# Amperage draw for Servo

I'm trying to find a battery that I can use for a project. Let's be hypothetical for a moment.

I have 5-8 of these Adafruit Servos, connected to this Adafruit Servo Shield. Leaving the Arduino itself out of the equation, how can I calculate the amperage draw that these motors and this shield will pull? And additionally, how can I use that with the Ah of a battery to calculate the time that the circuit could be on and in motion with the battery in question?

## 2 Answers

The datasheet for the device should always list a maximum or "stall" current. For this device, it's 2.5A.

You can use that for a worst-case estimate of battery life by multiplying it by the amount of time you want to run. For example, if you want 12 minutes of run time, then the battery needs to have

$$(2.5\mbox{A})*(12\mbox{min})/(60\mbox{min/hr}) = 0.5\mbox{Ahr}$$

Per servo that you have on your device. That is, if you have 5 servos, then you need a battery with a capacity of 2.5 Ah to run for 12 minutes. If you have 8, then you need a 4 Ah battery to run for 12 minutes.

Again, this is a worst-case scenario, if you try to run your servo at stall current for a long duration then you'll probably wind up burning them out.

How much capacity you actually need depends on the the duty cycle at which you're running the servos. If you're running an excavator then you might actually be running them close to stall current frequently. If you're trying to position a camera then you won't be.

My advice to you would be to go get a battery pack, any battery pack, and use that to power your system. Then, run the system as hard as you think it would normally be run. Compare the run time you achieved with the run time you desire. Then, adjust the capacity of the battery pack accordingly:

$$\mbox{Required Capacity} = (\mbox{Test Capacity})*\frac{\mbox{Desired Run Time}}{\mbox{Actual Run Time}}$$

So, if you had a 1.75 Ah battery and it ran for 5 minutes, but you want it to run for 20 minutes, then you need (1.75)*(20/5) = 7 Ah battery pack.

The most practical way current draw is to power up the arduino board with a power supply capable of measuring current draw. Each time the Servos are activated the additional current draw can be observer on the power supply with a digital read out like the one below.

For much more accurate calculation, find a power supply capable of logging current draw. Calculate the current draw in relation to time to obtain the Ah.

Alternatively you can you use a multimeter on supply line to record the current draw. A multimeter similar to 34410A Digital Multimeter, 6½ Digit is capable of data logging the current draw. 