A little different explanation on the "shunting" types of regulators.................. I think you generally will have excess power from the panel.. After four years of always adding more power, I ended up with a 40 watt panel and an outboard charger and just have way more power than I need even for a one week trip…
Hey Walt welcome back and a great explanation of the different type of controllers. I didn't even know about the shunt type I thought there was just the PWM and MPPT. I'm wondering if Jim's isn't a PWM since it is new.
Jim for a 12 volt battery to take a charge it has to be presented with over 12 volts. Your car alternator is doing this all the time. Most of them charge at between 14.0 and 14.3 volts also when the battery needs it.
How big of a panel does someone needs is a question with out a good generic answer. You need one to fit your needs. A 40...
http://purplesagetradingpost.com/sumner/macgregor/outside-22.html
....helped us, but no where near meet our needs with my CPAP machine and the frig that we now would never give up and our computer/chartplotter usage. With it we still ran the gen-set ...
http://purplesagetradingpost.com/sumner/macgregor/outside-15.html
...15-30 minutes a day. 180 watts...
http://purplesagetradingpost.com/sumner/macgregor2/outside-33.html
...now takes care of 90%-95% of our needs and we only have to run the gen-set occasionally now. Since we broke a panel on the last trip we have replaced it with a 100 watt panel for a total of 200 watts now.
It also depends on how long you go out, 1-3 days and you can have a net loss on the battery/batteries and make it home and charge them up again. You getting past 3 days is a different story. Unless you are using the outboard quite a bit, and it has an alternator, chances are, unless you aren't using much electricity you will fall behind with even a 40 watt panel. Especially if the sun conditions are not good.
The confusing part for me was the controller types and which one would work best for our needs. Like Walt said the PWM, the most common and cheapest, just connects the panel to the battery and does this in an off/on manner. Now here is where it gets confusing and you have to take your time to get through this part, at least I have had to.
Let's take say the 40 watt panel that we have in our array that is very typical of about any 40 watt panel. We know that Watts = Amps X Volts and from that know that Amps = Watts/Volts. From that it would be easy to surmise that our 40 watt panel puts out 3.33 amps (40 watts/ 12 volts = 3.33 amps).
That won't work as if you look at the panel specs it says that its max. amperage is 2.33 amps and the max voltage is 17.2 volts. So did they lie telling us that it is a 40 watt panel? No because 2.33 amps X 17.2 volts = 40 watts, so it is a 40 watt panel. Now the problem is that when we hook it to the discharged 12 volt battery with the PWM controller it is at 12 volts. So we still have the panel putting out 2.33 amps, like Walt said, but 2.33 amps X the 12 volts now means the panel is only 28 watts (2.33 X 12). So we just lost about 30% of our wattage.
If you look at the adds for the MPPT controllers they claim they can 'potentially' produce 30% more to the battery than a PWM. How they do that is that they take that 17.2 volts at 2.33 amps and convert it down, so that what the battery gets is now 3.33 amps at 12 volts. The 3.33 amps X 12 Volts = 40 watts. So they do more than just connect the panel to the battery they convert the amperage side to your benefit.
As the battery becomes charged their benefits diminish. If you are out multiple days and using the battery up and it is low in the morning (12 volts or less) then the MPPT can make your solar panel look to be 30% larger than the PWM. We chose the MPPT for this reason and also that we are limited to how much solar we can physically mount on the boat and want to maximize what we have.
For most people that go out 1-3 days the MPPT probably isn't worth the extra money. I will say though that panel prices are coming way down and since a 60 watt panel doesn't take up much more space than a 40 if you are thinking solar maybe think about a 60. Places like Solar Blvd....
http://www.solarblvd.com/Solar-Pane...60-Watt-12-Volt-Solar-Panel/product_info.html
..have them for as low as $159. When looking at panels be sure and get what they call a 12 volt panel. The specs will say it has a voltage between 17 and 18 volts. For instance they now have a 60 watt panel for only $99 on their site, but it is a 48 volt and is not going to work in our application.
Go solar and if you want a better explanation of PWM vs. MPPT go here....
http://windsun.com/ChargeControls/MPPT.htm
Sum
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