PREFERRED RELIABILITY PRACTICES. Practice:

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1 PREFERRED RELIABILITY PRACTICES PRACTICE NO. PD-EC-1103 PAGE 1 f 6 Octber 1995 Ni-Cd CONVENTIONAL SPACECRAFT BATTERY HANDLING AND STORAGE Practice: Flight prjects develp and implement handling and strage prcedures fr Ni-Cd flight batteries when applicable t minimize deteriratin and irreversible effects n flight perfrmance due t imprper handling and strage. Benefit: The prcedures described in this practice are specifically fr Cnventinal Ni-Cd batteries and are nt necessarily applicable t Super Ni-Cd batteries. Ni-Cd batteries are perishable and their ability t satisfactrily cmplete missin life is directly related t prudent handling and strage prcedures. The develpment and implementatin f apprpriate prject-unique prcedures based n a set f prven guidelines assure that the ptimum perfrmance f Ni-Cd batteries is nt degraded due t inapprpriate handling and strage. GSFC Prgrams That Used Practice: Slar Maximum Missin (SMM); Landsat; Earth Resurce Budget Satellite (ERBS); Internatinal Ultra-vilet Explrer (IUE); Csmic Backgrund Explrer (COBE); Gamma Ray Observatry (GRO); Tracking and Data Relay Satellite System (TDRSS); Natinal Oceanic & Atmspheric Administratin/Televisin Infrared Observatin Satellite (NOAA/TIROS);Gestatinary Operatinal Envirnmental Satellite (GOES); Upper Atmspheric Research Satellite (UARS); Extreme Ultravilet Explrer (EUVE) Center T Cntact Fr Mre Infrmatin: Gddard Space Flight Center Implementatin: The Ni-Cd cell is cnstructed f psitive plates (nickel electrdes), negative plates (cadmium electrdes), and a separatr material that is interleaved with the plates and serves t insulate the psitive plates frm the negative plates and retain the electrlyte. The plates are cnnected t the respective cell terminals which are attached t a cell cver and inserted in a steel case and welded shut. The electrlyte is nrmally 31 percent cncentratin f ptassium hydrxide GODDARD SPACE FLIGHT CENTER

2 PAGE 2 f 6 and is added thugh the "fill tube" which is fitted with a pressure gauge. After the cell satisfactrily cmpletes its manufacturing and acceptance testing, the fill tube is pinched ff and welded clsed. The manufacture f a hermetically sealed Ni-Cd is predicated n a delicate balance between the active material, the relative state-f-charge f the active material between the psitive and negative plates at the time the cell is sealed, the amunt f electrlyte placed in the cell at clsure, the prperties f the separatr material, and the free vlume allwed by the case design. The aerspace Ni-Cd cell, which has n free r excess electrlyte, is referred t as an "electrlyte starved" design. The primary prerequisite fr a sealed-electrlyte starved cell t perate safely is that the psitive plates be limiting n charge s that nly xygen is generated during vercharge. During charge sme f the current is utilized in the generatin f xygen gas and when in vercharge, all the current is used in generating xygen. This causes the cell pressure t increase t a level that is dependent n the recmbinatin rate f xygen at the negative electrde, the rate f diffusin f the xygen thrugh the separatr, the amunt f electrlyte in the cell, and the cell free vlume. The cell pressure at 20 C can typically be in the range f 50 t 65 PSIG. The negative plates f a cell cntain apprximately 50 percent mre capacity than the psitive electrde. Of this "excess" negative capacity, apprximately 60 percent remains uncharged when the psitive plates are fully charged. This uncharged material is referred t as "vercharge prtectin" and is required t prevent the plates frm becming fully charged and generating hydrgen gas. The remainder f the excess negative material is in the charged state when the cell is fully discharged and prvides ver-discharge prtectin. It is referred t as precharge. On discharge, when the cell vltage drps belw 1 vlt, the psitive plates are limiting, thereby leaving charged cadmium material t react with any residual xygen when the cell is cmpletely discharged. Typical pressure f fully discharged cells is 3 t 5 PSIG. A secnd reasn fr the psitive plates t be limiting n discharge is t prevent the effects f negative capacity fading, which ccurs during nrmal use, frm causing lsses in cell capacity. It is thught that capacity fading is related t the sizes f the cadmium crystals. It is mst imprtant that the vercharge prtectin is available fr the entire life f the cell. Shuld the negative plates becme fully charged, hydrgen gas is generated during vercharge and there is n effective mechanism within the cell fr the recmbinatin f H 2 gas. If a cell is ver discharged (ptential reversed) H 2 gas is generated at the psitive electrde at a rate dependent upn the discharge rate. Because f the limited free space in a sealed cell, a cell that is reversed can quickly build up pressure and rupture the cell case r battery package. The Ni-Cd cell is a highly cmplex, interactive electrchemical device where the present and future perfrmance is ttally dependent n its past histry. This histry includes the attributes and characteristics f the raw materials, the prcessing f these materials int cmpnents, the assembly f these cmpnents int a sealed cell, and all testing, handling, and strage.

