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A Solar Trolling Battery System for an Electric Outboard...?

Hi Vaughn,

Powering an inflatable with an electric motor…. One thing to keep in mind—the battery system really is the ‘motor’ and the type of system and quality of the batteries, along with the charging system, will determine the length of trolling time.

Firstly, a 12’ inflatable, like the Avon/Achilles type, does not have an efficient motion through water. If you have ever tried to row one of these vessels you will find the have no ‘carry’—no forward motion after the stroke of the oars. Moving, even at a trolling speed, will require more ‘juice’ for the electric motor. One way to overcome this is to place on board a bigger battery system set up in parallel mode—i.e. 2 batteries hooked up in parallel. At this point something like a pair of Gel Cell (calcium type, not lead acid type) as they have an ‘at rest’ voltage, after proper charging, of 13.8 to 14.2 volts. This is much higher than a lead acid type which after a good charge will have an ‘at rest’ voltage of 12.4 to 12.7 volts. The ‘strapping’ between the batteries should be thick cables. The minus cable to the motor should be hooked to one battery of the pair, while the plus motor cable goes to the other battery. This will effect a stable even ‘pull’ from the battery system while the motor is running.

As to a solar cell “charging” the battery system while in use… Your boat can’t carry enough solar cells—big enough—to do the deed (maybe someday—solar panel efficiency has gone from 7% in the mid 80’s to about 16% today). A better way is to ‘starve off’ battery demise. Look for an efficient VOLTAGE solar cell—not a solar cell that delivers a lot of amperes and hook it up to the unused plus and minus of each battery (meaning opposite the motor cables). You can put Ohm’s Law to use by keeping the battery voltage as high as possible (Gel Cells) and by pumping into the batteries a decently high voltage from the solar cell while underway—say 16 to 18+ volts. Remember, this solar cell must have diodes to prevent reverse flow of electrons or the batteries that will ‘cook’ the panel. Ohm’s Law says that the higher voltage equals less amperes drawn. As your battery system looses voltage, more amperes rush to the motor like a ‘ crash of rhinoceroses’ or a ‘knot of toads’ or a ‘shoal of bass’ and the battery system losses power--faster and faster--as voltage drops.

The last part of a good battery motor system is to have a proper charger. One should invest in a 3 or 4 stage charging system to properly get the batteries up to full capacity. These chargers take a low battery and hit it first with a relatively high voltage. Then, after 20 minutes or so, they lower voltage and begin to push in amperes. As the battery nears it’s topping off point, about 15% below capacity, the charger backs off again and trickles in the last of the amps. Now you have a full capacity charge on the system and are ready to go again—but the next day--as this takes time and will need to be done overnight.

Sounds like one could have as much fun as a 'pod of whales' after setting up a trolling system like the above. Let us know your progress and experiences.

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