1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-04-19a71 Center Point of Contact: JSCa71 Submitted by: Wil HarkinsSubject: Manned Space Vehicle Battery Safety Practice: This practice is for use by designers of battery-operated equipment flown on space vehicles. It provides
2、 such people with information on the design of battery-operated equipment to result in a design which is safe. Safe, in this practice, means safe for ground personnel and crew to handle and use; safe for use in the enclosed environment of a manned space vehicle and safe to be mounted in adjacent un-
3、pressurized spaces.Programs that Certify Usage: This practice has been used on the Space Shuttle Program, Orbiter, Apollo Command (2) nickel-cadmium secondary; (3) nickel-hydrogen secondary; (4) nickel-metal hydride, (5) alkaline-manganese primary; (6) LeClanche (carbon-zinc) primary; (7) zinc-air p
4、rimary; (8) lead-acid secondary pressure relieved cells or cells having immobilized electrolyte; (9) mercuric oxide-zinc primary and (10) lithium primary cells having the following cathodic (positive) active materials consisting of: (a) Thionyl chloride; (b) Thionyl chloride with bromine chloride co
5、mplexing additive (Li-BCX); (c) Sulfur dioxide; (external to habitable area); (d) Polycarbon monofluoride; (e) Manganese dioxide; (f) Iodine; and (g) Silver chromate.It must be noted that lithium-based cells are subject to extremely close review and are required to have seemingly excessive hazard co
6、ntrols incorporated in their usage. They can yield extremely high energies per unit weight and volume relative to other cell types. They have uniquely hazardous failure modes. For many types of lithium batteries, there is little comprehensive data which characterizes either performance or response t
7、o abusive or off-nominal exposure. The chemicals contained in them are usually either highly flammable, corrosive and/or toxic. In their various failure modes, they are subject to leakage, venting, or violent explosions accompanied by scattered shrapnel and toxic materials. Hence, no effort is spare
8、d in providing the utmost assurance that every known or suspected failure mode is prevented by effective hazard controls. Use of other types of cells is strongly encouraged wherever feasible. Weight and volume differences alone are not necessarily sufficient justification for use of lithium based ce
9、lls.Use of batteries of any chemistry, including those listed above, may require extensive testing, evaluation and use of source controls. Certification prior to flight is always required.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Many of the ha
10、zard controls associated with the batteries, enhance performance reliability, since the battery is designed to prevent hazards which are the result of failures. For example, the prevention of electrolyte leakage and grounding in a battery case which may cause a battery explosion also prevents aborte
11、d battery operation.The content of this practice is not intended to consider every conceivable contingency. There is no attempt herein to provide knowledge on the theory and electrochemistry of batteries, except as necessary to dictate a hazard or its control.General Battery Hazards Sources and Cont
12、rolsBattery hazards can generally be broken into seven categories. These are: (1) short circuits; (2) electrolyte leakage; (3) battery gases; (4) high temperature exposure; (5) circulating currents; (6) structural; and (7) charging.Practice No. 1. Flight batteries should not be subjected to short ci
13、rcuits.Rationale. Shorts can occur in the loads served by the battery through conductive electrolyte leaks between cells within a battery or by careless contact with cell and battery terminals. Internal shorts in cells of batteries which have been prepared for flight by effective procedures are rare
14、. A sustained short can result in extremely high temperature increases. Table I shows effects of shorting relatively benign alkaline-manganese cells and batteries through about 30 milliohms. Peak currents are reached in less than one second.refer to D descriptionD Table 1: Effects of Shorting Throug
15、h 30 Milliohms High temperatures can result in surfaces which burn crewmen (118 degrees F is the specification limit for touchable surfaces), meltdown of protective plastic structure surrounding the battery, release of noxious or explosive substances (hydrogen for example) or initiation of a fire. I
16、n addition to heating, a short circuit through an electrolyte leak can decompose water in the electrolyte to Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-hydrogen and oxygen, then provide the minuscule ignition energy (1-2 micro joules) to explode
17、 the hydrogen-oxygen mixture when the short circuit current terminates with a small arc at last contact. This type of failure is considered to have caused a momentary LM descent battery short circuit during the cis-lunar leg of the aborted Apollo 13 mission. Some obvious hazard controls had been omi
