# Solar power module .. continued

We will be using the KXOB22-04X3L solar cell from IXYS.

• Voltage (at max power output) – 1.5 V
• Current (at max power output) – 13.38 mA
• Max power output – 1.5 * 13.38 -> 20 mW

Let us calculate the upper limit on the amount of energy which can be produced per day by this cell assuming that the cell operates at the maximum power point for 8 hours.

20 mW * 8 -> .16 Watt-hours -> 576 joules

The V80H battery has a usable capacity of 80 mAh which translates to 80 * 1.2  -> .096 watt-hours -> 345 joules

Let us calculate the node’s power consumption. The CC1101 based node has a data rate of 38.4 kbps. Assuming packet size of 64, a packet and it’s ack will consume 20 milli-secs at max. Assuming average power consumption of (35 mA * 3 V) -> .105 W, energy consumed every time the RFD wakes up comes to .105 * 20 / 1000 -> .0021 Joules. Assuming the DC-DC converter operates at 70% efficiency, the RFD energy consumption comes out to .0021/.7 -> .003 Joules.  Let us assume that we are able to use just 1 % of the max possible solar output. This comes to 576/100 -> 5.76 Joules.  Number of times RFD can send data/receive ack comes to 5.76 / 0.003 -> 1920 which translates to a sleep duration of 86400/1920-> 45 seconds. Note that we are ignoring the energy consumed when the node is sleeping.

One thing to note is that if the node’s peak current consumption is say 35 mA, the battery’s peak discharge current will be (3 / 1.2) * 35 -> 87.5 mA, This is assuming the DC-DC converter operates at 100 % efficiency. Assuming efficiency of say 70%,  max discharge current (Ib) can be calculated as follows –

1.2 V * Ib * .7 = 35 mA* 3 V

Ib = 35 * 3 / (1.2 * 0.7)

Ib = 125 mA.

The V80H can sustain a continuous discharge of 140 mA whereas our requirement is  a burst discharge  (125 mA for 20 milli-seconds). We are covered.

Varta mentions sensor networks power supply as one of the applications for the V80H.

Let us look at the charge and discharge curves of the V80H. The curves (shown below) are given here – http://www.batterystore.com/content/Spec_Sheets/55608%20101%20501%20-%20V%2080%20H%20%20Product%20Information.pdf.

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As shown in the figure above, if the battery is continuously charged at 0.2 CA (which corresponds to 0.2 x 80 -> 16 mA), it will take between 7 to 8 hours for the battery to get fully charged.  If the battery is continuously charged at 0.1 CA (which corresponds to 0.1 x 80 -> 8 mA), it will take between 16 to 16 hours for the battery to get fully charged.In our design, the solar cell will be operating at 1 Schottky diode’s drop voltage above the battery voltage. The Schottky diode is used to prevent the battery from discharging through the solar cell. This Schottky diode (DB2J20900L) has a forward voltage drop between 0.3 V and 0.5 V (for currents between 10 mA and 500 mA). The solar panel will be operating roughly between (1.1 + 0.3 -> 1.4 V) to (1.4 + 0.3 -> 1.7 V).  The solar panel current should be around 11-13 mA on average (assuming good illumination). The figure also shows how the cell voltage changes when charged. It can go up to 1.5 V if charged at 15-16 mA.

Let us look at the discharge curves next.

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The figure above shows the available capacity at different discharge rates. If the battery is continuously discharged at 14 mA, it’s effective capacity is above 80 Ah (battery voltage drops below 1 volt after releasing 80 mAh of it’s energy to the load). On the other hand if the battery is discharged at a high rate of 140 mA, it’s effective capacity is only 16 mAh (battery voltage drops below 1 volt after releasing  just 16 mAh of it’s energy to the load).  These graphs are for continuous loads but a sensor node operating as an RFD  will mostly consume high current (> 100 mA) in very short bursts (compared to the time the RFD spends sleeping).  I need a setup to  monitor the V80H’s output voltage with a WiSense RFD as it’s load. I will configure the sensor node to send out data at different frequency (once every second, once every 10 seconds etc) until the battery voltage drops below say 0.9 volts. This setup won’t have the solar cell.  This data will help us decide the threshold level below which node should stop using the radio or perform any other high energy activity.