Since this hasn't come up in a while, I would like to add the following caveats to the discussion of electric trolling motors, batteries, and solar panels/chargers:
1) Calculate your total battery capacity at no more than 50% discharge. Even the best batteries will be very short lived if you repeatedly discharge them deeper than 50%. The best batteries for really deep discharge are Gel Cells, but you MUST limit the charging voltage according to manufacturer's specs (usually less than 14.1 volts), or you will ruin them in short order. AGM batteries will also tolerate relatively deep discharges, and some are more tolerant to higher (14.6v) charging voltages. Both Gel and AGM batteries have extremely low self-discharge rates, and can be left for long periods without consequence. 2) Using a smart charger (multi stage such as Xantrex/Statpower True Charge or equivalent) will dramatically extend battery life. If you are using traditional lead-acid batteries, equalize the cells periodically (better chargers h ave this capability) to prolong life. I have found the old-style Ferro-resonant type chargers to be a false economy in that they don't optimize the rate of charge, nor are they smart enough to compensate for temperature, etc. 3) Not all so-called 'Deep Cycle' batteries are. Most sold under this appellation (especially at car parts stores and discount chain stores) are merely slightly better car-type batteries. If you are serious about real capacity and longevity, buy one of the better brands that truly have different plate thickness and composition suited to long, deep discharges. I have found that the cheaper batteries are a false economy when compared to a properly maintained and charged true deep cycle or industrial battery. 4) Forget about charging your trolling motor battery with a solar panel unless you can wait several weeks (or even a month) between use. The Unisolar USF-11(of which I have on both my M15 and my Flicka) is a great panel, but you have to look at the numbers. Consulting the solar irradiance chart in the Unisolar manual, most of us in the northern half of the country aren't going to see more than 16 amp-hours/week TOTAL from this panel. Those in the sunny south will be lucky to get 20 amp-hours/week. You then need to multiply these figures by 0.75 to account for average efficiency lost to the battery/wiring/controls to get a realistic net number. Let's say you start with a fully charged group 31 battery with a total rated capacity of 105 AH. If you use the average electric trolling motor for 1.5 hours in a given day (at max output), you will deplete approximately 50-60 amp-hours. The Unisolar panel, under *ideal* conditions, produces 0.62 amp-hours. Derate this by 0.25, and you have about 0.465 AH. Even assuming five productive sun-hours per day (that is about the max you can expect, on average), you are talking 20 to 25 days to replace a 50% depth-of-discharge. I think you can see the trend. Let's say you use the trolling motor for just 20 minutes a day to get to and from your dock. It would still take the USF-11 more than five days to replace the energy used in one day. This is not to say that the panel is useless in this mix. They are great for topping-up small discharges, and for holding a battery at float (I use a charge regulator on mine now--note that float voltage for Gels and some AGM's is different from normal lead-acid type). The ideal (as I see it) is a good multistage charger with a solar panel for float. On the M15, I use my Unisolar to replace very shallow discharges while cruising from either an LED interior light or Davis 'Mega' anchor light. 5) If you are going to mount your battery (batteries) forward, and have your trolling motor on the stern, you are going to need some large gauge wire to stay within the recommended 3% voltage drop limits. Assuming a 20' run on an M17 (after snaking around bulkheads, etc.), you would want at least #6 wire for a trolling motor with a maximum draw of 40 amps, and #4 wire (smaller number is bigger wire) for one drawing a maximum of 50 amps. That's some pretty big (and expensive) wire!! Consider mounting your battery very close to the motor!! 6) If your motive is to save weight and or expense, a small, efficient four-stroke outboard may well prove the better choice. I agree that for small lake sailing, or for occasional use, the electric has its place, but I would argue that it is entirely inappropriate for a boat used for cruising, or when shore-side power is not available for overnight charges via a good AC battery charger. Sincerely, Scott Grometer M15 #478 'bebe' PSC Flicka (unnamed) _______________________________________________ http://mailman.xmission.com/cgi-bin/mailman/listinfo/montgomery_boats
