I’m building a robot arm on wheels. I’m using 4 MG996R which operates at 4.8V- 6.6V, stall current of 2.5A, and running current is 0.5A-0.9A. The DC BO gear motors operate at 3V - 12V and have a stall current of 700mA.
The local robotics shop says that if I buy batteries with the stall current rating total of my motors it will damage them and that I should follow the rated current instead. However, it is the opposite recommendation in another post:
Find a different shop or stop listening to that salesperson. They do not know of what they speak.
Do listen to @jremington. He is very knowledgeable.
One always needs a power supply that can supply more current than the circuit needs. A motor (or most any electronic component) will only draw the current that it needs. Think about your toaster. It connects to the line power grid with hundreds of amps available but does not explode.
Whoever told you that does not have the slightest idea what they are talking about. Do avoid that shop!
Loads draw current; the power supply must be able to supply that, or the circuit will not work as intended.
That said, do not exceed the voltage rating of the servos. And of course, servos are not meant to be stalled for any length of time, but they do briefly draw the stall current every time they start moving.
I suspect the shop guy is confusing the ability of the batteries to supply a certain maximum current with actually using that maximum.
As @groundFungus stated, you always want to have more current available than you would use. This will avoid unexpected voltage drops, erratic operation and brownouts.
However, you always want to operate motors/servos at their rated current, since that is the number the manufacturer says the motor will be able to use without damage.
Stall current is the maximum the motor will pull and will probably lead to damage if run for a length of time. The exact time depends on the motor, case, cooling, ambient temp, etc.
Any DC motor will, briefly, draw the stall current every time that it starts.
I would wholeheartedly agree with that.
Although the thought of having a local robotics shop is an intreating prospect. There are very few in the UK and those there are are only online things, as far as I know.
So I should still calculate the overall stall current for my batteries? Then are there any batteries that can give me 5-6V and 12A? I have a battery slot that fits 4 AA batteries.
I think you mean stall current for my motors/servos. They draw the current; batteries only supply as much as demanded, up to their own maximum capabilities.
You will find it hard to meet the requirements of 12A in a AA format. You'll have to look at LiPo cells. They will be 3.7V each, nominal. Can be as high as 4.2V when fully charged.
The discharge capacity is listed as 5C, 10C etc. 'C' is the cell capacity in amp-hours.
A 1700 mAh cell is 1.7 Ah. Multiply that by the 'C' rating of the cell, and this should give you the maximum current you can draw from that cell.
I don't think there are any AA sized batteries that can supply the ~10 A start/stall current required by four MG996R servos.
You will have nothing but trouble with an inadequate servo power supply, so save yourself the frustration.
Look for suitable battery packs from Radio Control vehicle suppliers.
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