Running off large solar array with small battery?

[SolarRat]

I will be slowly converting my pontoon canal cruiser boat to electric. I'm not ready to pull off the 50hp 4 stroke just yet but I found a good deal locally for some panels. I'm thinking of picking up 1120 watts worth (4x280w). I want to install the panels now while sourcing/building the new E-motor and lithium banks...I'm not in a rush so it may take 6 months or more.

The boat has a tiny 55 lb thrust bow mount trolling motor on it now that I mainly have as a backup in case the gas motor leaves me stranded. It is connected only to a single 12v 120ah FLA battery. The motor draws 50 amps at full speed (~2.5mph). I sometimes cruise the canals with the E-motor but range is obviously limited.

I'd like to temporarily be able to use the array to power the motor at full draw indefinitely as long as the sun is out. The problem is I don't want to charge the battery at any higher than ~15 amps. If I put 3 more batteries in parallel I could just use a 60 amp charge controller, but i don't want to waste money on LA batts I won't need later.

So, is there a simple and cheap way to allow the motor to draw a full 50 amps while the battery never sees more than 15 amps of charge? Is there a cheap controller that can do this, or a more expensive one that I will still be able to use after going to a 48v setup? Could I run the motor directly off a cheap 60 amp PWM controller (maybe using diodes to the batt so it always senses a battery, but can't charge it) , and have a second cheap 10 amp controller to keep the battery charged?

Thanks for any thoughts!

 

[efficientPV]

I've been thinking about doing this, but I have enough boats already. There are linear current boosters used for DC pumping motors, but they are expensive. I would design a module so it ran all the time on solar and only used the battery to get back in an emergency. Don't know if they are common high side or low which would nix the diode idea. It is not complex technology as it is only a big buck converter with maximum motor voltage set and that is decreased when panel voltage dips below a set voltage. Not much off the shelf unless you can modify it yourself.

 

[bulrush]

The boat has a tiny 55 lb thrust bow mount trolling motor on it now that I mainly have as a backup in case the gas motor leaves me stranded.

Have you tested your trolling motor in high winds and choppy water? I'm thinking it won't work on a pontoon of sizable mass.

 

[Mike Albert]

I will be slowly converting my pontoon canal cruiser boat to electric. I'm not ready to pull off the 50hp 4 stroke just yet but I found a good deal locally for some panels. I'm thinking of picking up 1120 watts worth (4x280w). I want to install the panels now while sourcing/building the new E-motor and lithium banks...I'm not in a rush so it may take 6 months or more.

The boat has a tiny 55 lb thrust bow mount trolling motor on it now that I mainly have as a backup in case the gas motor leaves me stranded. It is connected only to a single 12v 120ah FLA battery. The motor draws 50 amps at full speed (~2.5mph). I sometimes cruise the canals with the E-motor but range is obviously limited.

I'd like to temporarily be able to use the array to power the motor at full draw indefinitely as long as the sun is out. The problem is I don't want to charge the battery at any higher than ~15 amps. If I put 3 more batteries in parallel I could just use a 60 amp charge controller, but i don't want to waste money on LA batts I won't need later.

So, is there a simple and cheap way to allow the motor to draw a full 50 amps while the battery never sees more than 15 amps of charge? Is there a cheap controller that can do this, or a more expensive one that I will still be able to use after going to a 48v setup? Could I run the motor directly off a cheap 60 amp PWM controller (maybe using diodes to the batt so it always senses a battery, but can't charge it) , and have a second cheap 10 amp controller to keep the battery charged?

Thanks for any thoughts!

Your best option is to use more than one battery. With one battery you are limited by what that battery can continuously supply. If you connect 4 batteries in parallel, you will quadruple your potential output and your solar system can provide the 50 amps to run your trolling motor at full speed and will keep the current to each battery under 15 Amps.

 

[SolarRat]

Thanks guys, that's what I was afraid of. At first I thought this would be simple but soon realized it probably wasn't. I was hoping they made something cheap for this. A 60a buck converter might work but they ain't cheap either, for a temp solution. Just to get by for a few months maybe I'll see if I can pick up 4 junkyard lead acids to use as a dummy load, they shouldn't need much life left in them (will keep my good batt separate of course).

Have you tested your trolling motor in high winds and choppy water? I'm thinking it won't work on a pontoon of sizable mass.

