charmflex
Posts: 4
Joined: Mon Mar 22, 2021 8:47 am

Control 8 35kg servos using raspberry pi with external power supply, how?

Thu Jun 24, 2021 5:20 am

Hello. I want to build a quadruped robot with 8 high torque servos (operating voltage: 4.8v ~ 6v). I have two 18650 batteries, with total of 7.4v.
I want to power those servos with these battery, but since the voltage is 7.4v, which will damage those servos, I decide to use 5v regulator in the circuit. However, the servo motors will also draw a lot of current. From what I surveyed, 1A per servo motor would be common condition. So I would expect I need 10A in the circuit. Which and how many regulators should I buy, and how to build, in order to sustain 10A current?

In simple word, I need to step down the voltage to 5v from 7.4v, how should I build the circuit in order to run 8 high torque servo motors with external power supply?

Thanks in advance.

LTolledo
Posts: 5559
Joined: Sat Mar 17, 2018 7:29 am
Location: Anime Heartland

Re: Control 8 35kg servos using raspberry pi with external power supply, how?

Thu Jun 24, 2021 11:51 am

what is the ampere-hour rating of your 18650 cell?

and what RPi model are you going to use?
"Don't come to me with 'issues' for I don't know how to deal with those
Come to me with 'problems' and I'll help you find solutions"

Some people be like:
"Help me! Am drowning! But dont you dare touch me nor come near me!"

charmflex
Posts: 4
Joined: Mon Mar 22, 2021 8:47 am

Re: Control 8 35kg servos using raspberry pi with external power supply, how?

Thu Jun 24, 2021 1:50 pm

LTolledo wrote:
Thu Jun 24, 2021 11:51 am
what is the ampere-hour rating of your 18650 cell?

and what RPi model are you going to use?
The battery is 3000mah, I have two, so total is 6000mah. I use raspberry pi 4.
I checked out that battery has c rating. Does it mean that each battery can only output up to certain limit of current?

LTolledo
Posts: 5559
Joined: Sat Mar 17, 2018 7:29 am
Location: Anime Heartland

Re: Control 8 35kg servos using raspberry pi with external power supply, how?

Thu Jun 24, 2021 9:18 pm

if used in series to get 7.4v, the capacity theoretically remains at 3000mAh
if used in parallel at 3.7v, the capacity will theoretically go up to 6000mAh

3000mAh / 10A = 0.3h = 18 minutes theoretically, but you'll get much less than that
so not enough juice!
also I doubt a 18650 cell can give 10A without eventually exploding!... so its a safety concern!

get a bigger capacity battery, like the ones used for RC cars/airplanes.
some are rated 6000mAh at 7.4v
each servo need to have its own battery + buck converter
the RPi will also need to have its own battery + buck converter

or if you really want longer operating hours in a single battery, get an 8D instead...

start calculating the cost to do the project....

or re-think the requirements for the project.
"Don't come to me with 'issues' for I don't know how to deal with those
Come to me with 'problems' and I'll help you find solutions"

Some people be like:
"Help me! Am drowning! But dont you dare touch me nor come near me!"

charmflex
Posts: 4
Joined: Mon Mar 22, 2021 8:47 am

Re: Control 8 35kg servos using raspberry pi with external power supply, how?

Fri Jun 25, 2021 8:43 am

LTolledo wrote:
Thu Jun 24, 2021 9:18 pm
if used in series to get 7.4v, the capacity theoretically remains at 3000mAh
if used in parallel at 3.7v, the capacity will theoretically go up to 6000mAh

3000mAh / 10A = 0.3h = 18 minutes theoretically, but you'll get much less than that
so not enough juice!
also I doubt a 18650 cell can give 10A without eventually exploding!... so its a safety concern!

get a bigger capacity battery, like the ones used for RC cars/airplanes.
some are rated 6000mAh at 7.4v
each servo need to have its own battery + buck converter
the RPi will also need to have its own battery + buck converter

or if you really want longer operating hours in a single battery, get an 8D instead...

start calculating the cost to do the project....

or re-think the requirements for the project.
Each servo need a bulk convertor? Why not use 1 bulk convertor with all 8 servos?
I also dont mind if it run for how long, as long as the robot can move...

LTolledo
Posts: 5559
Joined: Sat Mar 17, 2018 7:29 am
Location: Anime Heartland

Re: Control 8 35kg servos using raspberry pi with external power supply, how?

Fri Jun 25, 2021 11:37 am

do as you wish, and see what happens...
"Don't come to me with 'issues' for I don't know how to deal with those
Come to me with 'problems' and I'll help you find solutions"

Some people be like:
"Help me! Am drowning! But dont you dare touch me nor come near me!"

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