Battery requirements for 4wd + Raspberry PI

Robotics and electronics discussion Electronics
1

Hi,

I have 4 motors and a Raspberry PI and I would like to use 1 single battery pack.
In particular, I have:

Total of ~15.8A

I am planning to use 1 single battery pack plugged in the motor controller so I can power the raspberry pi using the 5v out.

Main Question
I would like a battery pack that will last for 4h. Which specification I should be looking for (Volt, Ampere, …).

Additional notes
I am planning to buy a Li-ion battery pack with this spec: 7.4V, 2000mAh; Discharge: 15C
I have the following concerns:

    • The battery will last roughly 10min
    • I might burn the motors with that ampere
    • I am not sure I can power the motors and the raspi togheter
2

Hello.

You should probably be looking at the battery voltage (V), its capacity (Ah or mAh), and its discharge rate (C). I strongly recommend reviewing this “Understanding battery capacity: Ah is not A” blog post which will probably cover most of what you need to know for picking your battery, but here are three additional considerations to keep in mind:

  1. Your battery lifetime for something like this is largely going to be dictated by how hard you run the motors. The ideal way to make an informed estimate for this would be to run your robot for a while monitoring their current draw, but estimating that each motor will draw around 25% of stall current might be a good enough starting point.

  2. High instantaneous loads, such as when your motors first start up, will cause voltage drops. Since the RoboClaw 2x15 has a minimum operating voltage of 6V, it might be better to use a higher voltage supply to increase your safety margin there. However, keep in mind that powering your motors with a higher voltage will increase their speed and torque, while sacrificing operating lifetime. You can avoid this by limiting the applied duty cycle so that the average applied voltage they receive stays around their nominal voltage rating.

  3. Unlike your motor controller and motor which can handle a broader input voltage range, your Raspberry Pi strictly requires 5V for power. So, if you want your whole system to be powered from the same battery, you should consider powering the Pi through an appropriate step-down voltage regulator.

- Patrick

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3

thanks Patrick really helpful.

The motor controller outputs 5V. So, I believe I can just connect that to my RaspPI.

Here is the my calculation:
For the Dagu I can average 3.5A consumed by each motor = 14A
The Raspi consumes ~600mA + 2A (for a couple of USB devices) = 2.6A
Total of 16.6A

I found a Lipo Battery 5200mAh 7.4V 50C which should give me roughly 18min based on the above assumptions. (60/16.6) x 5.2

I hope the above calculation is correct.
Thanks

4

Basicmicro’s datasheet for the RoboClaw 2x15A says that it can supply up to 3A of 5V power, so as long as you do not have too much other stuff connected to your Raspberry Pi, powering it from the RoboClaw should be okay.

Your calculations look okay, though I will point out that if your motors are on average drawing 3.5A, then you are probably over-exerting them (though maybe you are just using a conservative value in your calculations). We do not make the motors that come with the Wild Thumper chassis, but we do make similar ones and our general recommendation for those is to keep continuous current draws to around 25% or less of the stall current.

- Patrick

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