Storing enough power for the whole night
Here we have gadgets that either use very little power, or that have large solar panels and lots of battery storage. In the latter case, they tend to be more expensive to make.
The photo above shows a simple circuit that has been running day and night for a little over a month when the photo was taken. The little motor has been whirring away without pause. You can see the blur of its little gear that is spinning too fast for the camera.
The motor and the battery are in parallel, so if there is no sun, the motor runs on battery alone.
The solar panel is connected to the battery and motor at ground (the black wires).
On the positive side of the solar panel, we have a tiny diode between the solar panel and the positive side of the battery. This diode allows current to flow from the solar panel to the battery, but it blocks any current from flowing the other direction. If we didn't have the diode there, at night the battery would turn the solar cell into a big infrared LED, and we would waste all our energy warming up the solar cell. You can see the diode in the lower right corner of the breadboard, a little blue device plugged into the positive row and the bottom hole of the first column.
The diode has to be placed in the proper direction. There is a black band at one end of the diode. This is the negative end, called the cathode. The other end, the positive end, is called the anode. We connect the positive side of the solar cell to the anode of the diode, and the cathode of the diode connects to the positive side of the battery, since it is connecting the positive side of the solar cell to the positive side of the battery.
I used a big solar cell here. It is 5 inches wide and 5.5 inches tall, and can put out more than a watt of power in full sun. The batteries are three 900 milliampere-hour nickel cadmium batteries, each 1.2 volts. That means the battery pack as a whole produces 3.6 volts.
The motor is drawing 27 milliamperes of current when it is running under no load (as it is in the photo). If I add a load to it, such as holding my finger on the gear, the current goes up as the motor fights against the load. If I push so hard that the motor almost stops, the current goes up to over an ampere.
Since the batteries are in series, we have 900 milliampere-hours of current available at 3.6 volts. If they were in parallel, we would have 2.7 amperes-hours of current, but at only 1.2 volts. The energy stored is the same either way: 1.2 volts times 2.7 ampere hours is 3.24 watt hours, and 3.6 volts times 0.9 ampere hours is also 3.24 watt hours.
So how much energy does our motor use in a day? It is drawing 27 milliamperes at 3.6 volts for 24 hours. That is 2.33 watt hours. That is 72% of our stored energy.
The batteries could run the motor for 24 hours even if we increase the load on the motor until it draws 37.5 milliamperes.
But the batteries don't need to last 24 hours. During the day, the solar cell can charge the batteries at the same time it is running the motor. A rule of thumb for solar panels that don't track the sun is to assume they get the equivalent of 6 hours of full sun per day. So the batteries are only needed for 18 hours. We could run the motor at a load almost twice as high, until it draws 50 milliamperes.
The solar cell is 130 millimeters by 150 millimeters, and produces 5 volts at 500 milliamperes in full sun. That's 2.5 watts. If it gets 6 hours of full sun, it produces 15 watt hours of energy. That means it should have no trouble charging the batteries and running the motor at the same time. Even if the day is cloudy, and the location doesn't actually get 6 hours of sun in a day, we have some energy to spare.
The drawback is that the solar cell cost me four dollars. The batteries cost a dollar each. So our eight dollar power supply is considerably more expensive than the little solar garden light.