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  #1  
Old 08-10-2010
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Trolling motor power consumption?

I have a Minnkota Endura 50 that I use on a 20' Mirage sailboat. I use it mainly to motor in and out of the Marina (15 minutes max. each way) but I occasionally use it to troll around the cliff areas to fish (sails stowed of course). In that case I usually keep it on the the number 2-3 setting. I just bought a 720 cranking amps/115 amp hours 27 series deep cycle battery to power it. How long can I expect to use the trolling motor before I discharge the the battery? Can I use the motor several outings before I have to recharge the battery if I'm not taking much power away each trip or will I have to recharge it after every outing? Also wondering if one ican use a small solar charger to recharge/ maintain the battery while it's not being used? The battery is pretty heavy and hard to load/unload on the boat if I have to take it home each time to charge. The boat is dry stored on the trailer with no AC power available.
I had been using a 525amp battery (24 series a starting not deep cycle) but I decided to get the right kind and size to make sure I'm not left stranded.
I'm open to any suggestions.
Thanks,
Steve

Last edited by SteveON; 08-10-2010 at 09:06 PM.
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Old 08-10-2010
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What is the amperage draw of the Minnkota? A Group 27 deep cycle wet cell battery has about 100-110 amp-hours at the 20 hour rate...or at a draw of about 5 amps per hour.

If the motor is providing 40 lbs. of thrust, that is about 2/3 of a HP or 500 watts or about 40 amps @ 12.6 VDC. This is based on the following:

746 W = 1 HP
58 lbs. thrust = 1 HP

If you're using the battery for .5 hours each time out, you're using 20 amp-hours (.5 hours * 40 amps).

The battery is rated for 100-110 amp-hours, but should not typically be discharged past the 50% point, so you have effectively 50 amp-hours, so ideally, you can use it for two trips before needing to recharge it. I'd point out that the 100-110 amp-hour rating is a 20-hour rating or at a 5 amp draw. With a 40 amp-draw, it is likely to be a good deal less, and you'll probably really want to charge the battery after every trip.

I'd point out that wet-cell batteries self-discharge during the warmer weather at a rate of up to 1% per day. So, if you leave the boat for two weeks, it will have lost possible 14 amp-hours from self discharge alone.

I would recommend getting a 30 watt solar panel to recharge the battery. A 30 watt panel, is about 2 amps per hour at 14.4 volts, and that will give you about 10 amp-hours of battery charging per day or so, especially if you're using a MPPT type charge controller. During the course of a week, it will return, on average, probably about 60 amp-hours to the battery, meaning that it will probably be able to handle your weekend usage of the boat each week.
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Last edited by sailingdog; 08-10-2010 at 10:33 PM.
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Sailingdog,
Thanks for your reply. Your answer makes perfect sense and I really appreciate the solar charger recommendation as well. I'll be search for a 30 watt panel with a MPPT type charger. Any suggestion where to start shopping? Ebay? Amazon? I might just have to suck it up until I find a setup and drag the battery home each time for now.
Thanks again!
Steve
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You can probably find a 30 watt panel with a charge controller, but it will likely be a non-MPPT type charge controller. It should work fairly well, but you're not likely to find a MPPT-type charge controller in an affordable kit.

A 30 watt polycrystalline or monocrystalline panel on eBay will run about $90. A non-MPPT charge controller will run about $20. I'd recommend you read the primer I wrote about Solar Power On Boats too.

You might also try Harbor Freight or Northern Tools. I got a 30 Watt panel that I use for winter battery maintenance at HF for not too much money and IIRC, it came with a small charge controller. I gave the charge controller to a friend, since I already have an MPPT one installed on my boat.
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Old 08-10-2010
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What SD said. Those numbers square pretty well with my experience using a 40# Motorguide and a Group 27 (115AH) flooded batt. There's a ton of variables in play, however, so YMMV.

For example, the Minnkota Endura 50 is an analog troller -- the 5 FWD/3 REV is your clue. That is to say, it achieves lower-than-maximum speeds by putting a bloody rheostat (dimmer-switch resistor) in line with the motor. The result is that analog trollers use nearly the same juice at any speed setting -- at low speeds, they burn off surplus amps as heat.

Digital trollers (like the Maxxum or Great White series) use electronic speed control. These can yield up to 3x improved run time at reduced speeds over analog models, tho at WOT there's no real advantage. They typically have infinite speed settings.

Another variable is discharge rate -- how fast you are pulling amps measured as a fraction of the battery's capacity. For most trollers, that's well inside the 8-hour discharge rate. Most batteries are labeled with their 20-hour discharge rate, which may be half again the 8-hr rate. What that means is the faster you discharge a battery, the smaller it behaves. So you should discount that 115 AH to maybe 80 AH as a derated capacity; then, as SD proposes, halve that to 40 AH for your functional capacity, and that means you get 1 hour of run time at 40A draw.

Now, there's a school of thought that says it's false economy to nurse batteries this way. That by limiting yourself to 50% DoD or less, you are leaving half your useful capacity on the table. Sure, driving a battery to its full 80% DoD may halve its life span -- but you will have gotten twice as much work out of it, so you come out even in the dollars-per-mile calculus. A decent deep-cycle lead antimony batt should withstand 1500 cycles between the eighties (80% DoD to 80% SoC). These contrarians, whom we might call the "Rented Mule" school, propose it's better to utilize more of the battery's capacity and accept a shorter service life. The advantage is, you carry around less battery mass in the meanwhile. And entropy being what it is, batteries decay even when babied or unused ... so why not ride em hard & put em away wet, buy new ones when these die?

A 30W mono- or polycrystalline solar panel will recharge your Group 27 in about 1 sunny day following 1-2 normal days' use, so it is a good match. You probably should add a charge controller, just a simple one, to keep from boiling the battery dry once it is charged. I got this one on eBay for $15 and like it just fine.

Cheers!
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Old 08-11-2010
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sailingdog-I'll take the time to read the primer you wrote and then start my search for the right panel for the job.

bobmcgov- thanks for your response. Lots to think about. Im of the school of thought that I would have no problem running the battery down to 80%. It has a two year free replacement warranty if it fails. I normally replace the batteries in my car, truck and motorcycles about every two years anyway. The heat here in San Antonio, Texas can be brutal on batteries.

I will probably have to wait until the first of Oct to buy a solar panel. I have a wedding I have to pay for comming up soon. Not sure my daughter would understand her wedding "gift" was a new solar panel for her dad's boat...LOL.
Steve
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A wise decision... women can be so sensitive about their weddings...
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Originally Posted by SteveON View Post
I will probably have to wait until the first of Oct to buy a solar panel. I have a wedding I have to pay for comming up soon. Not sure my daughter would understand her wedding "gift" was a new solar panel for her dad's boat...LOL.
Steve
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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