3 PAGE 3 f 6 Cnsequently, a cell r battery is classified as perishable and treated accrdingly. Because Ni-Cd batteries can be irreversibly degraded by imprper use and handling, the fllwing guidelines were develped fr the use f battery engineers in preparing prject-specific Battery Handling and Strage Requirement Dcuments: Guideline N. 1 - Flight batteries shuld be maintained in a discharged and shrted cnditin and stred at cld temperatures when nt required fr "critical" spacecraft testing. The electrchemical activity is at a minimum in the discharged state and when stred at the ptimum strage temperature f 0 C. A battery, stred discharged and shrted up t three years since cell activatin, is expected t prvide several years life f nminal perfrmance in rbit. Guideline N. 2 - Flight batteries shuld nt be subjected t extended spacecraft integratin and test activities. The pen circuit and intermittent use f Ni-Cd batteries during extended spacecraft integratin and testing activities are knwn t significantly accelerate the degradatin f batteries. Results frm cntrlled tests have shwn permanent and irreversible changes unlike anything bserved after several years f spacecraft flight peratins. Degradatin is bserved initially as an increase in cell vercharge vltage at lw temperatures which is indicative f lss in vercharge prtectin. Als, integratin and testing use prmtes significant cadmium migratin. Bth f these are recgnized as the dminate wear-ut mechanisms which determine battery life. Guideline N. 3 - The use f charged batteries after an pen stand shuld be initiated with a 3 t 5 minutes discharge prir t initiating battery charge. Typically the discharge is dne with spacecraft lad and in cncert with the spacecraft grund pwer cnsle. During nrmal cycling use, the battery is discharged fllwed by a recharge and sme vercharge. In this mde, there is always a partial pressure f xygen frm the vercharge with xygen recmbinatin ccurring at the negative electrde. In a relatively shrt time n pen circuit, the xygen recmbines and the internal cell pressure returns t a vacuum. Charging cells that are fully charged in the absence f xygen creates an "unnatural" cnditin, since there is n xygen available t react with the negative electrdes. Past experience shws that this technique reduces the effects f pen circuit stand n perfrmance. Guideline N. 4 - During perids when the battery is nt needed t supprt spacecraft test and peratins, it shuld be maintained n a lw rate trickle charge, (C/100).

4 PAGE 4 f 6 Trickle charge at lw rates is preferred t pen circuit stand fr a battery. While there are degradatin mechanisms assciated with trickle charge, data frm cntrlled tests indicate that this t be much less detrimental than pen circuit stand. Guideline N. 5 - Cling shuld be prvided t maintain battery temperatures at abut 18 C during spacecraft integratin and test peratins. Expsures t elevated temperatures accelerate cadmium migratin and separatr breakdwn which depletes the vercharge prtectin in the cells. Since the rate f separatr degradatin increases by a factr f 3 fr every 10 C rise in temperature, strict adherence t this guideline is advised. Guideline N. 6 - A battery stred discharged and shrted fr a perid greater than 14 days shuld be activated with a "recnditining cycle" prir t placing it in use. The recnditining cycle is perfrmed at 20 C and is defined as fllws: 1 a). Recharge at C/20 fr 40 hurs (+/- 4 hurs) b). Discharge at C/2 cnstant current rate until the first cell reaches 1.0 vlts. c). Drain each cell with a 1 hm resister t less than 0.03 vlts. d). Shrt each cell fr a minimums f 4 hurs e). Recharge battery at C/20 cnstant current rate fr 40 hurs (+/- 4 hurs) Guideline N. 7 - Charged batteries shuld nt stand n pen circuit fr mre than 14 days. Charging shuld be initiated nly after implementing Guideline N. 3. When cells are n pen circuit "self discharge" ccurs which results in the frmatin f large cadmium crystals. Cntrlled tests have shwn capacity lss f just under 1% per day at 23 C and abut 1.5% at 35 C. The self discharge rate f each cell is nt identical, cnsequently after extended perids f pen circuit, there can be an appreciable capacity lss and capacity divergence between cells in a battery. This can be remedied by discharging the battery as described in Guideline N. 3 and trickle charging the battery several hurs t bring all cells int balance. Guideline N. 8 - A battery shuld be "recnditined" if it has been n pen circuit, subjected t intermittent use, i.e., pen circuit, trickle charge, ccasinal discharge, etc., fr a cumulative perid f 30 days. Recnditining is effected by perfrming the fllwing sequence at 20 C: 1 C/*** indicates recharge r discharge rates in terms f name plate battery capacity, e.g., C/20 = 1/20 f the capacity listed n the battery nameplate

