# Smaller Battery for trolling motor?

#### baucat9

Joined Oct 27, 2014
28
Hi AAC. I have been pondering this thought for quite a while: Can a battery smaller than a marine deep cycle battery power a trolling motor? The reason I have though about this so much is the fact that a trolling motor alone costs basically $100 and a battery is at least another$75. I've been trying to find a motor that would be big enough to give a motor around 30-35 lbs of thrust but at the same time could be powered off of a small battery like the ones in model airplanes and cars. It would be nice if I could manage to get the battery to last 4ish hours on 1 charge. So I am basically looking to see if anyone has done a similar project and what their solution was and or some reccomendation son how to do this. All suggestions or information is greatly appreciated.

Thanks,
Shane

#### tracecom

Joined Apr 16, 2010
3,944
Deep cycle batteries are rated in ampere hours, which give you some idea about how long the battery will power a given load. So, look at the current (amperes) required by the trolling motor and divide that number into the ampere hour rating of the battery; the answer will tell you the best case maximum number of hours that battery would power that trolling motor. The best case is rarely achieved however because the battery is often overrated and because completely discharging the battery is very bad for it. If you divide the answer you just got by 2, that will be a better estimate of the number of hours that the battery should drive the trolling motor.

So, check the ampere hour ratings of the various types of batteries that are available, do the math as described above, and you will have the answer you need. What I think you will find is that the cost of any suitable alternative to a lead acid deep cycle battery will be much higher than a lead acid deep cycle battery.

I know that's not the answer you wanted, and maybe someone else here has a better solution.

#### Papabravo

Joined Feb 24, 2006
17,308
I seriously doubt that you will find a motor battery combination that will meet your expectations.

#### Dodgydave

Joined Jun 22, 2012
10,088
You need to find a motor and measure its current at the required power/torque output, only then you can select the battery to fit it.

#### Alec_t

Joined Sep 17, 2013
12,320
This thread gives some guidance on trolling motor power requirements.

#### baucat9

Joined Oct 27, 2014
28
Deep cycle batteries are rated in ampere hours, which give you some idea about how long the battery will power a given load. So, look at the current (amperes) required by the trolling motor and divide that number into the ampere hour rating of the battery; the answer will tell you the best case maximum number of hours that battery would power that trolling motor. The best case is rarely achieved however because the battery is often overrated and because completely discharging the battery is very bad for it. If you divide the answer you just got by 2, that will be a better estimate of the number of hours that the battery should drive the trolling motor.

So, check the ampere hour ratings of the various types of batteries that are available, do the math as described above, and you will have the answer you need. What I think you will find is that the cost of any suitable alternative to a lead acid deep cycle battery will be much higher than a lead acid deep cycle battery.

I know that's not the answer you wanted, and maybe someone else here has a better solution.
I'm not really going for a current trolling motor available, I thought I might design and build one with the smaller battery concept.

#### Papabravo

Joined Feb 24, 2006
17,308
I'm not really going for a current trolling motor available, I thought I might design and build one with the smaller battery concept.
Unless you are a precision machinist, I hardly think that is a practical idea. It is not just the motor and the battery but the drive train and the propeller. Are you sure this is within your capabilities?

#### Alec_t

Joined Sep 17, 2013
12,320
Depending on the troll speed you're aiming at, it looks like a 30-35lb thrust will need at least a ~0.3 HP (250W) motor. However you design/build a motor/drive for that power level, you are going to need a hefty battery to give you a 4-hr duration.

#### Papabravo

Joined Feb 24, 2006
17,308
Depending on the troll speed you're aiming at, it looks like a 30-35lb thrust will need at least a ~0.3 HP (250W) motor. However you design/build a motor/drive for that power level, you are going to need a hefty battery to give you a 4-hr duration.
From the Wikipedia article at https://en.wikipedia.org/wiki/Trolling_(fishing) :

Baits and lures are typically trolled at speeds up to 9 knots, though speeds up to 15 knots can be used, particularly when boats are travelling to different fishing areas. The speed at which the lure is pulled through the water impacts on the fishing success. The optimum trolling speed varies with different species of fish, with weather conditions and the time of year, and other conditions. Chinook salmon can be successfully trolled at higher speeds than more docile lake trout. For these reasons fishermen use devices that accurately track speed.[4] Trolling motors calibrate speed more accurately than large outboard motors. Trolling plates are also used with larger motors to slow the boat to the desired speed, although some anglers experience mixed results with plates.

9 knots = 15.19 feet/sec
If a standard bass boat weighs 1200 lbs and the fisherman weighs 180 lbs and the motor battery and gear weigh another 120 lbs that's a total of 1500 lbs. or 46.62 slugs. If we estimate the drag force on the boat as 12 lbs, then 18-23 lbs remain to propel the boat forward. Taking the bottom end of the range as 18 lbs of thrust this gives us 0.386 ft/sec/sec of acceleration meaning that it would take about 40 seconds to reach a velocity of 9 knots. That seems like a reasonable estimate of the power requirements for a trolling motor.

Further: The kinetic energy of of our bass boat is 0.5(46.62 slugs)(15.19 ft/sec)^2 = 5378 ft-lbs = 2.025 Watt hours. Now, can our 250 watt motor supply this enegy to accelerate the bass boat to 15.19 ft/sec? I think it can because (250 Watts * 40 seconds)/(3600 seconds/hr) = 2.77 Watt hours.

http://www.kencook.net/Setupart.html

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#### jpanhalt

Joined Jan 18, 2008
11,088
Cost analysis in this situation is complex and inaccurate because of the unknowns, but some approximations can be made. Presumably, you can get a marine-rated trolling battery for $75. I found that a 35 AH marine battery that cost$90. So, let's say your $75 battery is a real bargain and delivers 35 AH. You mention using a model airplane battery. For propulsion, they are usually lithium polymer; although, newer technologies are on the way. A "3S" lithiun battery produces a nominal 11.1V; a "4S" produces 14.8V. At HobbyKing ( known for its low prices), a 5 AH, 4S is$26; a 3 AH, 3S is $28. So, consider the cheaper, higher capacity battery. To attain 35 AH, you will need 7 of the$26 battery = \$182, which is 2.4 times the cost of the marine battery.

Where is the cost savings? Remember, the care of lithium batteries is more complex than lead acid batteries. The one thing you might save is weight. Is that really such a concern in a boat?

John

#### Papabravo

Joined Feb 24, 2006
17,308