Can a solar panel run a trolling motor?
The short video shows me and the wife sailing away across the bay in a solar powered inflatable kayak with electric trolling motor mounted at the rear. There are two 100 watt solar panels connected to the system, but there is also a lithium battery hidden under the rear panel behind the kayak seat mods.
The pertinent question is: Can you run a trolling motor off a solar panel by itself?
Traditional wisdom says no – trolling motors draw too much current – let’s find out!
In this post you’ll discover the process I went through in creating my DIY motorized solar kayak:
- How much power does a trolling motor take?
- What size trolling motor do you need for your boat?
- How fast will my kayak or canoe go?
- How much power will a 100 watt solar panel really produce?
- Do you need a battery for a solar powered trolling motor?
- Which batteries are best for trolling motors?
- How long will your trolling motor run with solar?
Kayak Trolling Motor Setup
What is the best trolling motor for a kayak?
I’ll focus on inflatable kayaks as this is my particular experience, but the this passage applies to all small boats whether rigid, inflatable, fishing or leisure. Prospective boat-owners have three pertinent questions relating to trolling motors:
- how fast do trolling motors go?
- what size trolling motor do I need?
- will a bigger thrust result in more speed?
All trolling motors go at the same speed, namely 5mph (8kmh) at top speed. Speed is related to propeller rpm and pitch. Trolling motors are designed to move a boat slowly to it’s destination. Fishermen, for example, want to move to another location, where there might be more fish, quickly and more importantly, quietly.
With the arrival of the leisure market and small inflatable craft, the trolling motor rapidly became the only means of propulsion, giving relief from paddling and allowing people just to float lazily along. Trolling motors became cruising motors! After-market propellers are sold but IMO are not worth it. The gains in speed are minimal and tend to put the motors under more strain than they are designed for.
The most important value for an electric trolling motor is the thrust in pounds-force (lbs/f). This number determines the weight of boat you can get up to speed reasonably quickly. A 30lbs thrust motor will not go any faster than a 55lbs thrust motor but will reach the top speed of 5mph in less time.
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For whatever style of boat you have a simple rule-of-thumb is that you need 2 lbs of thrust for every 100lbs of weight, which includes boat, equipment and passengers.
Here’s what the number look like for my Itiwit inflatable 3-man kayak:
- Kayak weight empty 17kgs (37lbs)
- Passengers 130kgs (286bs)
- Equipment 53kgs (117lbs)
- Total = 200kgs (440lbs)
- Thrust required = 440/100*2 = 8.8 lbs
Did that surprise you? For my streamlined and light boat I don’t need such a powerful trolling motor to cruise across the bay where I live in tranquil waters. Even an 18lbs model would do for an inflatable kayak trolling motor.
I decided on a 30lbs thrust Minn Kota brand motor. Why? Because men automatically size up to the nearest power size and a bit of extra thrust can be useful if there’s a wind, some waves or weeds under the surface. These motors have stood the test of time so perfect for my needs of reliability and long life.
How many amps does a Minn Kota trolling motor draw?
The 30lbs Minn Kota has 5 forward speed settings, which push the boat at different speeds and draw progressively higher currents from the supply. Speed is adjusted by turning the tiller handle. It isn’t a smooth transition as a 5 position switch in the Minn Kota body changes over connections varying the watts need to run the motor.
These two facts are important for two reasons;
- the solar panel output needs to be more than the current drawn by the motor
- each upward speed change causes a surge in current due to an instantaneous load increase in watts
I’ve found that my kayak speed at No 3 setting pushes the boat along at a speed not much less than top speed, but the current draw is up to 65% less – very important when considering running the trolling motor from a solar panel.
Minn Kota 30 lbs
Max Amp Draw
The above chart look straight-forward enough but it doesn’t reflect actual conditions. Those currents are maximum values which may be drawn in adverse conditions, and for which the motor was designed to withstand.
For example, the Minn Kota 30lbs motor running on my inflatable kayak pulls around 12 amps on speed 3 and 15 amps on speed 4. It just doesn’t need huge thrust in lbs to push it along. This means that we could use those values when calculating solar panel ratings.
When I’m in the kayak on a sunny day, I don’t really mind cruising at a lower speed – it’s a lazy, tranquil activity anyway. Keep that in mind as we move on to consider the solar panels output needed to run our trolling motor.
How much power does a 100w solar panel produce?
Solar panel manufacturers usually give the Standard Test Conditions (STC) panel rating in their promotions. They might advertise a 100 watt panel but is it really? What does that rating mean? It states that the solar panel will produce 100 watts at an irradiance of 1000watt/m2 at a specific temperature and atmospheric conditions.
Irradiance is simply how much sunshine energy falls onto a surface at any instant in time. Insolation is how much energy falls onto a surface over time, like per hour or day. The first thing to realize is that these are ideal conditions and are almost never seen in real life.
If you checkout the spec sheets for a solar panel you’ll find another rating called NOCT (Normal Operating Cell Temperature). The temperature and atmospheric conditions are different but the big thing is that the irradiance value is only 800watts/m2. This takes into account the fact that the sun doesn’t shine intensely all the time and clouds also pass.
Using this realistic rating already brings down the real solar panel rating in watts to 80 watts, all other thing being equal. Of course, if you live in a very sunny area you may be lucky to get 1000w/m2 consistently.
100 watt solar panel output amps
If you short the leads of a solar panel together through a multimeter on the right setting you’ll get a current called Isc or Short Circuit Current. This doesn’t harm the panel, due to it’s internal resistance. It is also an indication of the maximum power current (Imp) that the panel can deliver, which is generally between 91 to 95% of Isc.
