@greendragon If you build an 8S2P and stack the additional 4 cells on top of the other 4 then you only reduce the heigth by an extra 2.76". The total battery will measure 2.52" wide (the space is 3") 5.04" deep (the space is about that deep as I recall) and now a total of about 5.6" tall but maybe more depending on wiring etc.That still leaves about 3" x 5" x 5" for the BMS and all the boards etc. I think it's a winner.

The battery is 8S2P of 32700 LiFePO4 with capacity of 12,000mAh and a peak current of 3C or 18,000mA with a 3 sec pulse of 10C or 60,000mAh. That should handle all your motors I think.

Posted by: @greendragon

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Back to Subject of a Battery:

The following is the latest proposal on the table -

Using LiFePo4 3.2V 6000mAh Battery 32700 LFP Cells to Build a 4S2P 32700 battery pack

Here is the ink to the proposed Cells if you need more information about them.

https://www.ebay.com/itm/266279786466

If I understand correctly this will create a 12.8V at 12AH

It has been suggested that the battery should be more powerful due too unknown variables in load.

I can see this, and am a big believer in overbuilding if possible. It's not going to cost a ton of money to do this, and it would be easier to plan to do it before then after building the battery.

So how much more Voltage/AH would you suggest??

GreenDragon

The questions you are asking are good questions, but you have not given the details necessary to do those calculations.  Without knowledge of how the power will be used, how the hard/easy the bot will be driven, how much and how hard the other steppers / servos will be used, how heavy the bot will be, it is simply impossible to give you answer besides... 42.  I can easily tell you that using those same 8 cells in a 8S1P for 25.6V will work far better.  But since you are now considering individual cells as an option, you can run them either way.  I believe your choice of steppers will not be powerful enough to move the bot you're suggesting.

But really, since you do not seem to have any actual quantifiable information or hands-on experience with stepper motors and stepper motor drivers, purchasing batteries at this early stage seems to be completely ill-advised.  Your eventual design would be far better served getting some pieces and experimenting with them so you have a real-world feel for steppers strengths and weaknesses.  Note - the links are just for reference purposes.  Use vendors you like.  I would suggest starting out with:

• A couple of the Nema-17 stepper you suggest
• DRV-8825 stepper driver - https://amz.run/7UrU - These have given me great service, are easily configurable, capable of driving larger steppers if/when you'll need them, are commonly available anywhere and dirt cheap.
• A bench power supply - https://amz.run/7UrS - This will allow you to vary the voltage and/or current so you can fully test out configurations quickly and easily.  It also shows you how much current is being used so you can easily do the calculations, you are asking above.
• I would assume you already have a volt meter.
• The Mega

With these you can:

1. Feel just how much/little torque these steppers can supply
2. Try them at different voltages, speeds and accelerations
3. Measure how much current they'll use
4. See about wheel mounting choices.    
5. Learn about current limiting adjustments on drivers
6. Learn about micro-stepping of the drivers.
7. Learn to program them, synchronize two on the same side of the bot
8. Determine if the Mega is going to have enough CPU to simultaneously control your large number of steppers, servos and anything else. 

Questions/Details/Information:

1. - As you have surmised I do not own a Bench Power Supply so at this moment I am unable to run a lot of the tests you would like me to do.

2. - This is a pay as you go robot build. In other words I make a sale on eBay get some small money, and order a part.

3. I have all the motors in the master list, and have installed them. This was necessary for the design/building stage. Note: I used my understanding of engineering to design the frame of robot 42 to take the weight off the stepper motors thus allowing them to be able to move free and easy.

4. I dd some crude early tests with very small brush/gear motors and they were able to move things quite well. 

5. I have A multi meter, a few test leads, soldering iron/solder, assorted electrical connectors, assorted heat shrink wraps, wire cutters, regular batteries..

6. I bought "The Most Complete Starter Kit MEGA 2560 Project" from ELEGOO.COM . Came with the MEGA 2560 R3, a bread board and a bunch of other electronics stuff to learn how to use a MEGA 2560 R3. Note: I am still working on getting through all the examples it's a lengthy course.

7. I do NOT have any of the I2C TB6612 Stepper Motor PCA9685 Servo Driver V2 Shields. I thought it would be best to figure out the power supply first even if I do not buy it right away. Then I would be sure these would work. They are not cheep when you need 6 of them. I really thought someone on this forum would be well versed with them, but this seems not to be the case. I am also going back, and forth about buying a adafruit version that has quality parts. Normally I would just go for it, but that's more cost, and I would have to do quite a lot of soldering. (I am not against hard work. It's one of my medical problems that limits what, and how much I can do with my hands.)

8. How heavy is Robot 42? Drum roll.... 12 LB 2 oz without batteries, or electronics.

9. Robot 42 has Two Feet and 8 wheels

    Each foot has 2 Nema 17 motors each directly connected to a wheel.