5 PAGE 5 f 6 (a) Discharge at C/2 cnstant current rate until the first cell reaches 1.0 v/c. (b) Drain each cell with a 1 hm resistr until each cell's vltage is less than.03 V/C. (c) Shrt each cell fr a minimum f 4 hurs. (d) Recharge battery at C/20 cnstant current rate fr 40 hurs (+/- 4 hurs). (e) Repeat steps a, b, and c. (f) Charge battery at C/10 cnstant current rate fr 16 hurs ( +/- 4 hurs). (g) Repeat steps a, b, c, and f. Exercising the active material by peridic discharge f each cell fllwed by a lw charge helps retard permanent change in the crystal structure f the cadmium electrdes and frces electrlyte redistributins within the cell. The less frequent the recnditining cycles, the less effective the recnditining cycle is in restring the discharge vltage f a battery. Guideline N. 9 - Flight batteries shuld be discharged and cells shrted during shipment. Batteries shuld be packaged t exclude humidity and cntrl temperatures t 5 C (+/- 5 C) and the shipping cntainer shuld be equipped with temperature recrders t prvide assurances that flight batteries have nt been expsed t temperatures exceeding 25 C. A Ni-Cd battery can deliver very high currents if shrted. High currents wuld create a safety hazard as well as destry the battery due t the excessive heat that wuld be generated. Elevated temperatures enhance the rates f electrchemical reactin and separatr hydrlysis. Guideline N The final recnditining f flight batteries shuld be perfrmed at least 14 days prir t spacecraft launch. Upn cmpletin f the recnditining, flight batteries shuld be kept n lw rate trickle charge until launch. Use the recnditining sequence defined in Guideline N. 8. The recnditining cycle restres the battery discharge vltage t "like new" cnditin by enhancing the frmatin f small cadmium crystals and electrlyte redistributin. A cmplete discharge establishes capacity balance fr all cells within a battery. The lw rate trickle ensures that the battery is maintained at full state f charge fr launch. Guideline N The design f flight batteries shuld include the fllwing prvisins fr grund cnsle interfacing with the batteries while integrated in the spacecraft: a). Signal lines fr mnitring ttal battery vltage, charge and discharge currents, battery temperatures, and individual cell vltages b). Capability t charge and discharge the battery frm the grund test cnsle c). Capability t place a resistr and a shrting plug acrss each individual cell Capability must be prvided t mnitr the state f health f batteries and t discharge, charge,

6 PAGE 6 f 6 trickle charge and recnditin batteries withut pwering up the spacecraft in rder t meet guidelines t minimize degradatin f the batteries. Guideline N A lg bk shuld be maintained n each flight battery including the cmplete test histries f each cell, f the assembled battery, and f all integratin and test and launch site activities. Each lg bk shuld identify the prject and battery and individual cell serial numbers. Chrnlgical (date and time) entries fr all test sequences, summary f bservatins, identificatin f related cmputer stred recrds, malfunctins, names f respnsible test persnnel, and references t test prcedures cntrlling all tests shuld be recrded. Since Ni-Cd batteries are perishable, their ability t satisfactrily cmplete their missin life is directly related t their strage, their grund use, and handling. Histrical perfrmance infrmatin is required t ensure their flight wrthiness at launch time. The referenced dcument prvides additinal details cncerning the degradatin mechanisms f Ni-Cd cells and hw these mechanisms are affected by imprper grund handling. The reference als includes a synpsis f Ni-Cd cell design and evlutin ver 30 years f space flight n GSFC satellites alng with a chrnlgical review f key elements which influenced the current design f the Ni-Cd cells. Technical Ratinale: Ni-Cd batteries can be damaged and irreversibly degraded thrugh imprper use and handling prir t launch. The 12 guidelines prvided abve were develped ver many years f experience in the use, handling, and testing f Ni-Cd batteries. Fllwing these guidelines ensures that flight batteries are nt irreversibly degraded and have been prperly recnditined and prepared fr launch. Impact f Nnpractice: The impact f nt fllwing this practice wuld very likely be that flight batteries wuld be irreversibly degraded due t imprper handling and preparatin fr launch. This culd result in the failure f batteries t meet flight perfrmance requirements and als pssibly early catastrphic failures. References: 1) Handbk Fr Handling And Strage Of Nickel CadmiumBatteries: Lessns Learned - NASA Reference Publicatin February 1994