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the solar panel was drawn by the LT3652 to charge two pairs of lithium ion batteries in
series, as shown in Figure 4.23.
Figure 4.23: Block diagram showing power flow from solar panel, through the
LT3652 and into the batteries and voltage regulator.
Solar power is common for powering wireless networks, and many devices have
been proposed in the literature [116, 117]. The MSX-20, 462 mm x 501 mm
semicrystalline silicone solar panel, in conjunction with the lithium ion batteries, was
selected as the main energy harvesting unit. Under standard irradiance and temperature
conditions (1 kW/m2, 25°C), the panel typically generates 20 W peak power, with an
output voltage of 16.8 V and a current of 1.19 A. The choice of the solar panel was
made based on its large power output which would be able to continuously power the
WLAN sensor during summer daylight hours. For the WLAN sensor to operate
throughout any given day of the year, the solar panel and battery sizes would have to be
dimensioned accordingly.
The battery charger is a Linear Technology LT3652 monolithic step-down
(buck) converter IC. The charger incorporates a Maximum Power Point Tracking
(MPPT) algorithm to optimize power output under varying irradiance conditions, while
at the same time varying the output voltage between 5.6 V and 8.4 V, according to the
operational requirements of the battery. The battery pack consists of an array of four 2.6
Ah rechargeable lithium ion batteries (Cylindrical, Type: 18650, Tenergy Corp.), each
with normal operating range between 2.8 V and 4.2 V. These batteries have high-energy
density and a long discharge time constant. When fully charged, the batteries allow the
WLAN sensor to transmit data continuously for 19 hours, with the advantage of being
recharged whenever required, irrespective of the amount of charge in the batteries. Each
battery also contains an internal voltage protection printed circuit board, which is
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