Many times, and it doesn't work well! I've had a few different pontoons down here over the years and always threw my trusty old 55 endura on them just for emergencies, needed it a few times. I rarely go out in bad weather but often need to go against strong tides in my canal. The motor still moves the boat but power drains fast.

On this boat I have a Riptide 55 with Co-Pilot remote control. It has a PWM speed control making it much more efficient at lower speeds than my old Endura that has resistors to dump voltage. It's very undersized for the boat but it will still easily get me home from over 5 miles away on 1 battery (about a 10 amp draw @ 1.5mph). I only have this motor because I couldn't pass up the $30 yardsale price (it didn't work, needed new brushes).

My current plan is to get a junk 15hp outboard, remove the powerhead and adapt a 5-10kw brushless motor. I have a lot no wake zones here, so a cruising speed of ~4mph with bursts to 6-7mph is all I'm looking for.

 

[Rider]

My current plan is to get a junk 15hp outboard, remove the powerhead and adapt a 5-10kw brushless motor. I have a lot no wake zones here, so a cruising speed of ~4mph with bursts to 6-7mph is all I'm looking for.

I did this back in the '60s. I used an old car generator and jumped the field winding so it had tons of torque but little speed. But it did work.

 

[Loyalist]

I will be slowly converting my pontoon canal cruiser boat to electric. I'm not ready to pull off the 50hp 4 stroke just yet but I found a good deal locally for some panels. I'm thinking of picking up 1120 watts worth (4x280w). I want to install the panels now while sourcing/building the new E-motor and lithium banks...I'm not in a rush so it may take 6 months or more.

The boat has a tiny 55 lb thrust bow mount trolling motor on it now that I mainly have as a backup in case the gas motor leaves me stranded. It is connected only to a single 12v 120ah FLA battery. The motor draws 50 amps at full speed (~2.5mph). I sometimes cruise the canals with the E-motor but range is obviously limited.

I'd like to temporarily be able to use the array to power the motor at full draw indefinitely as long as the sun is out. The problem is I don't want to charge the battery at any higher than ~15 amps. If I put 3 more batteries in parallel I could just use a 60 amp charge controller, but i don't want to waste money on LA batts I won't need later.

So, is there a simple and cheap way to allow the motor to draw a full 50 amps while the battery never sees more than 15 amps of charge? Is there a cheap controller that can do this, or a more expensive one that I will still be able to use after going to a 48v setup? Could I run the motor directly off a cheap 60 amp PWM controller (maybe using diodes to the batt so it always senses a battery, but can't charge it) , and have a second cheap 10 amp controller to keep the battery charged?

Thanks for any thoughts!

Why are you wanting to restrict your LA battery to a 15 amp charge? I regularly charge my deep cycle FLA battery (4 6volt FLA in series parallel) with 80 to 90 amps on alternator and 20 amps on solar. I would still be able to charge a 12 volt LA with the same.

 

[SolarRat]

Why are you wanting to restrict your LA battery to a 15 amp charge? I regularly charge my deep cycle FLA battery (4 6volt FLA in series parallel) with 80 to 90 amps on alternator and 20 amps on solar. I would still be able to charge a 12 volt LA with the same.

The safe charging rate limit for LA is usually about 10-13% of of the AH rating. My battery is only 120ah, 13% = 15.6A.
Your 6v batts may be 225ah, which would be 450ah in your setup, 13%= 58.5A.

 

[Loyalist]

The safe charging rate limit for LA is usually about 10-13% of of the AH rating. My battery is only 120ah, 13% = 15.6A.
Your 6v batts may be 225ah, which would be 450ah in your setup, 13%= 58.5A.

There are different views on this depending on what method and time-frame you have allowed for charging your batteries. if you are charging from an engine driven system you would run into serious inefficiencies at 13%. I am using GC@ 6 volts which have the advantage of distributing current through more but thinner plates. This allows for faster CAR on bulk charge. They are more fragile than heavier storage batteries such as trojans but they cycle faster. Standard for marine engine charging is frequently 25%. In my setup as you describe to a "T" my 100 amp externally regulated alternator can theoretically just about match that 25% figure but in reality the temperature controls set by both battery and alternator temperatures probably come closer to 20%. The Solar output is closer to 5% but is continuous. If I keep the battery use to 50% SOC the charge parameters may be considered to be closer to 10% solar and almost 40% on the alternator. If we start talking lithium the CAR is 98% and very low internal resistance resulting in burnt-out alternator syndrome.