    In short - 4 wheels 4 motors. (if necessary I can even add 4 more motors to the other 4 wheels)

Note: the center of gravity is exactly over the feet, and thus I can slightly push this robot with my finger and it will move forward. My bedroom floor is not exactly flat, and so the folding table that Robot 42 stands on is at a slight angle, and so I have to put some wood blocks in front of Robot 42's feet to stop him from rolling.

I have watched enough YouTube video's on Nema stepper motors to make me believe that I have enough motor power to move this robot around.

Note: I never planed on having Robot 42 run around the floor as fast a RC car. A slow speed will be more then sufficient.

I am tired and need to rest.

I would be glad to answer any more questions you all have. - GreenDragon

@greendragon Until you can do some power measurements you are stuck at point 1. When I started into electronics after a 65 yr hiatus the first thing I attempted was a power supply. I never finished it as it was a special design that became obsolete due to a better part. I then gathered parts for two power supplies. I finished one and at least for now dropped the other. Recently I saw a different power supply design and I am gathering parts and modifying the design in my spare time.

I understand you are not 100%, but what you don't know is my heart stops beating all the time and I am waiting to hear from a cardiologist which will likely end up with me getting a pacemaker. I hear after that I will feel great!

Watch Bills video on steppers  HERE and read the article that goes with it so you can compare specs and determine what is best for you  HERE . After reading and re-reading as many times as required you should be able to figure out what you need. Buy ONE and test/trial it, or as I would do maybe buy the 3 most likely candidates and trial them all using a meter to make real world measurements. This is how an amateur engineer goes about designing things.

Your description of the wheels is confusing and points at a possible problem. Each foot is obviously rectangular with a wheel at each corner. Are they common shafts with some sort of gear box to allow a single motor to drive a pair of wheels? If the wheel circumferences are not equal and they will not be, then you will get scrubbing on one wheel at least. If the two axles are not connected then you will get all kinds of scrubbing and other out of synch problems. Others here may advise better/different than me, but I would allow one axle to just freewheel and either use 1 motor to drive a gearbox that drives one pair of wheels, OR two motors each driving a single wheel perhaps at two corners with a feedback loop to keep them in synch. 

Start with the feet and once that is working in a few months post some video.

Goodbye until then.

Posted by: @greendragon

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I do NOT have any of the I2C TB6612 Stepper Motor PCA9685 Servo Driver V2 Shields. I thought it would be best to figure out the power supply first even if I do not buy it right away. Then I would be sure these would work. They are not cheep when you need 6 of them. I really thought someone on this forum would be well versed with them, but this seems not to be the case.

You will find that most people on the forum that are working with bots use geared DC motors with encoders to provide critical feedback for doing positional related tasks.  To my knowledge, I'm the only one that has used steppers for driving force of a bot and @davee through no small effort has dug deep into the theory of steppers and drivers and shares his considerable EE knowledge.  

Posted by: @greendragon

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It's one of my medical problems that limits what, and how much I can do with my hands.

Anyone using stepper motors in the Nema-17 class and smaller will be taking advantage of 3D Printer economy of scale and using those drivers and will not be using the drivers you have specified.  Your selection is weaker, less configurable and way more expensive than the alternatives I have suggested.  There is no other redeeming quality about your specified part than its "snap-in" shield aspect.  It is targeted at first-time people that can't solder or use a breadboard.  However, if your condition prevents you from working with these small pieces, then that aspect trumps performance matters.

Engineering

I haven't seen any of that going on so far.  Even though I am a proponent of stepper motors, I would not have specified them and certainly not these tiny 17HS08-1004S steppers to drive a 15+ pound bot running at slow speeds.  My engineering background and practical experience with steppers tells me, you'll need magic to even get it to move and any differential steering will certainly cause the steppers to skip steps and come to a halt.  Geared motors would have been the logical / better choice. 

For your design to have any chance of working it will need gearing on the drive motors and will only be driving the outside wheels of each foot.  I hope your inner wheels are just free spinning and even then, they'll be scrubbing off a great deal of the marginal power you have available.  At the very least, you'll need Mecanum wheels which are also quite expensive.  I hope @zander's assessment wasn't correct or necessary, but I fear it might be.  It appears you have been more interested in the persona of 42 than the actual engineering to getting such an inefficient design to move.  As you are not listening to or using sound engineering principles, I wish you luck... and magic.

Hello:

I know my description of the wheel was somewhat confusing, and I am planing on taking pictures today and posting a few. hopefully this will make things clearer. 

I had been thinking of getting a bench power source as many have pointed out in this thread. I just really would have liked to put the money towards other parts, but I do understand that once and awhile you just need to bite the bullet, and buy the tool you need. I am sure everyone who has ever bought a tool can relate to this.

Special thanks to Inq for pointing out that very affordable power source.

Special Thanks to Ron and May God be with you.

GreenDragon

@Inq what aerospace company did you work for?