Kurt - questions on LiPo usage for DIY household robot

Dear forum,

Over the past couple of months, I have been building my first home robot. I do not have a background in electonics, so for many elements, it is a steep learning curve. I do have some background in IT and have some programmig skills.

So far, I’m quite happy with the result I achieved so far. The robot - his name is Kurt - is based on a triangular two-level chassis with 3 omnidirection wheels and 9-12 V DC motors with hall sensor encoders (the latter not being used yet).

The brain is a Raspberry Pi 4B with the following hats:

  • A motor controller HAT. The motors are currently powered by 6 1.5 Alkaline batteries.
  • A Grove Base HAT to connect sensors from the Grove system. Currently, I have US sonics for obstacle detection, a digital compass, and a 3-axis accelerometer installed.
  • A PiJuice UPS HAT to allow the robot to switch seamlessly from battery operation to USB/external power source operation and vice versa. I have a 5000 mAh and a 12.000 mAh lithium battery that are compatible with the PiJuice, currently operating on the smaller one.
  • A ReSpeaker 4-mic array with 8 LEDs for voice detection, direction estimation and visual feedback on voice commands using the LEDs.

In addition, a camera and a USB speaker are connected to the Raspberry.

Kurt is currently able to:

  • Understand voice commands and reply to voice commands
  • Move in different directions based on voice commands
  • Detect obstacles ahead and stop movement
  • Recognize faces using the camera and greet known people by their name

The final aim for Kurt is to move around the house autonomously. One element of autonomous movement is also autonomous charging. I’m planning to connect a wireless charging solution below the chassis that can provide power to the PiJuice to charge the battery and to operate the Raspberry while charging. The charger will be a DIY (3D printed) charging station. The homing will likely be implemented using infrared signals.

My main concern at the moment (and I’m hoping someone on the forum here can shed some light and maybe give some hints in the right direction) is how to include the motor power supply in the autonomous charging routine. Therefore, I would like to replace the 6 x 1.5 alkaline batteries with something rechargeable.

The obvious (?) choice would be a LiPo.

To power three 9-12 volt motors (working voltage is specified as 9V, load current 1200 mA per motor), I would say I need a three-cell LiPo with 11.1 nominal volts and e.g. 2200 mAh capacity (most of the time, after all, the robot is not going to be moving at all).

The big challenge is how to intregrate it into the existing system and autonomously charge it. the PiJuice is able to provde a 2100 mAh output at the battery voltage (so something like 4 V I would assume). Is it at all conceivable to somehow charge a second battery (i.e. the motor LiPo) with that current? For example, using a voltage regulator and a multi-cell charger board? I have not found any such device though, I only find single-cell charger boards, or large multi-cell stand-alone charger devices.

Then: Is it actually SAVE to create a robot like this? If you read about LiPos, you inevitably get to the many warnings about the danger of LiPos: NEVER charge it unobserved, ALWAYS store and transport in a LiPo safe bag. I’m a bit torn between being doubtful about the safety of such a robot in our household, which I don’t wish to burn down, and the notion that most of those warnings come from the RC racing / drone scene where much larger LiPos are used in mush harsher conditions. Also, there are so many household lithium batteries around in our phones, laptops, vacuum robots…

Speaking of LiPo safe bags/cases - everybody says “Use them for charging and transport”. Why not during usage? Is that because usually that is not possible due to space constraints, or is there a reason a LiPo battery should NOT be in a safe bag/case during usage/discharge, for example access to air? Otherwise, I would imagine putting the batteries of our robot into a LiPo safe case screwed on to the robot itself (under the decorative hat) for additional peace of mind.

Apologies for the lengthy text and looking forward to some insightful hints!



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