# Need to run a mobile robot on batteries

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

• stall current: 0,8mA, ?? 0.8A Jan 29, 2018 at 20:40
• 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. Jan 29, 2018 at 20:40
• stall current: 0.8A. Edited. Thank you. Jan 29, 2018 at 20:42
• Sure. I will post, as soon as I draw the diagram. Jan 29, 2018 at 20:43
• 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. Jan 29, 2018 at 21:25

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.

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.

• 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. Jan 29, 2018 at 21:28
• Is this a Stepper or Servo feedback design? WIth gears or? Jan 29, 2018 at 21:32
• Geared dc motors driven by DRV8801's. PWM frequency ~21.9kHz. Jan 29, 2018 at 21:41