Planning stage, solar power for boat.

First post from a novice. Need some advice by someone who can give me some guidance and best solution. Still confused how to think in a broader picture and what is best method.

I will be using 4 flexible panels, 160watt each (Vmpp: 20,52 V, Impp: 7,8 A)

I have to charge 2 separate battery banks, one is 12v 100AH and the other 24v 200AH. When weather permits I will run a bldc 24volt motor directly from panels and not the 24v bank.

So logic say I need at least 2 separate controllers (MPPT), one for the 12v and one for the 24v bank. Or maybe there is somekind of controller that can handle 2 banks?

If I connect all panels in parallel it will not work because I think the 24 volt will not charge and I can not run the bldc motor directly on sun.
Is using a dc2dc bridge something I can consider? Then I can use one bridge for the 24v bank and one bridge for the bldc motor?
Is this an option?

I could use one panel for the 12v but then its "idle" and useless when 12v is full making it not optimal.

What is the best method to achieve what I want?

A few things:
1. Flexible panels don't have a good reputation on the forum. Apparently they don't last long and underproduce.
2. Do you actually need two battery banks, or can you power your 12V loads from a dc to dc converter? If it's just small things like LED lights, I would ditch the 12V battery completely. If there are big things (winches, etc.) that run off the 12V, keep the battery and plan to keep it charged from the 24V system with a dc to dc charger. That way all your solar goes to the 24V system and you don't have to worry about a separate controller or switching, or anything like that.
3. Why would you want to run the 24V motor directly from the panels and not the battery bank? The panels will have variable output with clouds, sun angle, shadows, etc., but the 24V bank will be very stable. If the batteries are fully charged, you end up running directly off of solar anyway and if the batteries aren't fully charged you can run your motor AND charge the batteries. Maybe there's some downside I'm not seeing, but I feel like trying to exclude the batteries from the motor complicates things for no real gain.

fxSol said:

When weather permits I will run a bldc 24volt motor directly from panels and not the 24v bank.

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If tilt and orientation are perfect you may get 500-600 Watts from the panels. As another poster asked, any particular reason you do not want to use batteries as a buffer? Is your motor for propulsion? If so how much does it draw to move the hull at some reasonable speed?

fxSol said:

First post from a novice. Need some advice by someone who can give me some guidance and best solution. Still confused how to think in a broader picture and what is best method.

I will be using 4 flexible panels, 160watt each (Vmpp: 20,52 V, Impp: 7,8 A)

I have to charge 2 separate battery banks, one is 12v 100AH and the other 24v 200AH. When weather permits I will run a bldc 24volt motor directly from panels and not the 24v bank.

So logic say I need at least 2 separate controllers (MPPT), one for the 12v and one for the 24v bank. Or maybe there is somekind of controller that can handle 2 banks?

If I connect all panels in parallel it will not work because I think the 24 volt will not charge and I can not run the bldc motor directly on sun.
Is using a dc2dc bridge something I can consider? Then I can use one bridge for the 24v bank and one bridge for the bldc motor?
Is this an option?

I could use one panel for the 12v but then its "idle" and useless when 12v is full making it not optimal.

What is the best method to achieve what I want?

Click to expand...

From the research I saw most advise not running the motor direct as it under and over volts burning up the motor quickly. Most advise using the battery for a buffer for stable voltage.

Minimoose said:

From the research I saw most advise not running the motor direct as it under and over volts burning up the motor quickly. Most advise using the battery for a buffer for stable voltage.

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Probably because the motor is designed to be used with a motor controller to limit the Amps to give the user some control over power. Neither the battery or the solar panels will deliver more voltage than their specifications, but a battery can deliver a lot of Amps if the load is enough. That is not a problerm with solar panels. You have substituted a controller for for the sun limiting the Amps. I assume this is all theorectical and you do not have batteries or a motor to test. When you do that I suggest you find some way to measure RPMS, voltage and Amp draw. The RPMs and the propeller design need to be coordinated or the prop will either cavitate from too many RPMs or not be efficient for the speed of the boat.

I converted a garden shredder to run by a DC motor and the only way I could control RPMs was to vary the voltage by stacking different Lithium batteries in series until I got to a voltage that gave me the RPMs which I wanted. I think that voltage was 32 volts. Interestingly when I started out with only 16 volts the motor drew two many Amps and my BMS shut off. Increasing the voltage reduced the Amp draw. That was my solution to save the cost of a $400 DC motor controller. I think you will find that this is a much more complicated process than it may initially seem on paper.

Ampster said:

Probably because the motor is designed to be used with a motor controller to limit the Amps to give the user some control over power. Neither the battery or the solar panels will deliver more voltage than their specifications, but a battery can deliver a lot of Amps if the load is enough. That is not a problerm with solar panels. You have substituted a controller for for the sun limiting the Amps. I assume this is all theorectical and you do not have batteries or a motor to test. When you do that I suggest you find some way to measure RPMS, voltage and Amp draw. The RPMs and the propeller design need to be coordinated or the prop will either cavitate from too many RPMs or not be efficient for the speed of the boat.

