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Solar Charger for 12v Lift Motor - Brand Name vs. Amazon/DIY


95echelon

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I am looking to keep a group 31 size deep cycle 12v battery charged for a boat lift motor. Using my battery I only get about 5 lift cycles out of the battery and I average 2-4 cycles per day. Trying to figure out what I should do for charging, and I think I need at least 20w. The boat lift companies charge a TON for their solar panels, but I don't know if they are actually better than units you can get off of Amazon. Has anyone out there done a DIY Solar charger for a lift?

My motor: https://www.boatliftanddock.com/product/706/lift-mate-dc-12v-wheel-assist-motor

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Option 1: Kit from a lift company for $450

https://www.boatliftanddock.com/product/3782/20-watt-solar-panel-charging-kit-for-12v-high-use-systems

It seems that Solar kits from boat lift companies run $300-$500 for a solar kit that includes:

  1. Battery Tray
  2. Mounting Pole (that avoids rubbing on the cover)
  3. 10-30w Solar Charger
    1. Unclear what kind of charger they use but they claim it at least cuts off to avoid solar charging.

Upsides: Easy, fast, high quality. Comes with mounting arm and battery tray

Downsides: Price

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Option 2: DIY from Amazon or Similar for $150-$200

To Purchase:

  1. 30w Solar Panel Kit with Charge Controller and Pole Mounting Kit: $60-$100
    1. https://www.amazon.com/gp/product/B088KBQS8N
  2. Battery Box: $75
    1. https://www.amazon.com/dp/B003UMF44M
  3. DIY Mounting Arm: $20

Upsides: Price, comes with a charging controller.

Downsides: I cannot find a standalone mounting arm anywhere so I have to DIY that. Also, cheaper solar panels may not perform to expectations.

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3 hours ago, 95echelon said:

Using my battery I only get about 5 lift cycles out of the battery and I average 2-4 cycles per day.

Let's do some math before you buy anything.  You have a (I'll guess 110AH) battery that can run your lift five times before it gives up.  You want to run that battery up to four times a day and not have it give up.  I'm going to simplify a few things and say that your five uses take exactly one hour to complete for all five uses, so you are using 110 Amp Hours of battery in that time.  Dividing that by five uses, we get that you need 22 AH per use.  In order to keep the battery charged (which is your stated requirement), we need to put up to 88 AH into the battery every day.  At 14 volts, that's about 1,232 Watt Hours per day.

Before we continue, let's look at your solar situation.  I presume that you don't need to use your lift during the winter, so I'll guess that your season runs from basically May to October.  If this is wrong, it makes a difference since the sun really disappears for you after September and doesn't really come back until about April.  Consulting an insolation map (e.g. https://www.nrel.gov/gis/solar-resource-maps.html), we see that you have roughly 4.5 KWh/m2/day as a low during the two fringe months.  That's power from the sun, not power out of a panel.

As for panels, let's assume that your panels are typical construction and you aren't paying for the cutting-edge technology.  That means you should assume that the panel actually converts around 20% of your sunlight to electricity ("for free!").  So, we can adjust your insolation to about 0.9 KWh/m2/day of actual useful electricity.  Since you need 1.232 KWh per day and you get 0.9 KWh per square meter per day, you need about 1.3 square meters of solar panel to satisfy your requirement.  Remember that you want to make sure the battery stays full for up to four uses a day, and I assume that you mean during the fringe months also.

Two of these 100 Watt panels (that happen to be on sale) will probably do what you want:

https://www.renogy.com/100-watt-12-volt-monocrystalline-solar-panel-compact-design/

Run in parallel, you'll get about 10 Amps at around 18 Volts.  That should recharge your battery in about two hours of *full* sunlight.  Note that any shade that hits the panel will seriously decrease the output.  That includes shade from trees or flags as well as clouds.  Expect a 90% decrease in output with only 10% of the panel shaded.

You'll also want a charge controller that can take your current, so buy something in the 20A range.  One with max power tracking will wring the most from your panels.

But wait!  That's expensive!  So buy a second battery (exactly like the first) and put them in parallel.  Now your system can give you eight lifts per charge, and your required recovery just got cut in half.  Now you can get away with the 100 Watt panel and a 10 Amp charge controller.  Your battery cost is twice as much, though, and you will replace them every seven years or so.  A cheap 10 A controller might be this one:

https://www.renogy.com/wanderer-10a-pwm-charge-controller/

As you can see, you need to be very honest with yourself to fit a system to your use.  If you lie to yourself, you either spend too much money or can't get satisfied with the performance.  If you buy small, understand what you are doing.

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I will take those formulas and do some more diligence. This is SUPER helpful, thanks! 5 Lift cycles is a guess and I need to put a multimeter on to see what I consider the lowest voltage I am willing to risk. I really only need this to work from June - August so sun will be better, but the lift spends time under a giant oak and the predominant view of the sky faces north. A bit of 110v conduit along the dock and a 10a charger may do the better trick. Or I could have the charger in my basement and run 40 feet of DC cord to the lift (would have to figure out the gauge required).

The losing 90% of the output with only 10% shaded is a sobering metric.

