A Brief Course in Electrochemical Energy Storage€¦ · A little history . . . • 1800 Volta...

transcript

A Brief Course inElectrochemical Energy

Storage

Bob NuckollsSr. Engineer/SME

Raytheon Aircraft Company10 July 2004

A little history . . .

• 1800 Volta demonstrates to Napoleon the Volta pile, a primary, nonrechargeable battery.

• 1854 Sinstede uses the first time lead plates in sulfuric acid to store i.e.accumulate, electricity.

• 1859 Planté improves the capacity of the lead acid batteries with a techniquestill in use today.

• 1881 Faure discovers the pasted plate which yields a mayor breakthrough incapacity. A lead antimony alloy is used the first time to give strength.

• 1882 Gladstone and Tribe describe the so called double-sulfate theory i.e. thebasis of operation of the lead acid battery. Tudor operates a lead acid batteryfactory in Luxembourg.

• 1899 Jungner invents nickel cadmium rechargeable battery. Expensive andlimited in useage. New electrodes developed 1930s. 1940s brought a sealednickel cadmium battery that recombines internal gases produced duringcharge. Improvements have been made every decade since.

• 1907 A lead calcium alloy is patented.

• 1910 The iron-clad or tubular plate construction is introduced

• 1915 Willard introduces rubber separators.

• 1918 Shimazu describes the ball mill oxide.

• 1951 Lead calcium alloys are used in telephone exchange stationarylead acid batteries.

• 1958 Jache describes the gel VRLA battery.

• 1965 Polypropylene SLA battery cases start to be used.

• 1968 The maintenance free SLA battery is developed by Gates.

• 1980 Stationary VRLA batteries based on AGM technology aredeveloped

Lead-Acid: A very successfultechnology with over 120 years ofcommercial service.

Carbon-Zinc: Evolved fromLeclanché’s 1866 patent on a wetcell design using a liquid,ammonium chloride electrolyte. Adry cell version of the Leclanchécell was developed and perfectedin the 1880s. The carbon zinc drycell has remains much the same tothis day.

Duracell pioneered the AlkalineManganese Dioxide electrochemicalsystem nearly 40 years ago. Alkaline orAlkaline Manganese Dioxide cells havehigher energy output than zinc-carbonpredecessors. Other significantadvantages are longer shelf life, betterleakage resistance, and superior lowtemperature performance.

Ni-Cad: Nearly as mature as the Lead-Acid battery, Ni-Cads were the firstto offer drip-free, sealed energystorage technology.

Nickel Metal Hydride: Chemically, oneof the best cathode materials forbattery cells would be hydrogen.Discoveries in late 1960s showedthat some metal alloys had theability to store atomic hydrogen1000 times their own volume.

NiMh technology is rapidly replacingNickel-Cadmium as the portablepower cell-of-choice.

Electrochemical EnergyStorage Basics

(It’s all in the cells!)

What is a cell?

Definition of Cell:

Anode - Source ofelectron flow to theoutside.

Cathode - Sink forelectron flow from theoutside.

Electrolyte - Media forthe exchange of ionsin reduction-oxidationreactions at anodeand cathode.

What is a cell?

Every material in the universehas a position on the ladderof values for ElectromotiveForce when compared withother materials . . .

Lets explore the comparativedifferences between arudimentary cell using silver-copper and silver-aluminumelectrodes . . .

What is a cell?

Classic classroomelectrolysisdemonstration . . .

The “Lemon” Cell.

What is a cell?

Construction typical ofall flooded and “gelled”lead-acid cells.

A “battery” is a array ofseries connected cells(higher voltage) orparallel connected cells(higher capacity /current)

What is a cell?

An array of cellsassembles into a“battery” . . .

What is a cell?

Cutaway of Gates/Hawker/Enersys“jelly roll” cells which introducedrecombinant gas, lead-acid technologyto the marketplace in the late 60s.

What is a cell?

Gell-Cell Not!