I converted a garden shredder to run by a DC motor and the only way I could control RPMs was to vary the voltage by stacking different Lithium batteries in series until I got to a voltage that gave me the RPMs which I wanted. I think that voltage was 32 volts. Interestingly when I started out with only 16 volts the motor drew two many Amps and my BMS shut off. Increasing the voltage reduced the Amp draw. That was my solution to save the cost of a $400 DC motor controller. I think you will find that this is a much more complicated process that it may initially seem.

Click to expand...

No I don't have a boat. I watched several videos of people with different setups and the only one that burned his motor up didn't have a battery. Now granted he had a brushed motor and burned the brushes out. Maybe a brushless motor would fare better..I don't know. I used to have fishing boats with trolling motors and the whole works, but back in the day solar wasn't mainstream. I watched 2 different guys with unlimited range (as long as the sun was out)on small boats and thought it was interesting. Of course they had to keep a close eye on wattage of the motor and it was slow, but doable.

Minimoose said:

No I don't have a boat. I watched several videos of people with different setups and the only one that burned his motor up didn't have a battery. Now granted he had a brushed motor and burned the brushes out. Maybe a brushless motor would fare better..I don't know. I used to have fishing boats with trolling motors and the whole works, but back in the day solar wasn't mainstream. I watched 2 different guys with unlimited range (as long as the sun was out)on small boats and thought it was interesting. Of course they had to keep a close eye on wattage of the motor and it was slow, but doable.

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Think its the least problem getting the bldc motor burned when using solar. Heat can be a problem though generated by controller and motor by itself. Will use water cooling. Batteries will on the other hand deliver amps in masses....

Ampster said:

If tilt and orientation are perfect you may get 500-600 Watts from the panels. As another poster asked, any particular reason you do not want to use batteries as a buffer? Is your motor for propulsion? If so how much does it draw to move the hull at some reasonable speed?

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Reason is ego =) Its a dream of mine to run on pure solar. In perfect conditions speed will be around 4-5 knots. And if I can spare some life on the batteries its a plus. I will probably go rigid because of the increase in current which can give me more speed on the motor. But I calculated that 500 watts would be enough when it comes to propeller shaft rpm.

LakeHouse said:

A few things:
1. Flexible panels don't have a good reputation on the forum. Apparently they don't last long and underproduce.
2. Do you actually need two battery banks, or can you power your 12V loads from a dc to dc converter? If it's just small things like LED lights, I would ditch the 12V battery completely. If there are big things (winches, etc.) that run off the 12V, keep the battery and plan to keep it charged from the 24V system with a dc to dc charger. That way all your solar goes to the 24V system and you don't have to worry about a separate controller or switching, or anything like that.
3. Why would you want to run the 24V motor directly from the panels and not the battery bank? The panels will have variable output with clouds, sun angle, shadows, etc., but the 24V bank will be very stable. If the batteries are fully charged, you end up running directly off of solar anyway and if the batteries aren't fully charged you can run your motor AND charge the batteries. Maybe there's some downside I'm not seeing, but I feel like trying to exclude the batteries from the motor complicates things for no real gain.

Click to expand...

Reason for flexible is only one - weight. But I can change my mind to increase output and efficiency by using rigid panels. Just that adding weight on top will and can change stability.
12 volt I need for sonar, starter motor and so on so I can not remove it...

Reason to run it directly when weather permits is to increase range and to spare some life on the batteries.

I have piloted a solar powered boat built by the Bortons' company and it was really fantastic! https://www.solarsal.solar/

But of course they have a normal battery system and don't have the issues created when skipping that step.

Like some others in this thread I am skeptical about propulsion without the battery buffer. Imagine lurching every time a cloud crossed overhead for one thing.

But I have heard that there are a lot of people who use direct-coupled PV to drive exhaust fans on barns and greenhouses. Whatever they're doing to keep the fans from burning out at low voltage and also from burning out at high current would apply directly to your application too.

Minimoose said:

No I don't have a boat.

Click to expand...

Well you might learn something from this thread.

fxSol said:

But I calculated that 500 watts would be enough when it comes to propeller shaft rpm.

Click to expand...

At 24 volts that is still 20 Amps. Is that what the motor specs say will be the draw at 24-30 volts? It now occurs to me that the MPPT controller may be useful to limit current and give you flexibility to use larger solar panels, especially if it is programmable. With regard to the comment by another poster about the danger of low Amps burning out the motor, I think there are controllers specifically designed to run motors off solar panels. Years ago, I used one to run an attic fan.

Leeds said:

I have piloted a solar powered boat built by the Bortons' company and it was really fantastic! https://www.solarsal.solar/

But of course they have a normal battery system and don't have the issues created when skipping that step.

Like some others in this thread I am skeptical about propulsion without the battery buffer. Imagine lurching every time a cloud crossed overhead for one thing.

But I have heard that there are a lot of people who use direct-coupled PV to drive exhaust fans on barns and greenhouses. Whatever they're doing to keep the fans from burning out at low voltage and also from burning out at high current would apply directly to your application too.

Click to expand...

What a build! My dream started by watching this -
Its an old and heavy boat - still manages to cruise slowly on pure solar. The way boating should be =)

fxSol said:

still manages to cruise slowly on pure solar.

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They also use reduction pulleys to get the RPMs needed. I can not tell if the motor is brushless or not.