18、tted to save weight because such an event was considered unlikely. Apollo 14 and later LM batteries incorporated the controls.Special ConsiderationsBatteries must have circuit interrupters which are physically and electrically close to the battery terminals and are rated well below the batterys shor
19、t circuit current capability. Interrupters may be fuses, circuit breakers, thermal switches or any other effective device. The interrupter should be in the ground leg of batteries with metal cases so that battery grounds inside the battery case (usually grounded to structure) may be sensed and inter
20、rupted.All inner surfaces of metal battery cases must be coated with an insulating paint known to be resistant to the battery electrolyte. This procedure aids in preventing battery grounds to the case through electrolyte leakage. Cell terminals must also be protected from contact with other conducti
21、ve surfaces by potting or by non-conductive barrier (e.g., plastic sheets). The parts of battery terminals extending inside the battery case must be insulated from unintentional contact with other conductors and bridging by electrolyte leaks. The battery terminals which pass through metal battery ca
22、ses must be insulated from the case by an insulating collar or other effective means. The parts of battery terminals on the outside of the battery case must be positively protected from accidental bridging. This may be accomplished by using female connector, recessing stud-type terminals, installati
23、on of effective insulating barriers, etc. Wire lengths inside the battery case must be insulated, restrained from contact with the cell terminals and physically constrained from movement due to vibration or bumping.Practice No. 2. Preventive measures must be implemented to prevent electrolyte leakag
24、e.Rationale. Electrolyte leakage can be caused by excessive free electrolyte in vented (pressure relieved) cells. Inadequate design of electrolyte trapping or baffling provisions under covers of vented cells or leakage through cracked cell containers is a major cause for electrolyte leakage. Another
25、 cause for electrolyte leakage is faulty seals on sealed cells, and leakage of electrolyte forced through seals by cells overheating or over discharging.Special ConsiderationsExcessive free electrolyte in vented cells should be corrected by performing cell tests in which the quantity of free electro
26、lyte is reduced until the cell capacity begins to be reduced. These tests must be conducted on cells whose age and cycle-life exposure is nearly identical to that proposed for flight cells. This type of test applies mainly to silver oxide-zinc rectangular cells. The cell manufacturer Provided by IHS
27、Not for ResaleNo reproduction or networking permitted without license from IHS-,-,-generally specifies a slight excess of electrolyte be used because his cells are generally recharged several times by most customers. With increasing cycles for use, the excess free electrolyte is generally depleted b
28、y both water electrolysis and absorption in gradually expanding zinc negative. Cells used in space applications are generally used on their first to fifth cycle and do not require excess free electrolyte.Cell covers can also be designed to have a cylindrical “stand-pipe“ extend downward from the und
29、erside of the cell cover toward the cell plates, from the cell vent opening in the cover. When the cell is inverted in a gravity environment, the electrolyte level collecting on the inside of the cell cover is optimized not to rise above the opening of the “stand-pipe“. This represents the worst cas
30、e. All other cell positions, including the zero “g“ are better.Cells having free electrolyte, must be fitted with relief values in their vent ports, not just an opening and/or absorbent material. Relief valve opening pressures having a range from 3 to 35 psid and are a function of the ability of the
31、 cell case to withstand internal pressure without cracking. Some steel-case rectangular NiCd cells are considered “sealed“ because they use relief valves set to open at 100 to 200 psid. These are not the hermetically sealed, space-type NiCd cells which may also be used.If inputs are feasible at the
32、cell design level, micro porous teflon plugs or sheets may be installed on the vent opening on the underside of the cell cover. Such material, if not covered over the electrolyte, will permit gas to escape but will prevent electrolyte escape due to its small pores and non-wetting property.If it is n
33、ot possible to use the above controls, absorbent material, such as non-woven polypropylene or cotton wadding, should be used to fill the void spaces in a battery container or is placed directly over the cell vents. This is a less satisfactory control since electrolyte may be trapped against conducti
