BM-SYS - Modular Battery System

Hello everybody,

Today I would like to share with you the output of my thesis project in Industrial Design.

Before telling you about my project I would like to take a second to thank all the forum members for the support that the topics I found here gave me. I would not have been able to do this without your help and work in keeping this place active and a great source for newbies like me to get started into this passion we all share.

About BM-SYS

BM-SYS is an investigation on the current state of lithium technology and the resulting environmental dangers. The output of this research is a battery module system powered by li-ion cells that changes the way power approaches electronic devices.

With a single module that adapts itself thanks to the help of USB Power Delivery technology, is possible to share the same energy source though all our daily use battery powered devices. Futhermore the addition of a raw power output through and XT-30 connector enables the modules to be used a quick-swappable power source for electric vehicles.

The result of this approach is a detachable open-source lithium battery that can be easily serviced, repaired, and upgraded. To express the capabilities of the modules, a small family of objects has been designed: a lamp module, a Bluetooth speaker, and an electric skateboard.

Now for what you have probably all been waiting for…

About BM-SYS Board

To demonstate the capabilities of the system I decided to implement my modules to power an electric vehicle. As I have always been a skateboard enthusiast, I decided to create a dual motor electric longboard.

The board features 6 BM-1X modules ( each module bein 4S 1P - Molicel P26) connected toghether in order to archive a 12S2P configuration. Each module features its internal BMS and Balance Board as well as an XT30 Connector for that juicy raw power output.

For the ESC I used an old FocBox Unity I had laying around that thanks to the now supported VESC 5.0 firmware enabled me to control the modules safely and disconnect them when they reached the empty voltage level threshold.

Motor and mechanics wise, I used dual 6354 190kv Motors from flipsky, Caliber II trucks, Dickyho’s Motor mounts, a 15:36 gearing and 90mm Boa Wheels.

The board Is a blank I got from Sickboards and painted black.

About Open-Source

I will release the files for the battery modules and connector pieces as soon as possible on my github. The decision to release everything opensource is because I dream my battery module system to become a standard in the future for electric vehicles, and the only way to make this happen is by enabling users like me to make their own for free.

If you have any questions please feel free to ask anything!

Please bare in mind that this system is only a proof of concept Alpha stage, I plan to take this further and improve it in order to compete with traditional battery systems.


Thanks for posting this!
I look forward to reading more about the system architecture, your different modules, and your plans for BM-SYS!

I love the idea of a modular battery. Some real tough challenges but a worthwhile endeavor IMO.

bq77915? :grin:


LOL…I have so many questions but I’m trying to be patient and wait for your GitHub uploads before bugging you too much. :slightly_smiling_face:

One question though…your BM images show a straight-sided enclosure but the image of the underside of the board shows a necking down of the enclosures. Is this a different case for the cells or does each module plug into to a second part that necks down?


what’s the weight of this board?

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So, Its quite difficult to explain ahahah. Regarding the modules: the internal structure with the cells slots inside the plastic shell and locks both trough a plastic mechanism as well as though screws on the back. Then the modules get connected to each other through the black part on the underneath of the board. They slot right into their compartments and are held both with a tight tolerance as well as with screw clamps.

Its very difficult to explain LOL but I’m planning to shoot a small video to show you all how this works.


Approximately 9kg

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Seems like im not the only one loving battery modules :grinning_face_with_smiling_eyes:
Great work, dooood, i love it :smiling_face_with_three_hearts:
Edit: I think you re better off mounting the battery slots from sideways, so you could add much more modules and they are easy to reach and swap out, brooooo :grinning: :eyes:


Actually, my first prototypes were made sideways! I’ve then changed the configuration just for looks reasons but I’ll definitely take into consideration returning to the side mount!


Doesnt matter how it looks if its working!
Function first, then the form…I bet with a specially designed enclosure this will look more than great, dooood :hugs: :hugs: :hugs:

Doing it sideways would also mean that the board can flex a bit without anything breaking.
Would mean you could put them onto mountainboards too.

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How do you deal with packs at different voltages when you plug them in? Wouldn’t a low pack draw a significant amount of current from the high packs and cause damage?

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Just a word of warning, Probably shouldn’t be putting vesc logo on your stuff or you might wake the Frank.


Ahhhh….I understand now!
Your explanation and that side-mount image made me realize that what I thought was a necking down of the BM enclosure was really just the flanges of the base’s compartments covering part of the BM enclosures after they were inserted.


I’m guessing the built-in BMS’ will prevent damage but you might get some on/off cycling of the BMS until the voltages were close enough to not result in current flow that was over what the BMS would allow.

There’s not a ton of current flowing though for different voltage packs but it could still be a problem for the cell(s) being charged. A 4S module of P26A will have at least 90mOhm of internal resistance. That is 180mOhm for the two modules plus the resistance of the connections between them. Let’s call it 190mOhm total.

If one module was fully charged at 16.8V and the other almost empty at 12.8V you then have a 4V difference. That would allow about 21A of current to flow from the fully charged pack through that total of 190mOhm of total resistance…probably well within the capabilities of the modules.

I screwed up!
I was only thinking about the discharge rating of the P26A and not the charge rating of 6A max. A charging current of 21A would be very bad for the cells.

So it’s always best to have the modules fairly close in voltage when inserting them though unless some sort of method to balance the module voltages is used.


I’ve already removed the vesc logo after I’ve been informed by a friend too. I’ll edit the vesc logo out of the pictures too just to be safe

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Good points! To be on the safe side I always plug the modules in when fully charged and within 0.1V one to the others.

From my test I could observe that the modules discharge all equally leaving them after a ride within 0.05V one to the other. Futhermore the charging is handled directly throught the board via a standard 12S charger. In the same way as they discharge at a constant rate they also charge equally too.


21amps is less than what I would have thought. So this would charge the lower charged pack at 21amps? If it’s 1p that can’t be good for the cell, no?

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Arrgghh….I screwed up!
Yes, a charge current of 21A into a P26A is indeed a problem. Thank you for catching my mistake! I have updated my earlier post.

So it’s definitely better to have the modules at fairly close voltages. That would be about a 1V max acceptable (IMO) difference for these 4S1P modules using Molicel P26A’s. That would result in about a 5A flow of current between them when connected in parallel, still within the P26A’s max charge current rating of 6A.


I would make each module 99wh for easy air travel.


Always best to have the modules fairly close in voltage when inserting them though unless some sort of method to balance the module voltages is used.

Regarding this, I am now working on the Version 2 of the BM-SYS. The version 2 will have a ton of smart protection features like:

  • Internal Smart CPU
  • Per cell control
  • Per module Can bus
  • Smart equalization ( modules will equalize themselves if uneven )
  • Other?