DESIGN OF 250 A- HR SEALED LEAD ACID BATTERY CHARGER

Cyril Aloysius Quinto

Industrial Electronics, EE242

University of the Philippines - Diliman

Abstract:

This paper aims to come up with a sealed lead acid battery charger for a 250 Ampere-Hour, 96VSeries Battery Bank System comprised of 48 cells with a flat voltage of 1.7V/cell, 2V/cell nominaland a boost voltage at 2.4 V/cell. Imaxdc will be 25A resulting to a 10-12.5-hour charging time.It is expected to go on a trickle charging mode of 2A at the boost voltage. Protection circuitagainst shorted batteries is to be employed.

OVERVIEWThe basic lead acid battery is an old type of battery and has seen differentcharging methods over the years. The lead acid chemistry is fairly tolerant ofovercharging which prompts for cheap chargers.Unregulated transformer‐based chargers consists of a wall mounttransformer and a diode which normally delivers 13 to 14 Volts over a reasonablecurrent range which can charge a 6‐cell 12 V battery. However, as the current tapersoff, its voltage rises to 15 to 18V commencing the electrolysis of the water in thebattery. This is tolerable with sealed lead acid batteries since they can recycle thegenerated gasses as long as they are being overcharged at less than 1/3 of itscapacity. However, leaving the battery at an overcharged state for a week at even1/10 of its capacity will result to corroded plates. At times, taper chargers like thisare made to operate on either a constant current or constant voltage. Regulating thetaper charger is a better and cheaper alternative. The voltage is not allowed to climbhigher than the trickle charge voltage.A lead acid battery charger’s added challenge is to have some kind ofprotection from shorted batteries. A shorted battery has the potential to deliver aextremely high current in a short amount of time (5 to 15 ms). The absence ofprotection may lead to the battery becoming a fuse.Furthermore, the battery’s state of charge and temperature offer modifyingeffects on the internal resistance of SLAs. As they age, their internal resistances risedue to corrosion of the positive grid, changes in active material structure andelectrolyte dry out.In deciding on the amount of charging current, the total internal resistance isto be considered. The total resistance of a series battery bank is the sum of theirinternal resistances plus external resistances such as interconnection hardware andcircuit protection circuits.

With these requirements in mind, this paper looks into a plausible designwithout regard for cost.PRINCIPLE / THEORY OF OPERATION

Topology UsedThe requirement of 25A max charging current for a 96V series lead acidbattery bank prompted the use of a buck‐fed open‐loop interleaved full bridgeresonant converter. This is to guarantee lesser output voltage ripple, and a smallersize for the magnetics. Resonant switching was considered to minimize theswitching losses for the charger.Fig. 1 below shows the process of the blocks of the charger.

Power factor correction was not considered in the design. As an input to thebuck, two wein‐bridge rectifiers were wired in parallel. This is to properly handlethe input current. The rectifiers turn the 220Vac – 264Vac into a DC voltage with a10% ripple. The buck regulates the input to the open‐loop interleaved full bridgeresonant converter. The resonant converter results into a nearly sinusoidal currentwaveform.Battery SetupInstead of putting the 96V series battery bank into a single pack, the bank iscomprised of eight 12V 6‐cell batteries. A series battery system will normally haveboth weaker and stronger cells. One of these cells will be fully discharged before therest of them. For a 3‐cell battery, the total voltage will then be less than 1/3 of theexpected voltage, making it easy to determine. However, for a 10‐cell battery, thiswill only amount to 10% drop from the expected voltage.Each of the eight 12V 6‐cell battery’s voltage is monitored. The table belowindicates the expected voltages.Boost Voltage 14.4VNominal Voltage 12VFlat Voltage 10.2V However, there maybe cases wherein 5 cells are already in their boostvoltage while one is already dead. This will register a 2.4V/cell* 5cells = 12V

BRIDGE

RECTIFIERBUCK

INTERLEAVEDFULL BRIDGE

96V SEALEDLEAD ACIDBATTERYBANK

reading. A lot of possible scenarios may present a misreading which necessitates forthe charger to monitor the current in the battery bank. This scheme is shown in theschematics (battery monitoring and setup).Protection Circuits/ Trickle ChargingIn DC systems, a shorted battery can deliver an extremely high current in ashort amount of time. This current may be equal to the battery’s open circuitvoltage/internal resistance. The total internal resistance of a string of batteries isgiven by the sum of the individual internal resistance of the battery and the wireconnection between the batteries. For new batteries, both the state of charge andtemperature have modifying effects on the battery’s internal resistance.BatteryTemperature ChargeVoltage percell Charge Voltage for a12 Volt battery GassingVoltageper cell Gassing Voltage for a12V battery

