Calculating Motor Run Time
A common questions people have when trying to select a motor is “How long will this motor run out on the water?” To answer this question, it's necessary to know two things: a battery's amperage hour rating and a motor's amperage draw.
Amperage Hour Rating: Marine batteries are rated on a metric called “amperage hours”, which is an indication of how long a battery can supply a consistent amperage. The larger the rating, the more power the battery can hold, and the longer it will be able to power a motor at a given speed. For example, a 100 amperage hour battery is one that would be able to supply 25 amps of power for 4 hours (25 amps * 4 hours = 100 amp hours) before running out of energy. Alternatively, the same battery could also supply 10 amps of power for 10 hours (10 amps * 10 hours = 100 amp hours) before running dry You can think of the amperage hour rating as being similar to a gas tank in a car – the larger it is, the more energy/fuel it can store, and the longer it can power a motor.
Motor Amperage Draw: The second factor we need to compute estimated run time is motor amperage draw. A motor's amperage draw rating refers to how much amperage (or current) a motor draws at a given speed. A motor's exact amperage draw rating isn't always easy to find, but should be available from the manufacturer or, if it's a motor we stock, from a TrollingMotors.net representative.
In order to approximate how long a certain motor will run on the water, you simply take the battery's amp hour rating and divide it by the amperage draw. For example, for a motor that pulls 20 amps at medium speed using a 100 amp hour battery, the run time would be:
100 amp hour rated battery / 20 amp draw = 5 hour run time
Most manufacturers will only list a motor's maximum amperage draw at top speed, so you may need to estimate other amperage draws at various speeds using the max speed amperage draw as a baseline.
For a general idea of approximate amperage draws by motor size, please see the chart below. Please note that these are approximations only and results will vary based on exact motor figures, batteries used and water conditions.
Considering Weather Conditions
It's important to note that water and weather conditions can significantly affect battery performance. A motor drawing 20 amps to move a boat at medium speed in calm conditions may need 30 amps to move the same boat in strong wind or waves. If you plan on using your motor frequently in windy or stormy conditions, you'll want to factor this into your calculations.
Extending Battery Life
There are a few things you can do to get the most run time possible from your battery & motor combination.
Use Variable Speed Motors: Using a variable speed motor (vs. a fixed speed motor) generally results in significantly longer run times. Variable motors are more expensive – generally about $100 more than a comparable fixed-speed model – but are more energy efficient, especially at slower speeds. They are also much more convenient as they allow you to dial in the speed to the exact setting you want vs. having to select from pre-set speeds. For more details, see these notes on fixed vs. variable speed motors.
Buy a Larger Motor: Larger 24v and 36v multi-battery trolling motors are able to provide the same thrust as smaller motors with less amperage draw, which results in longer run times. If you need seriously long run time for your smaller boat, it may make sense to buy a 24v or 36v motor.
Don't Fully Deplete Your Battery: It's never good for a battery to be run completely bone dry. Doing this on a regular basis will almost certainly reduce the lifespan of your battery. If possible, try to recharge your battery(s) before they are completely empty. Using a battery life indicator – a handheld unit or one built into the motor – will help you monitor battery life as well as reduce the chances of unexpectedly running our of power miles from shore.
Remember The Off-Season: It's really bad for batteries to be left uncharged for months at a time, and will contribute to shorter battery life and reduced performance. During the off-season, it's a good idea to use a battery tender or battery trickle charger which keep a small amount of current running through your batteries. Alternatively, you can also re-charge your batteries every month or so to ensure they retain a charge and don't sit empty. Both options will significantly increase the life of your batteries.
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