Prismatic cells followed closely onthe heels of Gate’s jelly-roll cells.

The vent regulated, sealed lead-acid (VRSLA), recombinant-gas(RG), absorbed glass mat (AGM)products proliferated.

Prismatic SLA cells are availablein sizes from 0.5 to 1200 a.h.

These are manufactured in themillions for emergency lighting,portable power, uninterruptablepower supplies, etc. etc

Cells of these batteriescontain so little liquid thatyou can drive a nail intothem and they will notleak.

What is a cell?

Ni-Cad and Ni-Mh jelly-roll construction.

Lead-AcidElectrochemical Energy Storage

Basics

Lead-Acid: How it works . . .

Lead Acid: how it works . . .

The chemical reaction during normal lead-acid use is

Pb + PbO2 <-----> 2PbSO4 + H2O +2 electrons

Fortunately the reaction proceeds readily in either direction withoutmuch heat. The chemistry usually is written as two half cell reactionswhich makes it a little more clear just what is happening at each plate:

At the anode: PbO2 + 4H+ + 2 electrons---> PbSO4 + 2H2O

At the cathode Pb(metal) ---> PbSO4 + 2 electrons

(Equations above are not balanced)

As the battery discharges more water is produced which forces specific gravityof electrolyte lower.

During cell over-charge there is no more lead-sulfate left on the plates to beoxidized and reduced. Current forced through a solution must produce a reaction atboth electrodes. Since all materials used to store electrical energy has beencompletely oxidized or reduced (charged) something else happens. Further, it’s notgood for the battery and could generate a hazardous condition.

The net reaction in the battery during overcharge is:

2H2O + 4 electrons ----> 2H2 + O2

There are two basic cell types: vented and recombinant.

• Vented cells are flooded with electrolyte. Hydrogen and oxygen gasesgenerated during charging are vented from the cell container.

• Recombinant cells immobilize the electrolyte. Oxygen generated from thepositive electrode during charging diffuses to the negative electrode where itrecombines to form water

• The recombination reaction suppresses hydrogen evolution at the negativeelectrode so that the cell may be sealed.

• In practice, the recombination efficiency is not 100%. Therefore, a pressurerelief valve limits internal pressure to a relatively low value on the order of 2psig.

• Sealed lead-acid cells may be called “valve-regulated lead-acid” (VRLA) cells.

Lead-AcidBattery Fabrication

Lead-Acid Battery Fabrication

Lead Acid Battery Fabrication

“Buttered” plates are stacked and then cured for two weeksin a temperature-humidity controlled environment.

At this point, the capped battery sits overnight for sealingepoxy to set up.

Battery is pressure tested on a cell-by-cell basis.

Any detected leakage is cause for rejection.

•Cast lead plates (1% calcium reduceswater loss)

•Apply paste positive and negativeplates

•Aged two weeks

•Insulated and sleeved

•Soldered into temporary battery array

•Charge individual plates•Group sets of (+) and (-) plates forassy and then group again for weightmatching.

•Molded plastic cases and tops

•Install sets of plates in battery box.

•Weld risers onto plate arrays

•Hand-weld crossovers and terminalstraps

•Epoxy grooves in lid and set batteryupside down into lid

•Pressure test. If pressure test fails,battery is scrap.

•7x Deep cycle charge and capacitytests

•Pour out excess electrolyte

•Cap and wash

•Ip Test

•Elapse Time ~ 6 weeks!

Exploded view of aConcord lead-acid aircraftbattery . . .

Concord

Labor intensive. ~160 employeesfabricate 1000 units a day

Lead Calcium Alloy Plates

Cast Plates, cast pockets for activematerial

An array of plates are weighed on cell-by-cell basis for capacity matching.

Sets of plates are relatively loose fit inbattery box and placed individually byhand before inter-cell connections arehand welded.

Enersys (Hawker)

Highly automated ~560employees fabricate 60,000 unitsa day

Pure Lead Plates

Plates punched from long rolls ofchill-cast lead sheet

No porous plastic pocket overplate structure. AGM is onlyseparator.