‐20 °C * 2.67 to 2.76 16.02 to 16.56 2.97 17.82‐10 °C * 2.61 to 2.70 15.66 to 16.2 2.65 15.90 ° C * 2.55 to 2.65 15.3 to 15.9 2.54 15.2410 °C 2.49 to 2.59 14.94 to 15.54 2.47 14.8220 °C 2.43 to 2.53 14.58 to 15.18 2.415 14.4925 °C 2.40 to 2.50 14.40 to 15.00 2.39 14.3430 °C 2.37 to 2.47 14.22 to 14.82 2.365 14.1940 °C 2.31 to 2.41 13.86 to 14.46 2.33 13.9850 °C 2.25 to 2.35 13.5 to 14.10 2.3 13.8The charger design takes into account the temperature of each of the eight12V battery. Making use of the table from Powerstream shown above, thetemperatures of each of the eight 12V battery are monitored. Both the thermal,current and voltage monitors can be used to determine how much output voltage isto be offered on the battery bank.In case one of the cells in a 12V battery is shorted, besides the voltagemonitor, the temperature monitor will enable the short circuit protection to isolatethe 12V battery from the system. An SCR is to be triggered to shunt the current awayfrom the 12V 6cell battery.In the event that a shorted battery is detected, the DSP FBDisable from the DSP is asserted. This time, the regulation of the output is to becontrolled by the DSP by regulating via the duty of the buck and, hence, the output of

the open‐loop full bridge depending on the reading of the battery voltage monitorsand current monitor.Trickle charging commences as soon as the voltage monitors read a total of8*6cell*2.4V/cell= 115.2 V. At trickle charging mode, only 2A of current is to besupplied to the battery by adjusting Voutput of charger.Software Requirements DefinitionThe digital part of the project is not shown even in the schematics. But thebrief descriptions of the necessary drives, input and outputs to the controller aredescribed.Pin Name Configuration Pin to be UsedOUT A Output Gen Purpose I/OOUT B Output Gen Purpose I/OOUT C Output Gen Purpose I/OOUT D Output Gen Purpose I/OBattery Current Monitor Input ADCBattery Voltage Monitors Input ADCDuty Control/Fault Sec Output Gen Purpose I/ODuty Synchronization Input Gen Purpose I/OBulk_Detect Input Gen Purpose I/OInput to MUX Output Gen Purpose I/ODSP Temp Sensing Input Gen Purpose I/OOut A,B,C and D are inputs to the gate drive IC that will drive the interleavedfull bridge. Fig 2 below shows how the drives will look like. A dead time of 5% willprevent the shorting of the bus voltage. The phase difference of the drive lowers theoutput ripple current. Since the other rail begins to supply the current while theother rail’s current is going down.Battery current monitor is a feed from the sense resistor in series with thebattery bank. This is read by the DSP to decide when to cut off the output.The Battery Voltage Monitors consists of 8 feeds from the eight 12V batteriesin series. This monitors the duty needed by the buck to supply the full chargingcurrent of 25A or do a trickle charging in the event that the boost voltage of everycell is reached.

Fig. 2 Full Bridge Drives – A,B,C and D.Duty Control/ Sec Fault Control feeds the Isense pin of the buck controller toproperly control the duty of the buck. When high asserted, the buck converter isdisabled.Duty Synchronization gets the feedback from the buck duty to synchronizethe DSP duty control in the event of trickle charging or shorting of one of the cells.This is normally high. When the buck duty is high, this pin gets pulled to ground.DSP Feedback Disable disables the hardwired voltage regulation thatregulates the Vo of the charger at around 103.2V which corresponds to a chargingvoltage of 2.15V/cell. The need to disable this arises whenever the output voltageneeds adjustment to do a short circuit protection of one of the battery cells, tricklecharging or secondary fault. This is normally low. When asserted high, the dutycontrol FB disable is pulled low.Bulk_Detect detects if bulk voltage (input to the buck) is already around340V. This will signal the turning on of the open loop full bridge. This ensures thatthe full bridge will not overload the buck while the bulk voltage is not yet set atpower up or during operating conditions wherein the AC voltage has droppedconsiderably. This is a normally low pin. When the bulk is around the desired level,this gets pulled up to the DSP supply.The DSPGND and the OutputGND are the same. They are named differently soas to be carefully laid out as the DSPGND, being the control ground, should becarefully wired.Input to MUX dedicates 3 DSP pins to select which 12V battery system toisolate.

