Servo datasheets seldom mention current, from what I've seen.
Here's how to calculate the power of a motor:
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:
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
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.