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

Figure 3.16: Capacity vs. number of cycles (cycling to 80% DoD).The orbit of OUFTI-1 will have a duration of around 103 minutes. This corresponds to5,103 cycles in one year. A DoD of 30% or less is recommended for this kind of application.The ideal capacity for our battery is thus at least 324mAh0.3= 1, 070mAh.3.3.4 RedundancySince the battery is a critical and sensible component, it is wise to add redundancy. Therewill be two batteries in parallel, connected to the main power bus (the main power bus canbe called “the batteries bus”). Using two batteries for redundancy means that the CubeSatmust be operational with only one battery. A second benefit of having two batteries is thereduction of the DoD. The DoD will be reduced to 15% or less, which will increase the lifespanof the batteries.3.3.5 KOKAM batteriesThe choice of batteries is discussed in [3]. A very interesting model was the Varta Poliflex Li-Pol cells. They are no longer being produced, and the SwissCube team strongly recommendsnot using Varta batteries that are more than two years old. Contacts have been taken (byPhilippe Ledent) with several battery manufacturers, and KOKAM accepted to provide thebatteries for OUFTI-1.Two models of KOKAM batteries were purchased, the SLPB554374H and the SLPB723870H4,the characteristics of which are compared in Table 3.3.From an electrical point of view, the SLPB723870H4 (1,500 mAh) was more interestingthan the SLPB554374H (1,250 mAh) since it has a better capacity and a lower impedance.However, the second model (i.e. SLPB554374H) was chosen for mechanical reasons. Thethickness of the SLPB723870H4 was too important to place two batteries on the batteriescard.The SLPB554374H cell has a nominal capacity of 1,250mAh. The depth of discharge willbe 26% for one cell and thus 13% for two cells. The voltage range of the batteries, and thusof the batteries bus, will be 2.7V to 4.2V. The maximum consumed power has been estimated31