DESIGN PROPER

AssumptionsThe following are the assumptions for the design of the power circuit:1. Switches were considered ideal.2. It is assumed that the best fan cooling is made available.3. It is assumed that the switches are attached to heat sinks that were designedproperly as per estimated losses on the switches.4. The design was done without regard for cost and form factor.5. Auxiliary windings PVCC = 12V and the SVCC = 18V(master) is assumed.6. Use fuse to protect the circuits when all else fails.The calculation sheets are presented per stage. The buck control designshows a temperature rise of around 150degC while the full bridge magnetics(calculations made per interleave – supplies 12.5A) shows a 120degC temp rise. Theformula used to calculate these temp rise holds the assumption of operating thepower supply in convection.The switching loss calculations were based on the following.

Fig 3. Turn on T2 interval

Fig 4. Turn on T3 interval

Fig 5. Turn off T2 interval

Fig 6. Turn off T3 interval.These were obtained from Switching Power Supplies A to Z written bySanjaya Maniktala.The magnetics sheet shows cores and choke tried to improve the temp rise ofthe full bridge transformer and buck choke.Miscellaneous sheet include the gain calculations for the comparator designused for battery voltage and current monitoring, ladder design.Results and SimulationsSimulations were done on a Simetrix environment. Fig. 7 shows the simetrixcircuit used. Simetrix 4.2 and the e‐DVT ASTEC modeled parts were used.The interleaved open loop full bridge is modeled as E1 shown in Fig.7. The E1value is calculated from the Nsec/Npri = 6turns/ 17 turns = 352m of the full bridgetransformer. The output capacitor’s ESR is assumed to be 43mohm each.For the circuit’s gain phase plot, the gain can’t be increased any longer as thesimulator does not allow changing the R4(in the simulation schematic). The gain ofthe closed loop circuit is given to be OptoGain* R4/R2.Looking at the duty cycle, they are equally spaced. The slope compensationwas employed because at more than 50% duty, harmonic instability may occur.

Fig 8. Shows the ripple current per output capacitor to be 162.7mA

Fig 9. Shows the Buck Duty in Green, Triangular Green graph is the Buck currentVocharger(after FB) is in Red, Vinbuck in Purple and Vobuck in blue.

Output Cap Ripple Current 167.5 mArmsVocharger 109.6 VBuck Drive Duty 92 %1.88 Apk‐pkBuck Current 8.8191 Arms311.2 VVobuck 5.542 Vpk‐pkVinbuck 340 VTabulated above are the measured values of the circuit.

Fig. 10. Shows the Gain and Phase Plot of the Circuit.

