Battery required to power 8 servos

Projects General Guidance
1

Hi! I'm new to arduino and electronics in general, but I wanted to try and power 12 MG996R servo motors (which have a stall current of around 1.1 A from what I know) using a PCA9865.

But I stumbled across this question: What battery can power all of them?

To be more clear on the purpose of the project, they will be for a bipedal autonomous working robot, and all it will need to do is: Walk, Crab Walk and Puch. That's all. Now, the operating current of those servos is between 500-900mA, with an operating voltage of 4.8-7 V, so if we take an average of 700 mA, that would be 700*8 (MAX 8 servos will run at the same time) = 5.6A needed. What battery can supply 5.6-6A of current with at LEAST 6 V? What options do I got, that will weight LESS than 500grams? :sweat_smile:

2

you don't want to provide an average current, you want to be able to support peak demand...

3

Then that would be 1.1 A at stall, so 8 (MAX servos running at the same time) *8 = 8.8 A, but lets's say 9A

Any battery that can provide 9A with 6V, and weight less than 500g?

4

Note: the battery can have even a higher voltage, like >6V, but that only if Amperage doesn't change when I take the battery through a LM2596 (voltage step down). If say the battery specs are 9A, 12V and I take it divide the voltage by 2 with the module, then will the amperage get sliced as well?

This is why I was wondering if there is any battery in this range: 10A, 6V, ,500g, so that we don't need to apply any step up/down module to it

5

I would look at RC batteries. You'll probably see them at 9.6V or 11.1V (so you'll need a step down converter that can take the amps) with a high C to ensure it can deliver 9 amps

the weight will basically play on how long that battery will last...

2 Likes
6

Oh, so the step down will not only lower the voltage, but also the amperage (e.g. we lower the voltage twice, the amperage will be sliced twice too)? And can you recommend me an RC battery that can last for at least 10 minutes (with such a high power demand)

This should take it below 500g?

7

no the step down will only play on the voltage, but it needs to support high current demand

I don't have a specific recommendation

9A for 10 min would be a 1500mAH - so say 2000mAH to be fine - with at least 6C. There are many offerings at 7.4V 2000mAh @20C with 2 Lipo Battery in series.

an alternative would be multiple smaller batteries each powering a subset of the motors.

1 Like
8

Great!! What do you mean that it needs to support high current demand? Will the LM2596 be good enough? Or is there something better?

So a 7.4V 2000mAh battery? But isn't that the capacity of it? From what I know (still a beginner, might be wrong), capacity and the current it can provide to the circuit at any given time are different. Can they provide 9A of current at any time to the servos?

9

Also, what does "C" mean? (Would appreciate it a lot, I'm trying to learn)

10

the datasheet says 3A output load current so no, you need to be able to support 9. (also it depends on the rest of the board)

pololu might have some like the https://www.pololu.com/product/2866 (never used it)

11

The C Rating tells you the maximum current you can draw without damaging your LiPo battery. The formula is simple

Max Current = Capacity x C Rating

Say you have a 3500mAH (so 3.5AH) LiPo with 20C it means you can draw up to 20 x 3.5 = 70A for some time

if you draw this, because your battery can only provide 3.5A for 1 hour, you get a duration of 3.5/70 = 0.05 hour so 3 minutes

if you only draw 9A, you get a duration of (3.5 / 9) x 60 = 23 minutes

There are many articles on line on the topic, here is the first google hit

https://hobbyking.com/en_us/blog/reading-and-understanding-lipo-batteries/

1 Like
12

Oh, ok! But what about the RC batteries? From what I found these are LiPo batteries. I don't get it how their curent output is measured since, from what i found, I get just 7.4V, 3000mAh, 40C. How do I find the amps it can provide from this?

13

see the answer I was typing whilst you asked :slight_smile:

1 Like
14

Yes!!!! Thanks so much for all the help! I get it all now

15

Yeah, I saw it now. Thanks again, and have a good night/day! I think this thread can be closed

16

great - to close a thread you can mark one of the answer as a solution and the post will get a little tick mark

have fun

1 Like
17

Just a quick question, it can be from any manufacturer as long as the output current is >=9A, right? Will this one work too: Amazon.com

?

18

In fact, the MG996R stall current is 2.5A, so for 12 of them, budget at least 25 Amperes. 30A would be ideal.

You will also need a servo power distribution PCB capable of handling that current.

19

if you trust the manufacturer, yes sure. This could work. I've never used such model

20

Ok, I will see what I can get to work with that RC battery