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Basically, we are in vehement agreement. A buck converter is normally a component in an MPPT controller.
On Feb 11, 2020, at 1:31 PM, Ryan Meador <ryan.d.meador@...> wrote:
"Higher operating voltage panels are better" is an overgeneralization. You need to factor in the desired performance, costs of the other parts of the system, topology of your charge controllers, and your battery bank voltage.
If your panel voltage is higher than your maximum charge voltage, you'll need a buck controller to convert it back down (and you're sacrificing the power when the panel voltage is lower than the battery voltage, as you said). A boost controller will convert the panel voltage up to the charging voltage needed, so it's producing power from sunrise to sunset, including when shaded. If you want low-light performance and have a high panel voltage, you need a buck-boost controller that can do both (more expensive). You can get full performance from a boost-only controller for a low voltage panel. Or you trade performance for cost and go with the buck-only converter with your high voltage panels, sacrificing a bunch of power in sub-optimal lighting conditions.
- Buck controller
- converts panel voltage down to charge voltage
- required if your panel voltage is ever higher than your maximum charge voltage
- sacrifices power in low light conditions (morning, evening, or when shaded) because the panel voltage is below the battery voltage
- Boost controller
- converts panel voltage up to charge voltage
- collects all the power in low light conditions
- will not protect from overcharging if your panel voltage exceeds the maximum charge voltage
- Buck-boost controller
- A combination of the two, always ensures you have the correct charge voltage
- More expensive than either of the others
As you can see, a panel with a maximum voltage that is below your battery maximum charge voltage can achieve maximum performance with a cheaper boost controller. A high voltage panel is forced to choose between lower performance with a cheaper buck controller, or higher performance with a more expensive buck-boost controller.
You're correct that a higher voltage means you can use a smaller diameter wire, but you need to weigh that against the other factors. Kelly and I settled on a boost-only topology for our installation, since the 24V battery voltage is high enough that the wires aren't huge. On a 12V boat, we would probably go the other way.
Blocking diodes are a very good idea if your panels can ever be shaded, but they come with a tiny performance cost.
You also want to consider how many charge controllers you need. To get the best performance, you need a charge controller for every angle of solar panel you have. If they're all flat on top of an arch, you can get away with a single controller for all of them, but if they're curved on a bimini, you probably want one controller per panel. Additionally, more controllers gives you more performance when the panels are partly shaded because the panels can each find their own MPP.
For what it's worth, a recent issue of Professional Boatbuilder magazine considered the SunPower Maxeon cells to be the best on the market right now.
Ryan and Kelly
SM 233 Iteration
Boston, MA, USA
On Tue, Feb 11, 2020 at 11:28 AM Matt Salatino via Groups.Io <email@example.com
Keep in mind, that the marketing claims made by Victron (a very good company, by the way) fit most panels on the market today.
They all have to be “Marinized”, or waterproof, as they all exist in our ambient environment. No panels are designed for “indoor only” applications.
Another thing to keep in mind:
Higher operating voltage panels are better.
Most higher output panels operate at about 35-40 volts. This makes a much more useable panel, for a few reasons. One, the panel reaches a useable charging voltage earlier in the morning. An 18 volt panel might be able to charge batteries by 11:00 am, when it’s voltage is finally higher than the battery charging voltage. A 36 volt panel will be able to reach a chargeable voltage, maybe by 8:00 or 9:00 am.
Second, moving watts through a wire at 18 volts, requires a larger wire gauge than moving those same watts at 36 volts. At 36 volts, the wire can be half the diameter. This saves weight, and money, and makes for an easier installation.
This makes the job of the solar controller important. A simple PWM controller will waste much of the energy from the panels. Always go with a good MPPT controller. They cost a bit more (Victron makes good, economical units), but are more than worth it.
On Feb 11, 2020, at 10:41 AM, Joerg Esdorn via Groups.Io <jhe1313@...