Sets of plates are hydraulicallycompressed and pushed intotight fitting boxes by machine.

Inter-cell connections are spotwelded

Concord cont.

Dry charged cells are flooded andelectrically cycled to insure saturationof separators.

Cells are sealed after excesselectrolyte is poured out at theconclusion of deep cycle activationand testing.

Manufacturing cycle ~6 weeks

Enersys (Hawker) cont.

Electrolyte vacuum injected inprecise amounts and cells areimmediately sealed.

Battery is “activated” during firstcharge cycle.

Manufacturing cycle ~5 weeks.

Cost approx 2x that ofcomparable Concord

Coming over the hill . . .

• Concord has patented a new process for making lighter grid plates.

• Lead-clad ALUMINUM plates are being perfected. Chemically, theseperform light lead but offer lighter structure, stiffer plates and betterconductivity.

• This process promises substantial reductions in battery weight bysomething on the order of 20%.

• Recycling is more difficult . . . Aluminum does NOT mix well with lead in asmelting operation. These new products will not be able to utilize thecurrent recycle stream for lead-acid batteries.

Care and Feeding of LeadAcid Batteries

Lead Acid Battery Operation

• Batteries in airplanes serve three major functions:

•Crank engine(s). This is a relatively high power/low energy (perhaps 5 to15% of battery’s total charge is needed to get a turbine engine started.Even less for a piston engine.

•Filter / stabilize the operating system. The battery’s low internalimpedance provides the best filter for alternator / generator noise. Also,some alternators do not run well without a battery on line.

•Standby power in case of engine driven source failure. Here the batterymust be sized and maintained to assure duration of operation of equipmentessential for descent to landing.

• Engine Cranking . . .

• Getting the engine started is certainly a high-power event. Currentsdelivered by the battery at the beginning of a turbine start cycle is on theorder of 700 to 1000 Amps. It tapers to about 300 Amps at the end of a 20-30 second start cycle.

• While the current levels are high, total energy removed from the battery isa fraction of the battery total. The 200,000 watt-second start curveillustrated next represents less than 10% of the battery’s 3,000,000 watt-second capacity.

• Reciprocating engines start in a much shorter period of time on the orderof 5-10 seconds . . . Average current during the start cycle is 200-300Amps.

• A reciprocating engine takes about 40,000 watt-seconds, about 4% of thebattery’s capacity.

• While power levels are high, total energy requirements are rather modest.

Beechjet Start Curve: Piecemeal Integration

Shows 200Kw/Sec Start Cycle

The importance of controlling internal and external impedance . . .

Each cell can be visualized as many hundreds of individual cells-sites inparallel . . . Each one contributing a small energy storage capabilityand a moderately high source impedance.

E.g. A cell of 1000 cell-sites having individual source impedance of 1ohm combine to make a single cell with a source impedance of 1milliohm. When half of the cell sites die, capacity drops by half andsource impedance doubles.

Effects of internalimpedance becomeapparent when weconsider energydelivered to theexternal world atvarious dischargerates.

Here are typicaldischarge curves at25°C for a 24 Volt, 37Ah VSLA aircraftbattery.

• Standby Power. . .

• This is the battery’s toughest task . . . Most production aircraft with astandby power storage requirement call for 30 minutes of operation sansengine driven power sources.

•Unlike engine cranking, emergency operations are all but guaranteed totax the battery’s capacity to the limit.

•Unfortunately, battery capacity cannot be gauged from outside the batterywithout doing an actual capacity test.

Portable capacitytester/chargers do existbut they’re not the kind ofthing you find in theaverage mechanic’s toolbox!

Ideal charging voltagefor a battery istemperaturedependent.

Unfortunately, the onlyknown temperaturecompensatedregulators for aircraftapplications areavailable only to theOBAM aircraftcommunity.

Lacking the “elegant solution” regulator, the best compromise is to makemaintenance adjustments of bus voltage depending on current climaticoperating conditions.

