PV Gap

Emily Said:

Solar Boat Charging help?

We Answered:

You need to fit a second battery and automatic switching system, to stop your starting battery from running down.

Solar panels are good. They are not rated on their physical size, although this is relevant to the output. You will be surprised at the area needed just to provide enough current to justify a charge controller, on a 17 footer you will find this pretty awkward, although a solid Targa or Bimini top can be a good place to fit panels if you can support it well. At best a panel mounted on the cabin top with zero shade can only approach full rated output for around four hours a day should the sun be directly facing the panel. In practice a solar panel is superb on a boat for keeping batteries topped up while parked, but not much use at providing serious current to run a stereo - a panel the size you seem to be suggesting might be around 10 - 20 watts. That size panel would peak at around 400 - 800 mA into a 12 V load. That is perfect for maintaining the batteries, but no good to run anything off itself. It all helps though.

Minnie Said:

Wow, isn't solar power really unimpressive ?

We Answered:

Well, it sounds like you have not set the solar cells up properly. The sun creates a tremendous amount of power that is incident on the earth, about 1000 Watts per square meter. A reasonable solar cell should be able to convert about 10% of that into power, giving you about 100 W/m^2 or a 10cm by 10cm cell should give you 1 W (that only counts the active area of the cell). By contrast, a 9V battery, run in a way that would drain it in 4 hours would provide about the same power (1 W). Of course, no one uses up a battery that quickly, so a more realistic power from a battery is about 0.05 W. That means the 100 cm^2 solar cell should be able to power quite a bit more than a 9V battery. Therefore, it really sounds like you may have hooked up your system wrong.

The voltage that you get out of a solar cell is pretty small. If you wired them up in parallel, your output voltage would be small as well. And if you hooked them up in series, your output current might not be big enough. And if you hooked them up reversed... well... then it wouldn't work very well at all. I don't know how you hooked them up or how much surface area you have, or how you are trying to apply the electricity to your devices, or even how much you know about all this. But if you can't run a calculator on something that is bigger than the typical solar cell on a hand-held calculator, it sounds like you must have done something wrong. And given that you are trying to run a fan (often times these are AC devices that need 120 V), a LED (these are DC devices that need very little voltage but MUST be setup in the correct circuit and receive the correct current and voltage), and a calculator (these often run on batteries with out any external connections), it sounds like you may not realize what each device needs or what you are giving it.