Design and Implementation of On-board Electrical Power ... - OUFTI-1

Over Charge Prohibition 4.275 ± 0.025V/CellRelease4.075 ± 0.050V/CellDelay Time 1.0sec ± 300msecOver Discharge Prohibition 2.600 ± 0.080V/CellRelease2.900 ± 0.100V/CellDelay Time125 ± 37msecOver Current Prohibition 3A ± 1A (Depend on cell voltage)Releaseby ChargingDelay Time8 ± 4msecCurrent Consumption Operation Max. 5µ[A]Power downMax. 0.1µ[A]Table 3.4: Electric Characteristic of KOKAM Protection Circuit Module [20].3.5 Dissipation system (Shunt regulator)The role of the dissipation system is to prevent the voltage of the batteries to get over 4.2Vby dissipating the exceeding power. There will be one dissipation system, connected to thebatteries bus. The reliability of this system is thus critical. Its failure would result in the lossof the whole mission.The worst case is when solar panels output is maximum and no power is consumed in theCubeSat. We can show that the instantaneous power produced by our five solar panels willnever be higher than 5.5W. This value of 5.5W corresponds to an optimal exposure to sun, analbedo of 30%, operation at MPP [6]. The dissipation system must thus be able to dissipatea current of up to 1.31A (5.5W / 4.2V).3.6 Battery heaterThermal simulations were done by the THER team [7]. Results are shown in Fig. 3.22. Theypredict that, in absence of any temperature control measure, the temperature of the batterieswill fall to -21.4 ◦ C in a cold case, and rise to 36.3 ◦ C in a hot case. However, the recommendedoperating temperature of the batteries is between 0 ◦ C and 45 ◦ C. Therefore, a heater will bedirectly placed on the batteries to keep their temperature above 0 ◦ C.3.6.1 ControlThere are two possible solutions to control the heater. The heater could be controlled by theOBC, or could have an independent control. Table 3.5 shows conveniences and drawbacks ofeach solution.It was difficult to decide between these two solutions. After discussion with the OBC team,the solution of the independent control was selected. The control will take the temperatureof the batteries and let the current flow in the heater when the temperature falls below somepredetermined threshold.36