# Choosing a battery to power multiple high power servos with different specs

I am currently designing a suit that will be using multiple high power servos. There will hopefully be 2 sets of servos with 24 V 5 A, 12 V 3 A, 7.4 V 3 A, and 6.8 V 2 A. I'm having trouble figuring out how to power it. The best advice I've seen out there for dealing with multiple voltages is to uses the highest voltage and use a buck converter or something similar to scale down the voltage for the rest but I'm worried that with so many amps being drawn that it would be too much. I'm also not sure if 5 A on a servos specifications actually means it will be drawing 5 A or not, there seems to be a lot of weird stuff with servos and how they draw power and I just can't find a clear answer on what the best way to power these very different servos. I've thought about getting a separate battery pack for each type of servo but there seems like there should be a better solution out there. It only really needs to be able to run for like 15 minutes so Ah isn't as much of a concern for me as the max Amp problem is.

Any help or pointing in the right direction would be appreciated. This is my first post so I almost certainly messed something up so sorry in advance.

Are you talking about hobby type radio controlled (RC) servos for model aircraft or vehicles, or closed loop controlled servo motors for machine motion? Post a datasheet for the individual servos.

Can you post a link to the data sheet for your servos please and we can have a look.
The peak value is what is known as the stall current when the load on the servo stops it from moving. You also get a pulse of this current when a servo starts to move from rest. The actual current drawn when moving depends on the load it is driving, which is why you rarely see this figure in data sheets although there might be a graph of current V torque.

You have to make sure that the power supply can produce these peaks and it is normal to fit large decoupling capacitors across the servo to help handle these peaks.

Yes a buck converter can be used to generate different voltages from a single source. But with the currents involved these could be expensive. The good news about this though is as the voltage drop increases the current drain from your main power source drops. In practice they only have an 80 to 90% efficiency but that is better than burning the excess power in heat.

One thing you will have to consider with a rechargeable battery of this size is not to over discharge them, as it will damage Lithium batteries so they need monitoring electronically to make sure this does not happen.

There is a C rating to a battery which determines the maximum current one can produce. This is a multiplier on the Ah, so with a C of 4 and a 1Ah battery you can pull a maximum of 4A from it.

Please post datasheets for all these servos. That's the only way we will be able to work out what the numbers you're quoting actually mean. It's extremely unusual to specify servos in terms of voltage and current.

Steve

Hi, @curlybird
Welcome to the forum.

Please read the post at the start of any forum , entitled "How to use this Forum".

This will help with advice on how to present your code and problems.

I am currently designing a suit that will be using multiple high power servos.
There will hopefully be 2 sets of servos with 24 V 5 A, 12 V 3 A, 7.4 V 3 A, and 6.8 V 2 A.
I'm having trouble figuring out how to power it.
The best advice I've seen out there for dealing with multiple voltages is to uses the highest voltage and use a buck converter or something similar to scale down the voltage for the rest but
I'm worried that with so many amps being drawn that it would be too much.
I'm also not sure if 5 A on a servos specifications actually means it will be drawing 5 A or not.
There seems to be a lot of weird stuff with servos and how they draw power and I just can't find a clear answer on what the best way to power these very different servos.
I've thought about getting a separate battery pack for each type of servo but there seems like there should be a better solution out there.
It only really needs to be able to run for like 15 minutes so Ah isn't as much of a concern for me as the max Amp problem is.

Any help or pointing in the right direction would be appreciated. This is my first post so I almost certainly messed something up so sorry in advance.

Tom...

Here are the servo data sheets I could find and all the information I could get on the ones that didn't have data sheets listed. Thanks so much for the responses I really appreciate what y'all do.

20 kg servo data sheet.pdf (414.9 KB)
35 kg servo data sheet.pdf (392.5 KB)
60 kg servo data sheet.pdf (327.5 KB)
180 kg servo data sheet.pdf (409.5 KB)

What is the standard way of specifying servos by the way? I'd love to be using the common language. Would it just be the kg-cm rating or oz-in rating?

Stall current, the current needed when starting, is the figure You need to account for.

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