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Old 15-04-2018, 19:22   #1
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No battery solar system?

Hi all!
I am competing in this solar boat race and one of their rules was that for a sprint race and a slalom race, the batteries have to be disconnected. We have 4 panels, each rated at 64W(18V), a victron MPPT 75v/15amp charger (blutooth activated), 240W DC motor (up to 48V), 3000W PWM motor controller.

My question is, how should I configure this electrical system with solar power only? Batteries have to be disconnected and not operable.

Things that I've tried:
I put 4 panels in series (72V) and connected them to the MPPT charger. Then the load I connected them to the PWM motor controller. This was not ideal since the motor barely got any power(very slow), and the panels would over volt up to 76V which was past the 75V as advertised and system goes into overvolt protection.

I also thought of connecting the leads of the solar panels in 36V config (2 parallel, 2 series) directly onto the PWM motor controller but decided that it was safer to ask y'all for some help first. I am open to buy a new MPPT controller that goes over 75V and return the old one, only if that is the best thing to do.

controller link: https://www.amazon.com/Victron-Smart...ords=mppt&th=1

Solar panels (MSX64) link: https://www.solarelectricsupply.com/...arex-MSX64.pdf

Thanks in advance!
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Old 18-04-2018, 20:38   #2
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Re: No battery solar system?

I'm pretty sure an MTTP is not going to work when not connected to a battery. it likely needs battery voltage sensing to work and to turn on. but I could be wrong. a PWM would likly put out power all the time. not sure either.

but this is one the stupidest things I've ever heard a battery is one of the most important safety features of an entire boat. it runs your VHF and nav lights...

you'd probably need 500w of solar to run a 240w motor. panels will never get 100% power.
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Old 18-04-2018, 20:59   #3
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Re: No battery solar system?

It is for a competition so I don't make the rules...
We do have batteries (180Wh) but its only for an endurance race.
I've been thinking about using capacitors, how can I wire these onto the MPPT controller?
https://www.amazon.com/coolestore-Fa...et_feature_div
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Old 18-04-2018, 21:02   #4
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Re: No battery solar system?

Quote:
Originally Posted by smac999 View Post
I'm pretty sure an MTTP is not going to work when not connected to a battery. it likely needs battery voltage sensing to work and to turn on. but I could be wrong. a PWM would likly put out power all the time. not sure either.

but this is one the stupidest things ever heard a battery is one of the most important safety features of an entire boat. it runs your VHF and nav lights....
Easy, he said "solar boat race." With a 240w motor I don't think we are talking ocean going yacht here. Plenty of canoes, kayaks, Jon boats, and sailboats running around without any electrical system whatsoever.

But I would agree skip the mppt. Panels wired to 36V should be unable to harm a system set up for 48V, so seems worth a try. Also not an expert though.
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Old 19-04-2018, 04:24   #5
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Re: No battery solar system?

Without a battery you have no need of the mppt controller.

If your pwm motor controller will accept a wide range of input voltages you can connect the panels directly to it.

Otherwise, do away with both and connect the motor directly to the solar panels. Itís a boat race, so other than maneuvering that can be done with a simple on/off switch and bursts of power youíll want full power anytime there is sun, so why introduce all p the losses associated with the controllers?
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Old 19-04-2018, 10:40   #6
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Re: No battery solar system?

You need a specialized controller, those designed for charging batteries usually require batts to be connected.
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Old 19-04-2018, 10:41   #7
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Re: No battery solar system?

Check out EV hobbyist forums
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Old 19-04-2018, 22:31   #8
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Re: No battery solar system?

DC motors generally consume high current at low voltage at startup or stall and as they rev up the back voltage increases whilst the current demand falls. This being so you may get a better result from panels in parallel at startup switching to in series as the boat accelerate and the back emf increases.

I don't have a clue as to how you would implement such a scheme.
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Old 20-04-2018, 06:44   #9
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Re: No battery solar system?

I vote connect the panels directly to the motor control. MPPT and PWM are designed to keep voltage at some set point. Most DC motors don't much care what voltage, the more the faster.
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Old 20-04-2018, 13:23   #10
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Re: No battery solar system?

A supercapacitor *can* be defined as a battery, so you might want to expressly check into that issue with the rules committee. It doesn't matter if you win, if you are then inspected and DSQ'd on technical grounds.

Some MPPT controllers adjust their output voltage to lead the battery voltage, so if there's no battery...they would go into an error mode and presumably shut down.

Whether your controller does that, whether the supercapacitor would be allowed (in which case, you just hook it up as if it were a battery, presuming it is rated highly enough) kind of moot points, until you just try running directly off the solar panels. That might just be good enough, unless your motor would run faster with the MPPT's output settings.
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