# 1.5v solar cell to replace two D batteries

I just bought a very small aerator called "Metal Marine Bubble Box" that runs off a small DC motor. Not much torque needed. I tried reading the print on it and all I could read was 1.5v. This makes sense since it takes 2 D batteries which are 1.5v. With both batteries can run for 40 hours, but it can also run on 1 D battery for 20 hours. I know a D battery's current is about 25mA. I want to eliminate batteries.

I want to build a solar cell/panel that can run it instead of using batteries, or that also will charge the batteries while running it, or something like that.

I've seen someone create a small fan from a dollar tree/store solar garden light but couldn't tell exactly how they did it.

What do I need to build it (diodes, resistors, etc.)? I'm a noob and know nothing about circuits, voltage, current needed to run this tiny motor. I'm wondering if I have to perfectly match the voltage/current.

• Well it looks like a typical D cell is 14000mah, so if one lasts for 20 hours, then your current is actually 700ma, which is pretty high. At 1.5V, this is about 1W. There are a load of considerations that you'll have to think of, including the fact that you only get a few hours of full sunlight per day Commented May 16, 2018 at 22:36
• Thank you. I think you're right about the 700ma. So the calculation to find the draw/current is mah/hours?
– plao
Commented May 17, 2018 at 14:45

Assuming you only need 25mA (big if), then yes it's likely that you can do this easily, the same way others have. You may need a solar panel with a larger current output to charge the battery, and the device may not like the lower voltage of a rechargeable battery chemistry.

simulate this circuit – Schematic created using CircuitLab

The first is a very basic. Dollar store (And more expensive) solar garden lights at this time use a led driver IC that also does turning on and off based on light and boost circuit for driving a 3V led, but essentially the same as this.

The second is a better idea. A stronger solar panel charging multiple batteries in series, and a voltage regulator to bring down the voltage to what your device needs.

Even if you need more current, the same still applies. You will just need a much better/larger solar panel. Maybe 12V 3W? The regulator option will be required at that point.

• Is it okay to connect 5V panel across 3.6V battery ? Commented May 17, 2018 at 3:36
• Highly depends on the charge rate (voltage/current), the diode in front, etc etc. Typical charge voltage for NiMH is 1.4 to 1.5V, so 5V - 0.7v silicon diode Vf drop, puts its right at the 3 cell voltage. Commented May 17, 2018 at 5:44
• Thank you for the diagram and the two options. Can I just bypass using a rechargeable battery or batteries and hook a solar panel up to this DC motor?
– plao
Commented May 17, 2018 at 14:55
• If the panel has the voltage and current needed, then yes. You wouldn't even need the diode, just the panel. Of course, it would fade on shadows and stop working in the dark. Commented May 17, 2018 at 15:40