Most solar panels actually output somewhere in the range of 15-18 VDC, depending on the design, strength of sunlight and position of panel to the sun. A 12 VDC lead-acid battery requires 14.4-14.7 VDC during the bulk charging phase, depending on the chemistry and construction of the battery.
There are basically three types of solar panels out there. The polycrystalline and monocrystalline panels are rigid and usually aluminum framed with a glass cover. These are the most efficient of the panels, however they're heavier and more difficult to mount. The multi-cell construction of these panels leaves them vulnerable to panel shading problems—where even a small amount of shade on the panel will greatly diminish its output.
The third type is the amorphous thin-film solar cell panel, which doesn't have individual discrete cells as do the two previous types. These are usually vapor deposited onto a metal backing substrate and the panels are semi-rigid. Generally unframed and and often designed to mount right to the cabintop. They can also be walked on, unlike the two previous types. However, they are far less efficient, and generally require panels to be almost twice the surface area of the other type to get the same output. However, due to the fact that they're essentiall one large cell, rather than a group of smaller cells linked together, they are less affected by panel shading, and also generally work better on overcast days.
Heat is a major enemy of solar panels, and their output drops significantly as the panels heat up. Keeping them cool is an important part of keeping them running efficiently.
As for what is a better panel, and how much they should cost—that is really dependent on the installation. For instance, many of the big racing boats go for the amorphous silicon panels and have them fastened over the cabin top. This allows them to use the cabintop for power regeneration, but doesn't require that the area be "off-limits" as the use of poly/mono-crystalline panels would require—since you can effectively walk on most amorphous silicon solar panels without doing damage. A cruising boat, with dinghy davits would probably be wise to go with rigid mono/poly-crystalline panels, to minimize the panel size needed.
One thing that helps considerably is using an MPPT-type solar charge controller. MPPT, or Maximum Power Point Tracking technology is basically an charge controller with a high-frequency DC-to-DC converter and a CPU that tracks and monitors the battery voltage on a regular basis and adjusts the voltage and amperage of the output, trading excess voltage for increased amperage. They can increase effective battery charging by almost 30% in the best cases. The excess voltage of a solar panel is generally shed as waste heat, and is pretty inefficient when it comes to charging the batteries—by lowering the panels output voltage to the minimum level required by the battery at the given point in the charging cycle, the amperage can be increased, effectively increasing the number of amp-hours a battery will receive for the given time period. BlueSky and Outback make two of the better MPPT type solar charge controllers.
Please note, if your solar panels are larger than just a trickle charge or "maintenance" level charge, you really do need a charge controller of some sort to prevent the panels from frying your batteries. A good inexpensive charge controller that is often used by solar panel setups is something like the Flexcharge NC-25A, which can handle up to 25 Amps of charging current.
they all claim to charge car batteries but when you look at the specs they say 12 volts max!!
correct me if I'm wrong but it takes 14+ volts to charge a battery right???
so explain this and whats the better kind of panel and how much should it cost (per watt)???
