JT,
Wiring the panels in series increases the voltage going to the MPPT controller allowing longer runs from panels to controller or smaller wire guage. As TT mentioned the controller outputs the correct voltage for the 12v battery. The advantage with parallel wiring for panels is less effect from shading on the total output of the panels, which on a boat can be a real concern.
James
Agreed, however I don't think the shading impact on series vs parallel panels is so clear. It depends on a lot of things, including how shading covers the panel(s), blocking diode configurations, etc. I think the best defense against shading is to use multiple controllers, but that adds expense not only in controllers, but also in disconnect switches, fuses, breakers, etc. What I do know for certain is that shading is a problem on boats. There is just no escaping it on 90% of boats. The remaining 10% are just annoyingly fortunate.
As for fusing/breakers/switches, here are some guidelines. Also follow ABYC guidelines to avoid getting dinged by a surveyor. And besides, they are good guidelines.:
- Between the battery and the controller you MUST have a fuse or breaker, and it needs to be located as close to the battery as practical. This is to protect the controller and wiring from the high current capacity of the battery, not the other way around. I prefer a breaker because it makes working on the solar system much easier when you can easily disconnect the batteries.
- You do not need a breaker/fuse on the solar input side of the controller, but I think a breaker or switch is a really good thing to have. In land based system I'm pretty sure they are required. Once again it's about being able to work on the system safely by having an easy way to disconnect the solar panels from the controller. This is especially important in higher voltage systems. Pay careful attention to the voltage and current ratings of any breakers or switches, especially if you have higher voltage panels. It's harder and harder, and more and more expensive to find breakers and switches that have high DC voltage ratings.
- In some cases you need fusing between parallel panels/strings, depending on how many strings you have in parallel. This is to prevent all the surviving panels from dumping all their current into a failed panel. In the panel specs you will find a max fuse size spec. That's the most current that can run through the panels without creating a hazard. To figure out if fusing is required, you need go back to the max current that calculated earlier (125% of Isc). That's how much current each panel/string can produce, and you then need to figure out how many such strings/panels you can have before exceeding the max fuse rating. That's how many strings you can have if one fails before you need fusing. So let's say the fuse limit for your panels is 15A, and your panels can produce 7A for each string. Two strings will produce 14A and is under the 15A limit, so that's the max you can have dumping current into a failed string. In that case you can up to three strings before you need fuses, allowing for one failed string, and two surviving strings. If you have 4 strings, then three survivors can produce 21A which will overload the failed string and create a hazard. In that case you need to install 15A fuses in each string before they are combined together. And again, watch the voltage ratings. Breakers are available up to about 125-150VDC, and above that you have to use fuses, and particular fuses rated for higher voltages.
One other thing unrelated to fuses and switches.... Be certain to wire the battery negative on the controller before any other connections. More than one controller has been smoked by doing otherwise.
