Need Help Sizing MPPT Controller For Boat (Newbie)

montenido

Hi all, as a total newbie here, I really appreciate all the great advice and info available. I need to sift through some of the values associated with my panels to decide on the right MPPT controller. I currently have three panels, but might add two more.

I have three BP 175w 24v panels that I will be mounting on my arch. Here are some of the values for that panel: Vmp - 35.8a, Voc - 44.2, Imp - 4,9a, Isc - 5.4a.

I plan to mount the panels in parallel, to prevent shading issues. Right now I am thinking that my CC needs to service 12v batteries, accept input from 24v panels, accept 700+ watts coming in, and handle at least 25-30 amps. Is this right? If so, how do I sift through the gobbledygook in the spec sheets for the controllers?

Recommendations on specific controllers would be most welcome, along with any corrections in my reasoning or figures.

Thanks in advance, Bill

BB.

Re: Need Help Sizing MPPT Controller For Boat (Newbie)

Welcome to the forum Bill.

First, double check your BP solar panels are OK... I had similar (identical?) panels on my roof and they failed after about ~5-6 years. If you measure their output with a current meter (set DMM to 10 amp full scale, put in full mid day sun facing sun, and measure Isc -- short circuit current. My panels were about 1/2 of their rated output and had a very light "brown/rust coffee colored stain right above the junction box visible under the glass. BP did replace my panels with, then new "A" versions--but they are now out of the solar panel business--so I don't now what they will do now). If your panels look OK and test OK--then go to step two.

When making your mounts--try to make them "generic" enough that you can replace the panels later. Between the tempered glass (tools dropped on glass) and salt air--You may need to change them out over time (and new panels may not have the same dimension/electrical specifications).

MPPT charge controllers have the ability to control their output current--So you can "over panel" an MPPT controller to a degree without problems and get near full available solar output:

• 3x 175 watt panels * 0.77 panel+controller derating * 1/14.5 volts charging = 27.9 amp typical maximum

So, any controller rated at >28 amps @ 12 volts will be fine for your present needs.

Midnite, MorningStar, Outback all make fine MPPT type charge controllers. Midnite is coming out with their smaller/lower cost versions of their "high end" charge controllers "soon". Rogue has made some very nice 30 amp 12/24 volt charge controllers in the past and is "soon" releasing their next generation charge controller (should be very nice at a good price).

-Bill

Please note, my calculations were based on 3 175 watt panels... Solar Charge Controllers are rated based on their "output" current rating to the battery bank. And MPPT controllers take high voltage/low current from the solar array to low voltage/high current for the battery bank... So, if you want more than your present 3 BP panels--You will need a larger MPPT charge controller.

Also, I am guessing that you are using a 12 volt battery bank--if it is a 24 volt battery bank, we need to talk a bit more.

montenido

Re: Need Help Sizing MPPT Controller For Boat (Newbie)

Bill, thanks for your informative reply. Based on your feedback, I am planning to go with a Morningstar 45amp MPPT controller. That should work, right? I would also like to use a combiner to provide a breaker for each of the three panels and a single output to the controller. I see the breakers that are available for the combiners are quite a bit larger than 35-40 volts that each panel will max out at. Does that matter? Can I run the same sized wires for each panel to the combiner as what is coming out of the panels now, or should I go up in size? Finally, would I install fuses for each panel, and if so, would they be on the input side of the combiner? Would there be one larger fuse on the output side of the combiner between it and the charge controller, and then one more between the CC and the batteries?

I apologize for all the lame questions, but for some reason, this stuff is hard for me to absorb. Thanks to anyone who contributes to my project.

Cheers, Bill

vtmaps

Re: Need Help Sizing MPPT Controller For Boat (Newbie)

montenido wrote: »

I would also like to use a combiner to provide a breaker for each of the three panels and a single output to the controller. I see the breakers that are available for the combiners are quite a bit larger than 35-40 volts that each panel will max out at. Does that matter?

That is a voltage rating that you may not exceed. The higher the better. Choose your breakers based on current (amps).

montenido wrote: »

Can I run the same sized wires for each panel to the combiner as what is coming out of the panels now, or should I go up in size?

Your combiner should be near the panels, so you should not to increase wire size. By the way do NOT cut the wires that come off your panels.

montenido wrote: »

Finally, would I install fuses for each panel, and if so, would they be on the input side of the combiner? Would there be one larger fuse on the output side of the combiner between it and the charge controller, and then one more between the CC and the batteries?

The circuit breakers in the combiner are one per panel (or series string of panels). It is a good idea to have a breaker between the combiner and the controller which serves as a disconnect. When you connect the controller you connect the battery first, and then switch on the panels. When you disconnect the controller, first disconnect the panels, and then the battery. EVERYTHING that connects to the battery should have a fuse or circuit breaker.

--vtMaps

BB.

Re: Need Help Sizing MPPT Controller For Boat (Newbie)

Pretty much what vtMaps said...

You barely need a series breaker (usually something ~15 amps--check the data sheets for maximum series fuse) if you have three parallel panels (two panels feeding a shorted third panel).

The run from the solar panels may be home run to a combiner box in the cabin near the controller, or in a weather tight area near the array itself. Many times, the warranty on the solar panels can be voided by the mfg. if you cut off the MCx connectors (it may not matter in your case).

Personally, I think that a combiner box + another disconnect/breaker is overkill. Not needed for safety and having more devices (on a sailboat in the ocean) and the chances for failures/debugging issues--Keep it simple. Use the combiner box breakers to turn off the array if needed--And put a 60 amp breaker (45 amps * 1.25 NEC Derating = 56.25 amps, round up) for the battery to charge controller disconnect. That should be enough.

I really like to suggest both a Remote battery temperature sensor (optional with many solar charge controllers) and a Battery Monitor (and Victron is another good brand) if you are going to be using a lot of battery power when the motor/generator is not running) to keep an eye on the batteries.

And--to round out the battery kit--A hydrometer for measuring specific gravity and a "cheap" AC/DC Current Clamp Multi-Meter are pretty useful too.