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1. Module – 5 Power Electronic Converter for Battery Charging

2. 2. Termination methods such i » the opplicaiion end the en irunmenl where the battery is use. The con- ventional tcrniination methods that can be used arc mentions here: 7.2d.1 Time   Using lime is one of the simplest methods w hich is mainly used as a backup for fast charging or normally used for regut:ir charging for specific types of batteries. This nielhud is very simple and inexpensix‘e , brit because of diminishing battery capacity over iimc dcc to 8ging, the time should be reset lor e rcduccd c8pecity aged battery tn avoid nx’ercharging nf old hatteries . . 2d.2 Voltage As meotl'O Hed hefore , voltage can he u.sed as a terniinaiinn factrir , t.e ., term1 hating the charging prr›cess when the battery voltage reaches a specific value. This meihA hue Hume inaccuracies. becauw bit) t3QCn-circuit xu ) tage is ubt ained when thc baitcry is left disconncctcd for somc limc aticr charging. Thi.s is because chemical reactions hiippening • inside the battery need snme lime tu stabilize. Nc vcrthclcss . this mcihod i,s widcly uscd . In addition, this technique is usually used wnli c onstant curi'eni tec h n lque tn avnid overhenti rig daiiiage in the battery. 7.2d.3 Voltage l) rup ( dV / d"l ’}   In somc chcrn isiric.s likc • N1•Ct1 when chargcd using constant currcrn method . the vn1taq•e increases up to the fully charged state pnint and then the voltage hegins to ‹ lccrcasc . This is duc to ox y gcfi build-up inside the baticry . This decrease is significant, en ihe deri ' aiive of the vnltage versus rime cm he measured tn indicate overcharge. When this parrneter becunies negative it shuws thut we have passed ihc fully charged state and ihc lcmpcralurc bcgin to rim. After thi point the chfirg lng method cm be switched to trickle. or fioat charge. or tei'minoted completely.

3. Bv Ik Charge Curmnt Fig. 7.1 Churyin3 alporiihm ot s typical Lead-4cid battery d.4 Current In the last stages of charging, if cnnstani voltage method is used, the current t›egins to ‹Increase .as ihc battery reaches fully charge slam. A preset currcni valuc such as C/ ! II rate can be defined and is•hen the current goes t›elow this value the charging wtiuld he teriiiinated .

4. 7.td.5 Temperature In general, during charging the hattery temperature increases In sc›me extent, howc cr , extra increase in tcmpcraturc is a sign of overcharge. Using temperature . sensnrs highly addx to the cnst nf system. Nevertheless, for snme chemistries such as Ni-MH, methods such as Voltnge Drop is not recommended, xince the voltage drop ofier full charge siate is not sigoificont to be relied on. hi this case, tem- perature increase is a good indication of overcharge and can be used. 7.2.4 Cha rg ing Algorithm Charging algorithm can be defined as the combiruitiun uf what was mentioned up to here and controlling* all or part of the paramcicn affecting battery performance and llfe cycle in such a way tn achieve hartery pack ehargi rig .safely, efficiently, and terminated on time. Managing the charging procedure nf a high power battery pack with hundreds uf cells involves many issues as ah‹rady discussed in this chapicr . To control all of ihcsc paramclcrs . cfiicicnt and accuraic algorithms with reliable safety and backup circuits uru required. The trend luward fast charging with huge amounts of currcnt flowing to thc battery pack producing lots of heat requires occurnte rind reliahle supervi.sory cnnirol algnrlihns in ensui e safe charge. Managing such complex task can be handled with advanced control techniques like fuzzy logic, supervisory control, and decentral iced control, and so pn . I n general, each battery chemistry requires a unique charging algorithm. Depending un the algorithm it may be applied tu Anne uther types as well, huwe•er , this .should be carefully alone accordi rig to life cycle and safeiy issues. For precise battery charging the charge/discharge profile nf the battery prnvided by the manufacturer must be nod. However, the profile is valid fOf brand new bntieries , hence, snme technique s like data acquisition methods must be used tr › acquire the charge/discharge profile of the battery with deteriuraiion due to aginp . Novel techniques regarding this issue are being introduced in the literature every often [5].

5. As mentioned before Lead-acid batteries have mature technology and infra- stmclure that already exist, but they still ha›'e purr life cycles in the urder of .3(1(M(1(1 cycle.s . A lot nf ‘ efforts have been put into research fnr increasing the lite cycle uf )eai1-acid batteries because of their advantages such as luw cust and availability. This chcrnistry has a common charging algorithm which includes four different .stages or three based on the application, as indicated in Fig. 7,1. In the firsl stage a predefined constant current is applied to the battery pack wfiicfi charges lhe cels rapidly. in 1hi. siege the ceM voltages incmese graduol1v because nf SoC increase. Thi.s stage is called “Bulk Charge” stage. The pmcess is continued until a predefined maximum voltage is reached, Then values are all recommended by ltte monufaciurer in the Datasheet. In the next stage calletJ “Absorption Charge” stage, a constant voltage is applied to the battery pack. As tnc battery agcs . its inicrnal characteristics also changc , hcncc . an adaptive chnrging algnriihir could he used to take intn account these parameters variatinns . Experimental results show that ihe value of voltage of the third stage should be increased over lime to get the same amount of energ • as the batter ages [6—8]. The equalization stsge is the key part of this algorithm and has goat intiuence on the life cycle nf the battery. As mentinned hefnre , the vnltnge of this stage should be instead but this increase the current and mls the heat generated which has negative impact on the life cycle. One way tn get the sauie amnuni nf current w'ith luwer heat dissipation is by rising pulses uf current. Although this technique seems the same as pulse charging, ii is actually different bccausc the time intervals arc .significantly bigger than pulse charge rime periods which as in the range of kiln hertz. This method is called “ Currcnt Interrupt” or “CI”, This icchniquc has .shown slgniticanl I ife cycle improve in ens [9]. Using rhts algnrithm , the battery can reach 5() % of the initial capacity after 5t)t) cycles which is a significant improvement in life cycle. Although this algorithm is useful, ii puts the battery under strcss whilc it rcachcs thc cnd of litc because of permancntly increasing the uvervollage value. This algorithm can be implemented in an alternative way. Instcad of using this method for cach cycle. which puts high strcss on the battcry . it cnn be utilized every 1 tl cycles. This algoriihiTi is culled “ PoriiaI -Stare- nl - Rech:irge cycling (PSOR) ” 9 l >>3 approximately the same effect with the