Lead Acid Battery Charger Bill of Materials

Power Circuit

Circuit Code

Q1 FET-N 25A 600V IPP60R125CP

Q2 FET-N 25A 600V IPP60R125CP

Q3 FET-N 25A 600V IPP60R125CP

Q4 FET-N 25A 600V IPP60R125CP

Q5 FET-N 25A 600V IPP60R125CP

Q6 FET-N 25A 600V IPP60R125CP

Q7 FET-N 25A 600V IPP60R125CP

Q8 FET-N 25A 600V IPP60R125CP

C1 CAP-MP 82N J 400V PHE450

C2 CAP-MP 82N J 400V PHE450

C3 CAP-MP 82N J 400V PHE450

C4 CAP-MP 82N J 400V PHE450

C5 CAP-E M33 M 450V GU

C6 CAP-E M33 M 450V GU

C7 CAP-E M33 M 200V AXW

C8 CAP-E M33 M 200V AXW

C9 CAP-E M33 M 200V AXW

C10 CAP-E M33 M 200V AXW

C11 CAP-E M33 M 200V AXW

C12 CAP-E M33 M 200V AXW

C13 CAP-E M33 M 200V AXW

C14 CAP-E M33 M 200V AXW

C15 CAP-E M33 M 200V AXW

C16 CAP-E M33 M 200V AXW

D1 RECT-UF 30A 200V FEP30DP

D2 RECT-UF 30A 200V FEP30DP

D3 RECT-UF 30A 200V FEP30DP

D4 RECT-UF 30A 200V FEP30DP

D5 RECT-UF 30A 200V FEP30DP

D6 RECT-UF 30A 200V FEP30DP

D7 RECT-UF 30A 200V FEP30DP

D8 RECT-UF 30A 200V FEP30DP

D9 RECT-SIC SK 10A 600V C3D10060A

DB1 RECT-BR 25A 600V GSIB2560

DB2 RECT-BR 25A 600V GSIB2560

R1 10 mohm HECNUM

R2 RES-TKF 15R F W25 RN412

R3 RES-TFC 10K J 0W1 0603WA

R4 RES-TKF 15R F W25 RN412

R5 RES-TFC 10K J 0W1 0603WA

R6 RES-TKF 15R F W25 RN412

R7 RES-TFC 10K J 0W1 0603WA

R8 RES-TKF 15R F W25 RN412

R9 RES-TFC 10K J 0W1 0603WA

R10 RES-TKF 15R F W25 RN412

R11 RES-TFC 10K J 0W1 0603WA

R12 RES-TKF 15R F W25 RN412

R13 RES-TFC 10K J 0W1 0603WA

R14 RES-TKF 15R F W25 RN412

R15 RES-TFC 10K J 0W1 0603WA

R16 RES-TKF 15R F W25 RN412

R17 RES-TFC 10K J 0W1 0603WA

R18 RES-TKF 10K J 0W1 RC21

R19 RES-TKF 33R F 0.25W RN412ES

R20 RES-TKF 10K J 0W1 RC21

R21 RES-TKF 33R F 0.25W RN412ES

TX1 17T pri:6T sec FEE64/21/51 Cores

TX2 17T pri:6T sec FEE64/21/51 Cores

L1 55T HiFlux OD330 Core

Drive Circuit

R201 RES-TFC 10K F 0W1 RK73H1JT

R202 RES-TFC 10K F 0W1 RK73H1JT

R203 RES-TFC 10K F 0W1 RK73H1JT

R204 RES-TFC 10K F 0W1 RK73H1JT

C201 CAP-MCC U22 K 16V X7R

C202 CAP-MCC U22 K 16V X7R

C203 CAP-MCC 1U0 K 16V X7R

C204 CAP-MCC U22 K 16V X7R

C205 CAP-MCC U22 K 16V X7R

C206 CAP-MCC 1U0 K 16V X7R

T201 15T each winding,TDG-T12.7X7.8X5-TLS

T202 15T each winding,TDG-T12.7X7.8X5-TLS

IC201 IC-SM DRIVER AP239TR

IC202 IC-SM DRIVER AP239TR

Control Circuit

R301 1.3kohms

R302 39kohms

R303 1kohms

R304 75kohms

R305 30kohms

R306 10kohms

R307 91kohms

R308 30kohms

R309 3.3kohms

R310 10kohms

R311 30 ohms

R312 100ohms

R313 10kohms

R314 360000ohms

R315 1000ohms

R316 1000ohms

R317 10000ohms

R318 1000ohms

R319 10000ohms

C301 1U 10V

C302 330pF

C303 4.4nF

C304 1pF

C305 1uF

C306 470pF

C301 1U 10V

IC301 IC-SM PWM AS3843D-8

IC302 OPTO-CPL SFH6156-2

IC303 IC-SM DRIVER AP239TR

IC304 TL431 CLPR

IC305 OPTO-CPL SFH6156-2

IC306 OPTO-CPL SFH6156-2

IC307 OPTO-CPL SFH6156-2

Q301 BC817-25 (NPN)

Q302 NMOS FET rated 2A

Q303 BC817-25 (NPN)

Z301 9V zener diode

Miscellaneous Circuit

IC401 IC-SM REG AZ1117H-3.3E

D402 DIODE-C 0A2 75V BAS16HT1G

R401 47ohms

R402 RES-TFC 2R2 F W063 RK73H

C401 CAP-MCC 0U1 K 25V X7R

C402 CAP-MCC 10U K 16V X7R

C403 CAP-MCC 10U K 16V X7R

C404 CAP-MCC 10U K 16V X7R

T1-T8 15T each winding,TDG-T12.7X7.8X5-TLS

R403(a-g) 30kohms

R404(a-g) 100ohms

SCR1-8 25A rated

IC 3 input multiplexer

Thermistor THMTR-C 10K J 0W21 TSM1A

R405 10kohms

Battery Setup For all eight of the circuits

Ra 1000ohms

Rb 1000ohms

Rc 200ohms

Rd 100ohms

Re 200ohms

Ca 100nF

X1-4 IC-SM QUAD OPAMP LM2902KPWR

X5-8 IC-SM QUAD OPAMP LM2902KPWR

X9 IC-SM QUAD OPAMP LM2902KPWR

2-DSPs IC-SM DSP MC56F8037VLH

REFERENCESManiktala, Sanjaya. Switching Power Supplies A to Z, Newnes, USA,2006.Butler, Dan. Lead Acid Battery Charger Using PIC14C00, Microchip Technologies,USA.Lenk, Ron. Practical Design of Power Supplies. John Wiley & Sons, 2005.www.powerstream.com.Martinez, R and Formenti,J., Design Tradeoffs for SMPS Battery Chargers. TexasInstruments Notes.