How to calculate electric trolling motor runtime?
Electric trolling motors are most often rated in horsepower (HP). One electrical HP is equal to 745.7 watts (W). A 1 HP motor “cannot” use less than 746W (rounded). If the motor draws 1000 W, the motor would be 75% efficient (accounting for the prop drive shaft and balance of system losses). It would be best if you always accounted for the efficiency of the motor and system when calculating your battery energy storage (capacity) needs. Horsepower is a unit of power, while kilowatt-hours is a unit of energy. To go from horsepower to kilowatt-hours, you need to know how long the power is needed.
So, the “general” power conversion of HP to required watt hours (Whr) of energy storage is HP divided by motor efficiency times 746.
A 1 HP motor that operates at an efficiency of 75% would require 994.66 Whr of energy to run for one hour. (1Hp / .75% x 746W = 994.66Whr).
Converting HP to Amps (A) is the next natural step you will take as you attempt to determine the battery capacity you need to meet your expectations. Because HP is a measure of electric power equal to 746 W per horsepower, converting horsepower to amps can be done by diving the Whr result by the system operating voltage using Ohm’s Law of Amps x Volts = Watts or Watts / Amps = Volts.
At 12V the Amp draw would be (1Hp / .75% x 746W = 994.66Whr / 12V = 82.8A). If you wanted to operate a 1HP 12V trolling motor for 1 hour, you would need an 82.8AH 12 V energy (battery) source.
At 24V the Amp draw would be (1Hp / .75% x 746W = 994.66Whr / 12V = 41.4A). If you wanted to operate a 1HP 24V trolling motor for 1 hour, you would need a 41.4AH 24V energy (battery) source.
At 36V the Amp draw would be (1Hp / .75% x 746W = 994.66Whr / 12V = 27.6A). If you wanted to operate a 1HP 36V trolling motor for 1 hour, you would need a 27.6AH 36V energy (battery) source.
At 48V the Amp draw would be (1Hp / .75% x 746W = 994.66Whr / 12V = 20.7A). If you wanted to operate a 1HP 48V trolling motor for 1 hour, you would need a 20.7AH 48V energy (battery) source.
What becomes evident is The higher your operating voltage, the lower your amp draw. This becomes important as you work to determine the size and voltage of the trolling motor you ultimately use and the size of the battery you will need to achieve your desired runtime.
To calculate electric trolling motor runtime, you will need to know your battery bank's Amp Hour rating and the motor's average amp draw: AH Battery Rating / Amp Draw = Run Time.
- A deep-cycle battery will maintain its operating voltage better than a dual-purpose or starting battery.
- A lithium battery will maintain a high voltage that does not sag, causing the motor to draw more amps.
- As voltage drops, the amp hour draw climbs (Ohms law).
- You cannot discharge a High-quality deep-cycle lead-acid battery to more than 50% to 80% without causing damage.
- You cannot discharge a high-quality dual-purpose lead-acid battery to more than 17.5% to 30% without causing damage.
- You cannot discharge a high-quality starting lead-acid battery to more than 1% to 3% without causing damage.
- A lithium battery can be discharged to 100% Depth of Discharge (DoD) without consequences.
Calculating what size trolling motor you need
Sizing or determining what trolling motor is right for you and your boat requires calculating the thrust necessary to get you and your boat moving. Thrust is measured in pounds (lbs.) and measures how powerful a motor is. Thrust is similar to torque in a car in that it is the determinator for the pull but not necessarily for speed. Essentially the larger and heavier the boat, the more thrust will be required. Thrust becomes very important if the electric motor is guided by GPS functionality to fix your position or run an actual course.