34、ve parts by the absorbent material which may also be flammable. Internal surfaces of metal battery cases must be coated with an electrolyte resistant paint as well.The required prelaunch stowage of batteries in any space vehicle, has to be oriented in an “upright“ position relative to gravity so tha
35、t any free electrolyte is forced by earth gravity and the launch acceleration into the cell plates and separators and away from the cells seals or vents. This configuration decreases the chance of an in-flight leakage from occurring. If electrolyte is added at the initial design level of vented cell
36、s having free electrolyte, extension of the separator material beyond the cell electrolyte is required. This extension provides additional volume for capillary capture of the electrolyte, which then may require acceleration forces larger than 1g for dislodgment. In-flight maneuvers nearly always pro
37、vide significantly less gs of force.Practice No. 3. Flight batteries utilizing aqueous-based electrolytes should not be stored in enclosed spaces.Rationale. Hydrogen gas, mixed with air or oxygen is flammable or explosive over a wide range of Provided by IHSNot for ResaleNo reproduction or networkin
38、g permitted without license from IHS-,-,-concentrations (e.g., 3.8 percent to 94 percent in air). Accumulation of hydrogen in enclosed spaces containing oxygen must always be prevented. Aqueous electrolyte cells subjected to charging, will generate oxygen as the charge nears completion, thus providi
39、ng oxygen where none may have existed before (due to nitrogen purging). Whenever a flammable/explosive mixture of hydrogen and oxygen exist, an ignition source is presumed to exist although one may not be obviously identifiable. This condition can occur because energy required for ignition is on the
40、 order of 1 or 2 micro joules.Special ConsiderationsThe traditional means of avoiding hydrogen accumulation is to provide continuous air ventilation at a rate sufficient to continuously dilute evolved hydrogen below the 3.8 percent flammability level. For example, a lead-acid or silver oxide-zinc ba
41、ttery on over-charge is considered to evolve hydrogen at the rate expressed by the following equation:Q = 0.016 NIWhere: Q = cu. ft. H2/hr. at 1 atm. and 77 deg F N = no. of cells in battery I = charging current in amperesThus, a battery of 20 cells on charge at 3 amps evolves:Q = .016 x 20 x 3 = 0.
42、96 cfh H2To dilute the hydrogen to about 2 percent concentration in the ventilating air, the air flow must be:0.96 / .02 = 48 cfh The value Q may be corrected for temperature and pressure by multiplying it by the value:K = 1.415(T+460) / P Where: P = actual pressure in mm Hg T = actual temperature i
43、n degrees FProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-In practice, it is rarely feasible to ventilate hydrogen in normal Orbiter battery applications. Hence one or more of the following controls must be exercised whether or not charging is perfo
44、rmed on board the Orbiter.a. Avoid charging the battery in the habitable spaces of the Orbiter.b. Do not seal battery cases or provide low pressure relief valves (3 to 15 psid) on the case.c. Minimize the volume of void spaces inside the battery case by design or by adding electrolyte-resistant, non
45、-flammable filler such as potting material.d. Prohibit any component inside the battery case which may provide an ignition source, such as arching between relay contacts.e. Purge the battery completely with dry nitrogen (or any other inert gas) as soon as the battery is installed in the Orbiter.f. M
46、inimize the exposure of the battery to high temperatures.Practice No. 4. Do not expose flight batteries to high temperatures.Rationale. High temperatures is construed to mean temperatures higher than 120 degrees F. Some batteries can safely and successfully operate at temperatures well above 120 deg
47、rees F. Some cells, notably silver oxide-zinc, are subject to thermal runaway. At high temperatures, silver oxide decomposes, yielding oxygen. The release of oxygen oxidizes zinc in the negative plates, resulting in heat evolution and an increase in cell temperatures which increases the silver oxide
48、 decomposition rate. This is different from the mechanism of thermal runaway which can occur during constant potential charging of NiCd cells.Special ConsiderationsPerform a thermal analysis on the battery and its surroundings to verify probable battery temperatures under load and non-load condition
49、s. This practice is particularly necessary for high energy, high power batteries that are installed with equipment stowed in the Orbiter payload bays. Do not operate cells at loads above those set as maximum by the battery manufacturers. Provide adequate short circuit protection (See practice no.1, Short Circuits). If the thermal analysis conducted on the battery shows that it will become cold enough to require heat inputs, electrical
