Fig. 7. The efficiencyof the converter goes up as |Ibt| increases, as the gate driveand switching losses have a less impact. There is close to 9%drop in efficiency for the buck-boost operation, as a resultof additional switching losses. The switching nodes of theconverter during a smooth transition from the boost to buck-boost mode are shown in Fig. 8. In both modes, S2 is keptclosed during the time toff , in order to keep the inductorterminals grounded.

TABLE IVWSN PROTOTYPE SPECIFICATIONS

Parameter Value Unit

Battery Nominal Voltage 3.6 VBattery Capacity 350 mAhUltra-capacitor Voltage Range 3.6 - 5 V# of PV Panels in the Array 11Nominal Generated Power (STC) 179 mW

−15 −10 −5 0 5 10 15 2070

75

80

85

90

95

100

Ibt

(mA)

Eff

icie

ncy (

%)

Vuc

= 4.7 V

Vuc

= 4.5 V

Vuc

= 4.3 V

Vuc

= 4.1 V

Boost Mode

Buck−Boost Mode

Fig. 7. Measured buck-boost converter efficiency at Vbt = 4.1 V, and fordifferent values of Vuc.

Vx1

Vuc

Vx2

Vuc

Vbt Vbt /2

Vbt /2

Vbt

Vbt

Fig. 8. Smooth transition from boost mode to buck-boost mode in the buck-boost dc-dc converter.

In order to verify the effective operation of the developedpower management scheme, the device was left to run au-tonomously for nine consecutive days, starting from 12:00 pmon day 1, and finishing at 12:00 am on day 9. The targeted daysoffer a representative sample of varying weather conditionsthrough which the WSN must remain powered. The datareceived from the RF transceiver during this nine-day time-frame is illustrated in Fig. 9(a)-(e). Each day spans the full24-hour period. In order to conserve power, the sample-ratefor the data points is doubled when Voc > 1 V, or mode Iis activated, compared to mode II, in which Voc < 1 V. Asa result, the sunny days (days 1-4) appear wider in Fig. 9.

0 500 1000 15000

2

4

6

8

10

12

14x 10

4

Lig

ht

Sensor

Outp

ut

(Lux)

Sample Number

DAY 1 DAY 2 DAY 3 DAY 4

DAY 5 DAY 6 DAY 7 DAY 8 DAY 9

(a)

0 500 1000 15000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Voc

(V

)

Sample Number

DAY 1 DAY 2 DAY 3 DAY 4 DAY 5 DAY 6 DAY 7 DAY 8 DAY 9

(b)

0 500 1000 15003.6

3.8

4

4.2

4.4

4.6

4.8

5

5.2V

uc (

V)

DAY 1 DAY 2 DAY 3 DAY 4 DAY 5 DAY 6 DAY 7 DAY 8 DAY 9

4.3 V

4.7 V

4.5 V

voltage step−up due toincrease inirradiation

4.1 V

(c)

0 500 1000 15003.2

3.3

3.4

3.5

3.6

3.7

3.8

3.9

4

4.1

4.2

Vbt

(V

)

Sample Number

DAY 1 DAY 2 DAY 3 DAY 4 DAY 5 DAY 6 DAY 7 DAY 8 DAY 9

(d)

0 500 1000 1500−20

−15

−10

−5

0

5

10

15

20

I bt (

mA

)

Sample Number

DAY 1 DAY 2 DAY 3 DAY 4 DAY 5 DAY 6 DAY 7 DAY 8 DAY 9

(e)

Fig. 9. (a) On-board light sensor data, (b) Voc, (c) Vuc, (d) Vbt, and (e) Ibtfor the nine-day test.

Vuc is properly regulated within its desired limits based onthe input conditions, as shown in Fig. 9(b). During night-timeand cloudy conditions, Vuc is intentionally increased to 4.5-4.7 V while this range is reduced to 4.1-4.3 V when mode I isactivated. The excess of the solar energy charges the battery insunny days (days 1-4), as noted by the increase in Vbt, shownin Fig. 9(b). Ibt,ref is controlled to either +15 or -15 mA(positive Ibt means the battery is discharging), as the buck-boost converter yields its highest efficiency at this current.Days 5-8 are cloudy days, resulting in a net negative energy,as confirmed by the downward trend of Vbt, as well as thedata from the light sensor and lower Voc values. The WSNcan remain operational for 8 consecutive very cloudy days,which is a good margin for sustainable operation in most ofthe geographic locations.

The estimated average power consumed by the WSN mod-ules is listed in Table V. The required average power is about3 mW. The bi-directional dc-dc converter losses are less than0.1 mW on average.

TABLE VWSN MODULES POWER CONSUMPTION IN ACTIVE MODE

Module Power Consumption Time Activatedin Active Mode (mW) per Day (%)

Basic WSN Operation 1.120 100Sensors 4.389 8.9

GPS 92.4 0.55RF Transceiver 198 0.04

IV. CONCLUSION

WSNs are traditionally implemented using a single energystorage element, usually a battery. In this paper, a novel powermanagement scheme for a hybrid storage system consisting ofthe traditional battery and an ultra-capacitor was presented.The introduction of a low-ESR ultra-capacitor gives the WSNthe capability of periodically activating high-power loads thatwould otherwise reduce the long-term battery life, or theoverall system efficiency. The battery serves as the back-upstorage for the night-mode operation or when the harvestedPV input power is too low. The storage elements are inter-faced through a 96 % efficient bi-directional dc-dc converter.The converter operates in peak-current mode PFM, whichsuccessfully regulates the battery current, while limiting theconduction losses in the converter. The power managementscheme ensures that the ultra-capacitor is kept charged at alltimes. Continuous operation was demonstrated over nine days,with an average power consumption of slightly above 3 mW.

V. ACKNOWLEDGMENT

The authors would like to acknowledge the valuable supportof the Natural Sciences and Engineering Research Council ofCanada, and the Ontario Research Fund. The authors wouldalso like to thank Texas Instruments for their support, as partof the Engibous Design Summit Analog Design Contest.

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