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Motor Current Calculator


This is a JavaScript page that lets you calculate the motor current your vehicle will need.

You will need to enter some data for your machine. Some cells can be left at zero (or their default value). Others must be filled in for the calculation to make sense – these are tinted.

  • A robot could enter a gradient of zero or some small number, with a fast acceleration, maybe 0.5 seconds, or even less.
  • A golf buggy could use a steep hill, maybe 30%, with a longer acceleration time, maybe 5 seconds.
  • A Loco would have a small gradient, 1% perhaps, a large overall weight and a long time to accelerate.

All controllers have current/time specifications. E.g “one minute rating”. This should tell you whether the controller you want will give the current peaks you calculate below.

The page is here for you to experiment with to get the feel of how the values will affect your vehicle’s performance. The cells to experiment with are:

  1. Vehicle Speed. This is the speed the machine would go when unloaded and with full battery voltage on the motor, i.e. without any controller. Enter the value either in miles per hour, Km per hour or Metres per second. Make sure the calculator know which you have chosen!If you cannot easily measure your vehicle’s speed, you should calculate it. See our Vehicle Road Speed Calculator.
  2. Vehicle Weight. This should be the full laden weight under the worst condition you wish to use the vehicle.
  3. Passengers. If the Vehicle Weight does not include passengers, enter the number of Adults and Children you wish to carry. If they are included in the overall weight, leave these set to zero.
  4. Nominal Battery voltage. Usually 12, 24 36 or 48v, but enter whatever you wish, to see how it affects the answer.
  5. Weight of one battery. This may be already included in the overall vehicle weight, if so leave it as zero. However, you may be toying with doubling the number of batteries, which will increase the overall weight (unless you use smaller batteries). It’s here for you to experiment with. If you enter anything here, this must be the weigh of one 12v battery as that’s the normal amount you will want to alter.
  6. Motor current on level ground. This is a difficult one as you usually cannot guess or measure it until after you’ve finished making the vehicle. But it does affect the calculation, so is really required. If in doubt, make a guess or use our figure. It’s likely to be a fairly small part of the total.
  7. Hill climbing ability. Your machine must be able to climb some sort of gradients. For locos, 1% is usual, 2% quite extreme. For a golf buggy, 30% is about the worst. Try various values and see the results on the motor current. Gradient here is the sine of the angle of inclination, or the rise per length of travel along the surface.
  8. Length of hill. You do not need to enter this but, combined with the vehicle’s top speed, it tells you how long you will need the peak current for. This can give you good indication of the controller you need.
  9. Acceleration. Time (in seconds) in which you want the controller to accelerate the machine to full speed.

Data Entry area
What is Vehicle Speed (at full motor speed)?
What is the Vehicle Weight
How many Adult Passengers? (Av’ge 12 stone)
How many Child Passengers? (Av’ge 5 stone)
What is the battery voltage?
What is the weight of one 12v battery?
Motor current on level ground?
Hill climbing ability: Gradient in percent
Length of hill.
Acceleration. (Time to reach top speed) Seconds
Output area: No need to fill in below!
Total mass of all of the above is: Kilogrammes
Vertical Speed is speed/gradient(Vv)

At top speed, rate of climbing is:

metres per second
Power for climbing. (Motor Current needed to climb)
Power= Mass x G x (Vv)

where G is the acceleration due to gravity, 9.80 metres/sec2

So the power for climbing is: Watts
And as this is at full speed

the motor current is:

Kinetic energy. (Motor Current needed to accelerate)
KE = 1/2 x M x V2
So in this case, KE at full speed is: joules
But this speed increase took place in the acceleration time you entered.

So to get Joules/second, divide by the acceleration time.

Now one Joule per second is a watt. We are accelerating from zero to full speed (zero to full battery volts) so divide by the average motor voltage (i.e. by half the battery voltage) to get the

motor current needed for this acceleration: Amperes
Total Motor Current
Add the three currents: Amperes
That on level ground + that for climbing + that for accelerating
Time duration of Current
You chose an acceleration time of Seconds
You will climb the hill you specified in Seconds