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10 minutes ago, 95echelon said:

The losing 90% of the output with only 10% shaded is a sobering metric.

Indeed.  I have verified it myself, too.  I have a solar vent system with four PC fans on a 50W panel.  The fans get about 14v in direct Summer sun here, but that drops to a crazy low around 8v (under load) when I create just a few square inches of shade with my hand.  Seeing a meter change is one thing, but hearing the fans get quiet is another.

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Also, keep in mind that I calculated worst case, and you can probably optimize your use.  For example, maybe you drop the boat once and lift it just enough to keep it safe, then lift it once.

I have three off grid solar installations.  Managing my expectations is the trick with each one.  Well, managing my wife's expectations is the real trick, and is harder to do.... 

She still isn't on board with the fact that the big (4.4kw inverter) system has no problem running the 1.5 ton 240 vac air conditioner and the 240 vac deep well during the day, but it struggles to run the ac over night on anything more than eco mode due to battery capacity.  

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12 hours ago, justgary said:

Also, keep in mind that I calculated worst case, and you can probably optimize your use.  For example, maybe you drop the boat once and lift it just enough to keep it safe, then lift it once.

I have three off grid solar installations.  Managing my expectations is the trick with each one.  Well, managing my wife's expectations is the real trick, and is harder to do.... 

She still isn't on board with the fact that the big (4.4kw inverter) system has no problem running the 1.5 ton 240 vac air conditioner and the 240 vac deep well during the day, but it struggles to run the ac over night on anything more than eco mode due to battery capacity.  

I wonder how the lift companies get away with selling 20 and 30w panels. I do have a slip I built on my dock so I can just leave my boat off of the lift during the day if I want to, but my lake is super busy so I often ask myself if I should really keep the boat rocking in the waves that much.

In my situation, would you consider just hardwiring a charger or push forward on the solar route? As I said I think my first step is measuring voltage drop, etc. I could also check the motor brushes to ensure efficiency. 

Maybe the solution is to sell this for 300 bucks and then spend the 800 bucks for a 110v model, subsidized by the 300.

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I just put 1100w of solar on my RV.  Most days I'm seeing a "peak" charge of 550-650w.  There's some tree shade that happens but it was eye-opening to see how little power I'm actually generating.  Most of the blogs say to expect a maximum of 5hrs of direct (effective) sun per day. 

The lift suppliers aren't expecting peope to use their lift 4-5 cycles per day.  My guess is that they don't even anticipate that amount of usage per week.

If it's feasible for you to run 110v lines for a trickle charger, that's probably the route I would go.  You would only need 4 amps/hr and you'd be filling your battery every day (even if you're using it 4-5 cycles.)  

Otherwise, if you have the space for panels I'd put in a pair of 250w panels (santansolar.com has them for $40/ea as blems) and the appropriate MPPT controller.  This would give you tons of overhead to charge both the lift battery as well as allow for a maintenance charge on the boat batteries (which I consider essential as well.)

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2 hours ago, Fffrank said:

I just put 1100w of solar on my RV.  Most days I'm seeing a "peak" charge of 550-650w.  There's some tree shade that happens but it was eye-opening to see how little power I'm actually generating.  Most of the blogs say to expect a maximum of 5hrs of direct (effective) sun per day. 

The lift suppliers aren't expecting peope to use their lift 4-5 cycles per day.  My guess is that they don't even anticipate that amount of usage per week.

If it's feasible for you to run 110v lines for a trickle charger, that's probably the route I would go.  You would only need 4 amps/hr and you'd be filling your battery every day (even if you're using it 4-5 cycles.)  

Otherwise, if you have the space for panels I'd put in a pair of 250w panels (santansolar.com has them for $40/ea as blems) and the appropriate MPPT controller.  This would give you tons of overhead to charge both the lift battery as well as allow for a maintenance charge on the boat batteries (which I consider essential as well.)

I never expected that. I store my boat in the lift and often i will run out when I get to the lake to grab Ice and fill my tank if it's low. Then i'll run to the marina for donuts and coffee oftentimes in the morning. During the day it's always a couple rounds of watersports or a trip to the sandbar, then an evening surf or wakeboard followed my a sunset booze cruise for the adults. Sometimes i'll order dinner from one of the marinas as well and picnic in the boat! Even without marinas available I would be going out 2-3 times a day just for sandbar and watersports.

Sounds like a lot of boats aren't getting used much! I guess that's why I keep racking up 100+ hours each 3 month season.

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I DIY'd solar for my boat lift last year, but I have a 24V system.  I'm pretty sure I have a 30W/24V panel plus a charge controller, and I just fab'd up a post mount for the panel out of EMT and some aluminum angle.  I probably saved ~$400 putting it together myself over a Lake lite kit.  I'm surprised you only get 5 lift cycles out of a battery though (unless the wheel crankers use more juice/slip more).  You do cycle more than we do though.  I'm usually at most 2 per day, as I'll just leave the boat moored at the end of the dock if we'll be using it a bit, but I'm also on a small lake.

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Lifting the boat about six inches between uses will keep it from swinging much and save a bunch of energy compared to a full lift every time.

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