6. advantage of less stress on the battery. This algorithm has been claimed to enable the battery deliver up to 80 fi of initial capacity even after 780 cycles [9] which is a really nnticeahle impmveiuent in life cycle. As can be sccn thcsc complicaicd algonthms cannot be donc using simplc PI, PID cnntrnllers . They requlre DSP- hased controllers in he pmgr mmahle z'ith numerical data dependent on the battery chemistry, state of health, and other factors. Continuously, different algorithms are being proposed and tested for improving life cycle t›f the batteries. This is a vast research area and is currently under dcvclopmcnt . getting a lot of attention as EVsfPHF-.Vs become popular and nvailable in ihe iiiai ket . Charging from Grid for practical purposes, connection of the carport to the grid is highly desirable, as such a connection would serve a number of useful purposes. The grid would primarily act as extended storage by collecung excess energy produced by the PV aixay . This will relieve the carpurt frum the task uf locally sturing energy in batteries or flywheels. Second, the grid can supplement any deficiency in PV generatinn , during cloudy dey.s or ct night, for instance. Finally , stnring energy during peak hours of grid utilization and the availability of connected batteries during the same period seems to be ihe optimal solution for the utility company for the many potential benefits described in [35J, namely power factor improvement, backup power during grid failures, and peak shaving•. Aboul the last point, it is well worth pausing on the realization that stand-alone installations, such as microgrids , must allow locally generated power to be stored in battery banks. as well as being converted to AC. in order to drive neighboring loads, At the same time, power must elso be allowed to now finn the batteries to the AC line at timex when PV generation ix lacking. li appears then, that at leaxt three out of the four power flow paths described above also apply tn stand-alone sysrei»s . The order of importanc e for the power paths is noi the same: nevertheless, it is still conceivable that mony findings wiihin this chapter mny be applicable to mtcwgrids as well. Keeping thix fact in mind, this chapter will, henceforth, fc›cus snlely on PV grid-tied HV charging applications.

7. As aforementioned in ihe introductory chapter, the grid could perform the funcHon of “storing” excess energy produced by the PV resource of the caiport , thus eliminating ihe need for fecal stonsge (batteries and flywheels). In fact, the sce - nario is somewhat more complicated than that. It is well-known that intermittent 7.3.3 Local Kictribution Confi8uration As a final note. ihe solutions considered in the previous sections assume the use of single-phase connections. The main appeal of the three-phase system is the enhanced power capacity, the fact that this power is delivered end absorbed without any line- frequenc y compone nts , and the elimination of third hnrmnnic ciirrents hi the distribution raid gruuniling wiring. These features are quite attractive and include the advantage of potentially eliminating undesirable pul - .eating charg lng current to the EV battery. On the other hand. ihe .single-phase configurntion allows for less expensive and simpler distributed inverters. Espe - cially considering the fact thai PV cen only provide 5 kW of power to the inverter, and that intervening DC/DC converters need In be added In can eliminate any pulsating current, it appears that a single-phase system is much better adapted t‹r this application and will be assumed throughout this chapter.

8. 9.8 Isolated Bidirectional DC-DC Converter Flgure 9. J7 Typlcal voltage and curFent wavefoi wi fa Pt > nP , : (a) waveforms fbr /( rt ) <0, :b) » wrorms for boundary conditions I”( tg = 9; (c) waveforms for i ( rr ) >g.

9. 9.8 Isolated Bidirectional DC-DC Converter In s‹ime applications, galvanic isolation between the battery arid the load tmits is necessary a nd desirable [23L Figure 9. 16 shows a full bridge i.solated bidirectional DC—DC converter. In Figure 9. 1 fi, the primary bridgt inverter sw'iix;he6 dt 20— SO kH z, with 5tr$ duty ratio, The output of tha primary is a square wave vultaga which is applied to the primary winding of the isolation transformer. The secondary winding of the transformer will therefore have a square wave voltage, Wlthout any control at the gatint ; of the secondary bridge convgrter , the voltage r›f the boundary of the transformer is recn fled thrriugh the four freewfieeling diodes. The output voltage will fluctuate with load conditions and the primary vr›ltage . 9&I Basic Principle ar¥d Sbeady ' Stata Opara€lons Steady state operations of isolated bidirectionalDCi —DC coavertem have beea studied la detail elsez'here {1, 3, 6]. In thia section, we complement these studies by distinguishing the operating m.odes of isolated bidirectional DC— DCi converters according to the phase ahift angle, load conditions, and output voltage. In this analysis, the dead-band and switching dynaMlcs will be neglected but Will be analyzed later. In the following analysis, the turns ratio of the transformer is o. the transformer primary voltage R V„ and the switching frequency is/,. For the convenience of analysis, we definr T, as one half ot the switching period, that is, T -— 1/(J,). The duty cycle or phase shlfi is based on a half perio‹t D -- tel T,. Therefore , DT ts the phase shift between the two bridges. Further, Jg is the current of the leakage inductance of the secondary winding. The output voltage of the secondary bridge is *z-