The following recommendations come from Concord’s user guide onlead-acid battery application.

The ideal battery chargingphilosophy maintains voltagecommensurate with presentbattery temperature untilrecharge rate drops to lessthan 1A at the whereuponvoltage should be steppeddown to something on theorder of 13.5 (27.0) volts.

A stepped down maintenance voltage would be just high enough to preventloading the battery but too low to put any significant charge on the battery.This charging philosophy would promise nearly ideal battery service life byoffering fastest practical recharge while protecting the battery fromovercharging.

Care and Feeding of Lead-Acid Batteries

Care and Feeding of Lead Acid Batteries . . .

• A comprehensive study was recently conducted at RAC to determine why wewere suffering large warranty losses on batteries installed in customer aircraft.

• The study looked at end-to-end battery handling issues from the time a cellplate is fabricated until a battery is no longer suited for service.

• A major fraction of the costs were traced to poor warranty policy . . . RACwarranty was set to a value much greater than the battery manufacturer’swarranty. Most of the cash bleed was fixed with a more realistic batterywarranty policy.

• The study identified a number of areas where battery handling can beimproved.

• Take advantage of just-in-time deliveries offered by batterymanufacturers to reduce number of batteries in inventory -AND- time thatbatteries sit on the shelf.

• Concentrate battery delivery and storage in smaller area. A survey ofSAP showed that we had batteries in storage in dozens of different placeson the square mile.

• Vast majority of handling induced battery failures on square mile occureither on experimental flight test aircraft or batteries neglected in storage.

• Most production lines already use “tool batteries” . New policies andprocedures have been developed to store customer batteries in racks atthe end of assembly lines.

• Tool batteries will be used until aircraft is ready for delivery to flight test.

• Number of storage locations for batteries on the square mile reduced to atiny fraction of the current condition.

• Few opportunities for improvement were identified after the customerbattery was installed on the aircraft: Battery failure rates from time-of-installation to time-of-delivery was quite low . . .

• Opportunities for improvement

• Work with field service organizations to avoid handling damage onreplacement batteries.

• New policies and procedures for ordering stock warehousing.

• Develop first-in-first-out handling procedures.

• Develop monitoring techniques for ALL life-limited parts includingbatteries.

• Acquire tools and conduct training on battery maintenance.

• Conduct training for folks who handle batteries to improveawareness of the fragile nature of stored batteries.

• Work with manufacturers to improve data gathering on field failures. Thissame data offers a fall-out opportunity to improve battery performancebased on real-life numbers on how aircraft batteries are used. More onthis later.

Long Term Battery Storage:

A number of conditions affect the magnitude of leakage current in batteries.Two of the strongest influences are:

• Storage temperature

•Batteries stored in warm climes and un-controlled warehouseenvironments are especially subject to increased rates of leakagedischarge. Batteries stored in Canadian warehouses do very well.

• Free oxygen dissolved in the electrolyte.

•Recall that the major difference between Concord’s flooded, ventedproducts and their sealed, valve regulated produces is the cap. Oncethe cell is sealed off from the environment, the percentage ofdissolved gasses in the electrolyte drops to a very low value. Thissimple isolation of the cell environment from ambient atmosphereresults in markedly low self discharge rates.

Long Term Battery Storage:

EVERY battery suffers fromsome degree of internal self-discharging leakage. Thismanifests itself as a low level“load” on the battery that willeventually produce a totallydischarged battery.

A “Battery Tender” type of “smart charger” will charge initially at some levelthat insures a charge top-off . . . Something on the order of 14.4/28.8 volts.When charge acceptance current drops below some small value, the outputvoltage drops to 13.0/26.0 volts so that the “Battery Tender” will just offset theleakage currents.

TENDER"

FOR 13/26SUPPLY SETREGULATED

LEAKAGEI L

EAKAGE R

117 VAC

"BATTERYINTERNAL RES

0.75A Battery Tender from Deltron (www.batterytender.com)

Value Assessment of Concordvs.. Enersys

(Getting Past the MarketingHype . . . Beware of pink

bunnies)

Getting Past the Marketing Hype . . .

