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I made a 4wd mobile robot with an 4 DoF robotic arm on it and currently in process of designing a power board for my robot. I would like get a detailed road map on designing a power board for the robot. Here is a brief overview of my hardware:

  • The robot has 8 joints which each of the motor runs with 12V (stall current: 800mA, nominal run: ~200mA),
  • The control board works with 5V (nominal current draw: 500mA, max: 1.5A),
  • Overall(averge) energy consumption of the robot is around 6 Watts (max 18Watts).

Here is what I have done so far for powering the robot:

  • I am using 2 ncr18650b battery in series with a cheap of-the-shelf battery management system (BMS), so I do not kill the batteries while over-running the robot,
  • Nominal voltages of these two batteries are 7.2V where I use one boost converter for driving the motors with 12V and a buck converter for driving the controller board with 5V,
  • I have to unplug the batteries when ever I need to charge them (since I do not have a charging circuit for the batteries yet).

Since I was working with the rest of the robot, I just built this battery setup in two days and it was working and enough. Now I would like to focus on developing a professional power management module. Here is simply what I need (need to understand) for designing the power circuit board:

  • I would like to power the robot with 2 18650 batteries in series,
  • the batteries (in series) should be charged without unplugging (from a 12V DC input port),
  • the robot can also be powered on while charging the batteries,
  • I would also like to monitor the batteries,
  • I would both want to supply 5V for controller board and 12V for the motors from the same battery unit. I encounted a problem with my cheap dump setup which; if motors draw much current from boost converter (12V), buck converters 5V drops. So, I would also like to prevent this happening.

As I am going to through the ICs and datasheet, I assume I need:

  • a 2S battery charging IC with a power path management (eg: bq24075) to also power the robot while charging the batteries,
  • a 2S battery management IC for balancing/protecting/monitoring the batteries (which are in series),
  • a boost converter for 12V,
  • a buck converter for 5V,

I am fairly experience with designing circuits but never worked on a power management circuit for batteries. I will appreciate if you give me detailed road map or resources for reading (schematics/articles etc.).

EDIT

A conceptual block diagram enter image description here

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  • \$\begingroup\$ stall current: 0,8mA, ?? 0.8A \$\endgroup\$ – Sunnyskyguy EE75 Jan 29 '18 at 20:40
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    \$\begingroup\$ You are on the right path. You need to make a conceptual block diagram and post it for review. Check out Linear Tech, they make all the parts you would need. \$\endgroup\$ – EE_socal Jan 29 '18 at 20:40
  • \$\begingroup\$ stall current: 0.8A. Edited. Thank you. \$\endgroup\$ – pmundt Jan 29 '18 at 20:42
  • \$\begingroup\$ Sure. I will post, as soon as I draw the diagram. \$\endgroup\$ – pmundt Jan 29 '18 at 20:43
  • \$\begingroup\$ The block diagram is a good start but needs more detail. For example what is Battery Monitoring interface, I2C? You have 12V going into a battery charging block but I do not see it can be used power the system while charging the batteries. Power Path is just shown as wires, there must be more to it. \$\endgroup\$ – EE_socal Jan 29 '18 at 21:25
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Define Efficiency, Features for Active or Passive Balance, cost, complexity, time budget, spare parts , temperature rise, thermal design of each block in addition with electrical design with max load while charging and operating. ( which could be almost twice the 1C current rate or ~ 6.7A !! and that BMS chip is limited to 1.5A)

So ALWAYS start with Specs before you waste too much time.

Battery background https://www.digikey.com/en/ptm/t/texas-instruments/introduction-to-battery-management-part-1/tutorial

If starting power at full voltage is 0.8A*12V = 9.6W per motor.

What software restrictions limit 5 motor starts at full acceleration?

I suggest you have PWM control V/f of motor slew rate controlled to limit current which will also improve stability.

Assuming you have done this and peak power is 18W with 2 cells at 3350 mAh typ each @ 3.6V = 12Wh * 2 = 24 Wh max then your max load will be slighly more than a 24/18 = 1.33C load rate. Which seems reasonable for experimentation at max load.

Batteries will be very well balanced (<<1%) when fresh and degrade very slightly then exponentially before EOL (end of life) Meaning that charge balance by cell monitoring is necessary for internal charging.

Try to keep between 20% and 90% State of Charge by useage. 90% is shortly after going from CC mode to CV mode. But if you have a good BMS chip, follow their advice. I only suggest smaller DOD range to extend battery total Wh*charge cycle life span. Using 50% will double battery life capacity.

Using a good Full MOSFET bridge with a good BMS and good software for slew rate control helps on the servo side while braking also recycles charge to battery. It depends on your requirements.

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  • \$\begingroup\$ Thank you for your precious comment. - Today I have not answer for efficieny, balances and temperature, - Since I am under a research budget, so I do not have a restriction for budget and complexity, - I have all the time until I create a working prototype, - Nothing limits the motor currents for now (the robot may hit a wall so all wheels may stall, the arm may try to lift a heavy object so all the joints mamy stall), - I am controlling each motor with PWM and monitoring each motors current draw. \$\endgroup\$ – pmundt Jan 29 '18 at 21:28
  • \$\begingroup\$ Is this a Stepper or Servo feedback design? WIth gears or? \$\endgroup\$ – Sunnyskyguy EE75 Jan 29 '18 at 21:32
  • \$\begingroup\$ Geared dc motors driven by DRV8801's. PWM frequency ~21.9kHz. \$\endgroup\$ – pmundt Jan 29 '18 at 21:41

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