godivaPrima:

Servo datasheets seldom mention current, from what I've seen.

Here's how to calculate the power of a motor:

P=Tω

where :

P= power in Watts

T=torque in Nm

ω=angular speed in rad/s (ω is lower case omega)

Now spec sheets usually give Torque in kg∙cm and speed in seconds/60 degrees so we need a few conversions.

A kg∙cm is 0.1 Nm so divide the kg∙cm by 10 for Nm

60 degrees is pretty close to a radian which is 57 degrees so let's not quibble. So ω in 57degrees/s is the inverse near as dammit of the speed they quote in s/60degrees, so we want 1/speed.

Therefore for a motor of X torque in kg∙cm and Y speed in s/60degrees:

P=Tω = (0.1X)(1/Y)

Example: TowerPro SG90 micro servo has torque X= 1.8kg∙cm and speed Y = 0.12s/60degrees, so:

P= 0.1X/Y = 0.1x1.8/0.12= 1.5W

Now, we also now that:

P=VI or I=P/V

where P=power in Watts, V=volts and I=current in amps.

The TowerPro page quotes the above torque and speed at 4.8 volts, so:

I = 1.5/4.8= 0.3A.

But that assumes very good conversion of electrical energy to mechanical, so probably call it 0.5A (at 4.8V), and remember this is a *micro* servo.

A beast like a PowerHD has a torque X of 35kg∙cm @ 6V and a speed T of 0.2s/60degrees.

So P= 0.1X/Y = 0.1(35)/0.2 = 17W @6V

And I=17/6 = 2.8A best case with good efficiency, perhaps budget about 4-5A maybe?

So if your servo datasheet doesn't quote current, now you can figure it out from what they do tell you, the torque in kg∙cm, the speed in s/60degrees, and volts; remember to add some wiggle room for inefficiency.

That was extremely helpful! Thanks so much for that!

The specifications for my servo are:

Specifications:

Speed: 0.23 sec/60° @ 4.8V; 0.19 sec/60° @ 6V

Torque: 44 oz-in (3.2 kg-cm) @ 4.8V; 57 oz-in (4.1 kg-cm) @ 6V

Dimensions: 1.6 x 0.8 x 1.4 in (1-9/16 x 13/16 x 1-7/16 in) (40 x 20 x 36 mm)

Weight: 1.3 oz (37 g)

Connector: "J" type with approx. 5 in lead

So,

P = (0.1X)/(Y)

X = torque [kg cm] = 4.1

Y = speed sec/60 = 0.19

P = (0.1 * 4.1) / (0.19) = 2.16 [W]

I = P/V [amps]

I = 2.16 / 6 = 0.36 [A] or 360 [mA]

Considering efficiency maybe 5 [A]?

Did I do all that correctly?

What kind of mobile battery can provide this current?

I have been considering some NiMH batteries as that seems to be the most popular.