Hanermo asked that we not measure battery energy by Ampere Hour.
Let me observe that we often confuse ourselves and others by misuse of terms that have well established definitions readily available in this internet age.
It has been a long time since most of us took physics.
Let me try to clarify key concepts and terms used in electric motor physics.
1. Volt. Measures electromotive force (emf). The force that could be applied to accelerate electrons if they were released. A form of potential energy. Terminal voltage is a parameter used to rate batteries.
2. Ampere. Measures the number of electrons passing a point in a second. A form of kinetic energy. Short circuit current is a parameter used to rate batteries, chargers and power supplies. It is the maximum current that can be sourced from the rated device. Usable current is always smaller.
3. Watt. Measures Power, the rate at which work is done, Joule/Second.
1 Watt = 1 Ampere * 1 Volt = 1 Joule/Second
745.7 Watt = 1 Horsepower
4. Joule. Measures energy stored or mechanical work completed.
A motor operating at 1 watt = 1 Joule/Second does 3600 Joules of work in an hour.
3600 Joule = 1 Watt Hour = 1 Ampere * 1 Volt * 1 Hour
1 Horsepower Hour = 745.7 Watt Hour
An Ampere Hour is a measure proportional to Energy when voltage is held constant; because battery terminal voltage falls with increasing current and as it is discharged, ampere hours is only loosely proportional to battery energy.
5. Approximate behavior of electric motors.
Torque is proportional to current. Increase current to increase torque.
Angular velocity is proportional to emf. Increase Voltage to increase rpm.
Instantaneous Power into an electric motor, Win = Volts * Amperes
Let Wout = Mechanical Propulsion Power out of a motor-transmission for a boat.
Wout = Win - electrical resistance loss - mechanical friction loss - propeller slip
James C. Rock
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Posted by: jcelrock@hotmail.com
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