It is my understanding that both a regular (speed coil) and a digital trolling motor constantly use the same amount of amperage when electric current is being sent to the motor. The difference is that a regular trolling motor draws the full amperage constantly and allows you to run at different speeds by reducing the voltage going to the motor using a speed coil that acts as a big resistor and wastes part of the power as heat.
A digital trolling motor has an electronic switch that turns on and off rapidly sending a pulse of power to the motor. The faster the switch opens and closes, the faster the speed of the motor. So less amperage is used at slower speeds because the current to the motor is being switched on and off and the amount of time electricity is turned on is reduced. At full speed, a battery charge would probably last the same amount of time for both a regular and a digital trolling motor. The slower you go, the longer the battery charge will last on a digital trolling motor vs a regular (speed coil) trolling motor.
You will get more distance out of a battery charge on a regular trolling motor if you only run it at the top speed. With a digital trolling motor you probably get about the same distance at any speed but you can use the trolling motor for a longer period of time by running it a a slow speed. There are other variables, wind, water current, etc but the amount of time you are sending electicity to the motor is the main thing that determines how long your battery charge lasts. With a regular tolling motor you are constantly sending electric current to the motor, with a digital, you are sending short bursts of electric current to the motor.
I know it is harder on my trolling motor, but I leave my trolling motor on speed 5, 4, or 3 and use a Big Foot switch to control my speed by manually turning it on and off with my foot. It lets me use my trolling motor for a longer period of time from one battery charge. It also probably scares the fish because my foot is not as smooth as a digital trolling motor or even using the speed handle on a regular trolling motor. Not a good idea for crappie but white bass and cat fish don't seem to mind.
After look at the Internet, I think my basic concept about how the digit speed control works is right but in reality it has a lot more happening to make it practical.