November 2004, Issue 172

Wi-Fi Sunlogger

CHARGE NiMH BATTERIES

One strategy would be to run the system only when there is enough power to power it directly. However, that doesn’t provide much run time, and it won’t allow you to collect data when the sun isn’t near its peak capacity.

Another strategy is to use a buffer battery to store energy when the sun isn’t out. I chose a 3.6-V NiMH battery with 1,200-mAH capacity. The main reason I chose a 3.6-V system is so I can power 3.3-V electronics directly from the battery without using a regulator. I use a Schottky diode to drop about 0.3 V from the battery system. Most 3.3-V electronics actually have a wide operating range. For instance, the processor I use has a range between 2.8 and 3.6 V. However, the CompactFlash wireless card can only operate within 10% of its nominal 3.3 V. Nevertheless, the operating range is suitable for a 3.6-V battery system.

We can safely charge a NiMH battery with a trickle charge of C/10. The charging efficiency is around 66%, so it would need about 15 h to charge completely. Because the battery is rated at C = 1200 mAH, the maximum charge current of 100 mA from the solar panel is extremely safe. The battery’s capacity size depends on the type of duty cycle you expect from your system. The average solar insolation values give you an averaged value. It’s entirely possible that on some days the actual solar insolation is less than the average. The battery capacity then dictates how long the system can still manage to run. In my case, the battery could bridge the following: 

or approximately 25 sunless days, which seems more than adequate. A smaller battery with this panel would suffice.

Because the panel’s maximum current output is 100 mA, it’s safe to charge a 1,200-mA battery directly from the panel. The maximum charge time is limited by the length of the day. It’s less than 13 h in most cases. I use a Schottky diode to protect the panel from reverse current when the battery is powering the circuit.

The processor core is powered from the battery through a 2.8-V regulator, while the CompactFlash adapter is driven directly from the battery through a diode to drop voltage from the nominal 3.6 to 3.3 V (see Figure 1).