Performance of various AA cell brands at room temp

Alkaline vs.. Photo Lithium at

Room Temp and -20C

•Bottom line of study on AA Alkaline batteries: Irrespective of intensity andflavor of marketing hype, the “best” isn’t a lot better than the “worst” andthe “lowest cost” is not the “worst” yet offers the best “value”.

• We know that lead-acid capacity is a function of mass of reactants. Lead,lead-oxide, sulfuric acid, water, lead-sulfate, and to some lesser degree,mini-reactions that affect charge/discharge efficiency and water loss. Justhow “bad” can a lead-acid battery be?

• Greater number of thinner, pure lead plates has an obvious advantage interms of lowering internal impedance of the cells but since most of ourcustomers are obligated to set service life based on capacity, is there acost-of-ownership advantage to the higher cost of thin, pure-lead plates?

• A question yet to be answered is whether the “premium” construction ofan Enersys (Hawker) battery translates directly into additional service lifecommensurate with the increased cost of the battery.

• Current best recommendation for the Owner Built and MaintainedAircraft (OBAM) community is to by the least expensive product you canfind and change it out often . . . Like every annual inspection.

• For light aircraft running dual batteries, this means that you can put a newbattery in the main battery slot every year and rotate the main battery intothe auxiliary battery slot.

• For a cost of about $40/year:

• The main battery is less than 1 year old and it’s stand-by capacity isassured.

• There are no batteries more than 2 years old and the auxiliarybattery can be depended on for backing up a light, ignition load (2A orso) for duration of fuel aboard.

• Two batteries in parallel offer 34 a.h. cranking performance forsuperior engine starting.

• The jury is still out on battery brand selection philosophy (Concord versusHawker) based on real-number economics and physics.

• There is consideration for developing a “black box” for aircraft batteries.

• A small (0.5” x 1.0” x 2.0”) module mounted in the head-space of thebattery would measure and record voltage and temperature every 10seconds for two years.

• When a battery is taken out of service, the “black box” can be easilyremoved and sent back to manufacturer for evaluation. One can easilydeduce number of flights, number and difficulty of engine starts. If,when and for how long a battery was deeply discharged and stored ina discharged state, etc.

• When batteries are pulled for warranty adjustment, the manufacturerwould have hard data on potential abuse of the battery.

•Same device might include an LED warning light that illuminatesbelow 25.0 volts . . . Battery in storage can say “Charge Me!”

• There is substantial anecdotal information suggesting that a majorsource of battery-killing stress in field is failure to shut of hot-batterybus accessories in some models. This will run the battery downcompletely. Batteries stored in this condition are VERY difficult torecover. The battery “black box” would record these events.

• When the battery makes it past the warranty period, data gleanedwould provide hard data feedback on battery performance and batteryusage. This type of information would be invaluable to themanufacturer in making process tweaks to design to maximizeperformance.

• This data would also cut through the fog of marketing hype andpermit considered recommendations of one brand over another withrespect to overall battery performance and best value for cost-of-ownership issues.

• At this time It’s not clear that capacity based, service life of pure-lead,thin-plate Enersys products will outperform Concord cast-plateproducts a factor of 2:1

Summation on Batteries . . .

• Batteries are like houseplants: Peak performance is achieved withoptimum control of deleterious and helpful environmental conditions.

• When a battery is fully discharged and allowed to sit, irreversibledamage MAY result . . . Not all batteries can be recovered from thiskind of abuse.

• Service life is strongly influenced by owner, pilot and maintenancebehavior. A battery “likes” to be moderately challenged often.Batteries stored for long periods of time will do better with consideredattention . . . Like Battery Tenders.

A Brief Course in Electrochemical Energy Storage

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A Brief Course in Electrochemical Energy Storage€¦ · A little history . . . • 1800 Volta...

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