The panel specs also include a value called Vmp which is the voltage at which maximum power is delivered by the solar cells. For example, I have a Dokio folding panel with Vmp of 16 volts and Imp which is 95% of Isc i.e. 4.5 amps. This means that the maximum power output of this panel is:
- 4.5 amps x 16 volts = 72 watts.
However, we never really know what this value is at any time because irradiation changed by location and local conditions. I use 2 x 100 watt flexible solar panels for my solar kayak project and I want to investigate the possibility of running the trolling motor without a battery.
In theory, 200 watts should be able to run an electric trolling motor without battery support – we’ll soon see!
Can a solar panel run a trolling motor without a battery?
I’m going to hook up two solar panels to a load without a battery to but with an extra element in the circuit – a 12 volt buck converter. A buck converter regulates a d.c. voltage between 12 and 60 volts into a steady 12 volt supply, or thereabouts.
One advantage is that there is no danger of over-volting a 12 volt rated load and also a buck converter will transform any voltage over 12 from the panel into current i.e. it maximizes the current. This model is rated at 20 amps but as the maximum current possible from 200 watts of solar is 16 amps, we should be OK.
For this test the two solar panels will be connected in series to give a healthy voltage of 42 V into the buck converter. If it doesn’t work out I’ll connect them in parallel for 12 volt output. The maximum current draw of this 30 lbs Minn Kota trolling motor is 30 amps on top speed but out of water it will only draw 2 to 3 amps.
I’ll add another d.c. load in parallel with it so the current draw in total will be about 14 amps. If the panels can supply this then we could run the trolling motor at speed 3 or 4 when mounted on a light boat such as a kayak or jon boat. I’m using a motor speed controller to ‘soft start‘ the motor, just in case the solar panels can’t handle the surge current.
Here’s the circuit diagram below:
Procedure: Run the trolling motor in open air – at full speed the current will be low (2 or 3 amps). Increase the voltage to the dummy load connected in parallel until it starts to draw current also.
Note the volts, current and watts in the circuit to determine if 200 watts of solar panels will run a trolling motor without the need of a battery.
Results: The trolling motor runs in free air and draws 2.3 amps. As the dummy load was brought into the circuit the available voltage decreased as the current increased.
Final V and I measurements:
- Current 4.9 amps
- Voltage 7.2 volts
- Watts 4.9 x 7.2 = 35.38 watts
Conclusion: In less than optimum insolation conditions 200 watts of panels would struggle to propel this motor. It’s difficult to tell without maximum sunshine as the voltage is critical for motor speed. (I will try this test again later in summer conditions and update the post.)
In any case, relying on solar panels alone to run a trolling motor isn’t a great idea – on a cloudy day the boat will go nowhere! We need some kind of other voltage source i.e. a battery.
What size battery for trolling motor?
Normally, quite a big battery is need to run a trolling motor for any length of time. A 90 Ah deep-cycle lead-acid type is not uncommon, which weigh in at around 30kg (66lbs). I used on of these for a while, but not for long – just too heavy to cart around.
Fortunately, lead-acid isn’t the only game in town:
I then moved on to a smaller lead-acid (44Ah) but this gave the problem of short running time hence the thought of solar panel backup. Mounting 200 watts of solar in parallel with the battery extended its running time a lot and also meant that I could use an ordinary auto-battery, as it very rarely discharged more than 5 to 7% (higher discharges will damage an auto battery.)
However, the life of any lead-acid battery is measure in 5 to 7 years even with the best care and I wanted something as near permanent as I could get. Besides, at 11kg the 44Ah lead-acid was still a tad heavy to lug around.
Lithium Phosphate (LiFeP04) Batteries
The answer is to use lithium battery technology. There are several type and lithium phosphate suits trolling motor use just right – see below:
- Lightweight – one third of the weight of similar capacity lead-acid
- Inherently deep-cycle – can be discharged 100%
- Long life – 2000 cycles at 80% DOD, 5000 cycles at 50% DOD
- Safe technology (their cousin, the li-ion variety, have been known to over-heat)
You can compare the different battery type in this way:
- 90Ah deep-cycle recommended discharge = 50% or 45Ah (will be less due to temperature and amount of current drawn)
- 44Ah lead-acid auto battery recommended DOD = 5 to 10% or 5Ah
- 30Ah LiFeP04 DOD can be 95 to 100%, or 30AH
The lithium battery has almost the capacity of the 90Ah lead-acid and it weighs 10 times less! That’s the one for me. My recommendations are to use an auto battery with solar backup for short hops (you’ll never discharge it enough to damage it) and if you intend to cruise then use a 30Ah LiFeP04 with 100 or 200 watt solar panels.
How long will a 12v battery run a trolling motor?
The table below shows the extra run time you can get by connecting 150 or even 300 watts solar panels to your trolling motor setup – this is what I use on my motorized inflatable kayak.
30 lbs Trolling Motor
Deep-cycle Pb Battery Size (Ah)
Hours Running Without Solar Panels
Hours Running With 150 Watt Solar Panel
Hours Running With 300 Watts Solar Panels
It always best to use a solar charge controller to charge trolling motor batteries. Not only does it ensure battery safety but also ensures maximum efficiency of the solar panels output power.
Can a solar panel charge a trolling motor battery?
Solar panels are a great option for charging trolling motor batteries but not everyone wants to go to the trouble of setting up a complete system. A good and quick way to do it is to connect up a 100 watt solar panel in parallel with the battery and the load. The panel will also help to deliver current when needed and also charge up the battery when the motor isn’t running.
This is a great arrangement if the motor is running for short periods, such as when fishermen move from one location to another looking for the fish. If the the panel is connected through a 13.8 volt buck converter as shown, then the battery will never overcharge, it will always be on float voltage.
Eventually, it will need a boost charge to charge it fully, but even this isn’t necessary, as lower charge levels ensure longer battery life.