Part 3-battery Packs For Thermal Extremes

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Part 3-Battery packs for thermal extremes Nihal Kularatna Source credit: Vanzwol, J., Designing battery packs for thermal extremes, PET Magazine, July 2006, pp 40-45 Main components of a typical battery pack • • • Unique design considerations are required for extreme temp battery packs Range may be as wide as -400C to +800C Cells are the main elements which gets seriously affected by temp [ Not the electronics associated in general] Rechargeable packs •SLAs can operate within about -400C to +700C •NiMH between -20 0C to +700C •Li-ions for high temp packs •Most Li-ions between -20 0C to +600C •There are new chemical formulations of Li-ions which can extend range to -300C to + 800C •Li ions require greatest degree of protection •Thermal shut down •Exhaust vents •External circuits for overcharge/over discharge control 1 Li-ion temperature behaviour •Performance of Li-ions tend to drop below 00C •Voltage droop at 2.0 A is more pronounced at -200C •Cell electrolyte will freeze with further temp drops •If stored at -500C irreparable damage may occur under certain conditions to the internal separators •Storage temperatures affect the subsequent performance of Li-ion •Under optimal storage temp of 200C a fully charged cell will have only about 1% self discharge •However at elevated temp of 600C or a 12 months period it can rise to 40% A cell stored at 600C, for 12 months at 50% SOC could have a recoverable capacity of 90%. Reason for storing cells at 40- 50% SOC! Primary batteries provide better range •When rechargeable cells don’t perform ok, lithium primary cells becomes the option •They have •approx 3.6 V nominal voltage •optimal load currents less than 5C •average energy densities around 260Wh/kg •Negligible self discharge •Can operate down to -400C •Some chemistries acceptable are •Lithium thionyl chloride (LiSOCl2) •Lithium manganese dioxide (Li MnO2) •LiMnO2 uses a solid cathode •LiSOCl2 uses a liquid cathode Liquid cathode systems suffer voltage delays at lower temperatures 2 Temperature monitoring requirements in wide temp battery packs • • Monitoring of temperature at different points will be quite important for high temp battery packs Specific (thermal) design principals need be used – – – – Pack circuit should have a thermal sensor to disconnect cells at a specified temperature [ When charging or discharging occurs at high rates, temperature rises occur, and can be very dangerous] Placement of electronics is also critical , particularly for heat generating components such as FETs etc Packs should be designed with vent holes to dissipate the generated heat Multiple vent holes may be needed • Heater may be embedded in the pack to warm cells prior to use • The host device can be designed to pulse discharge cells prior to use [ Technique is applicable in systems where cycle is predictable ] Supercapacitors may be embedded in the pack for immediate energy supply! – • This can be powered from the main cells or by an external source like a charger or another battery pack Placing thermal sensors in appropriate places can be important 3