Power Design

1. Primary Battery

1-1.Batteries

There are many kinds of batteries, but probably Ni-Cd battery or Ni-H2 battery is good for CanSat. Researching about these batteries is under making.

We find Ni-H2 secondary battery in the web as follows:
( These parameters are not ascertained by experiments. )

        - Voltage: 1.2 V
        - Capacity: 650 mAh (min. 600mAh)
        - Size: (Diameter) 10.5 mm x (Height) 44.5 mm
        - Weight: 13.0 g
        - Cost: $4.0  

At now, we cannot judge whether this is the best selection, and plan to keep on researching. But it is expected that to find batteries that can meet every requirements is not so difficult.

1-2 Life time analysis

Rough analysis concerning about the CanSat's operational life time using only primary battery is given here.

      Assumption:
        - Battery: 1.2 V, 600 mAh primary Battery
        - Continuous power consumption: 25 mW 
        - Power consumption to communicate: a 50% converter, 
          a 50% efficient 100 mW transmitter and 50 mW for other 
          electronic controls. ( Total 300mW )
        - Communication time: 5 minutes/day  

After all, 625 mWh/day is required. CanSat's operational life time is

        (1.2 x 600) / 625 = 1.15 days  

Operational life time must be proportional to the number of batteries. ( For example, 2.30 days using 2 primary batteries. )

2 Secondary battery with Solar cell

2-1 Basic of Solar Cell

1. Cell can be cut as we like. But do not cut the electrodes.
2. Voltage of the cell depends on the material. Area of the cell and the direction of input light affect on the current we can get from the cell.
3. Diode must be attached to every one series to avoid the back current.
4. Put cells parallel to get more current, and series to get more voltage. If one cell is shaded in the series circuit, little current can flow and shaded cell doesn't work as a conductor. (We did experiment about this.)

2-2 Basic of Secondary Battery

1. Some Li battery is dangerous to handle;
   

      --Cobalt type    : dengerous, easily explode by short or back current.
      --manganese type : safe. 

2. Ni-Cd type and Ni-MH batteries are safe.
3. Each type of battery has its appropriate charge circuit, but general one charges to a certain level in constant-current mode, then switching to the constant-voltage mode. There are some IC which have such functions.
4. Charging efficiency of secondary battery is more than 0.9 in general. Li type has the efficiency of almost 1.0 and Ni-MH type has 0.9.

2-3 Power Capacity

Assume AA Ni-Cd batteries are used as the secondary power source. The specification of AA Ni-Cd battery is as follow:

        --Voltage 1.2V
        --Capacity 700mA  

If we use n batteries, the total capacity will be

         1.2 x 0.7 x n = 0.84n (Wh).  

2-4 Power Consumption

There are 2 operating modes;

        mode-1 : Full-operation
        mode-2 : Minimum-operation  

Cansat enters mode-1 when the satellite is in sight from the ground station. All mission conponents and communication equipments are operative in this mode. In other case, Cansat enters mode-2 in which only receiver is working. Now, assume that mode-1 consumes 300mW and mode-2 consumes 25mW. This is the same assumption as what we show in privious section;

        mode-1 : 300mW   5min. / 1 orbital period
        mode-2 :  25mW  

Then, considering that the period of approximate orbital altitude of 300km is 90 minites, the power consumption per 1 orbital period is calculated as follow;

        0.3 x (5/60) + 0.025 x (85/60) = 0.060 (Wh)  

Without solar cell, the lifetime of Cansat is estimated as;

        0.84n / 0.060 x (90/60) = 21n (h)  

2-5 Solar Cell Area necessary for Cansat

To balance the input and output of the power, consider using solar cell. The assumptions below are introduced for following calculation;

        --Power generation efficiency : 0.1
        --Attachment efficiency       : 0.7
        --Circuit efficiency          : 0.8  

Then, the power generation is calculated as follow;

        W = 1358 x 0.1 x 0.7 x 0.8 x S cos(a) = 76 S cos(t) (W)  

where 1358(W/m^2) is the sun coefficient near the earth, S(m^2) is the area of solar cell, t is the average direction of the sun. The Power Cansat can get in one orbital period is estimated as;

        P = (90/60) x (1/2) x 76 S cos(t) = 57 S cos(t) (Wh)  

Assume Cansat needs the power generation per one orbital period of 0.08(Wh) to meet the power consumption per one orbital period of 0.06(Wh) with some margin.

        57 S cos(t) = 0.08
           S cos(t) = 0.001 (m^2)  

Now, we can get the following relations between solar cell area S and the average sun direction t;

        S       cos(t)       t(deg)
        0.001    1           0
        0.002    0.5        60
        0.005    0.2        78     <-- mount type
        0.010    0.1        84
        0.100    0.01       89     <-- thin film type 

In this case, it can be seen that mount type is enough to meet the power input/output balance.

But if the power consumption becomes a little bit severer, we must consider another type like thin film.

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