BM-SYS - Modular Battery System

I mean just to throw it out there I had an idea to make a button wired to all modules that when pressed activates an RGB light showing the charge. Quick and easy way to check if all your packs are good and would double as some dope lighting given they are exposed. Idea works much better if they are printed in clear filament.

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yeah, 10s modules should work great

Iā€™d love to know how youā€™ll do this. An inductor can work if you can figure out how to make it small.

This is a must. I was planning on adding it but then I could not make it in time for my final presentation.

Simple button as you said and 4 LEDs, old MacBook style.

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Also this is for good measure.

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I was really liking my idea with RGB lights and clear housings, giving a cool display while riding. Seems a little unnecessary for an early version though I would admit.

Iā€™m now working a v.1.1 with 21700 cells. I really belive they will replace 18650 in popularity in the future.

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@Okoku
Considering the applications you have noted for your design, I would imagine that the scope of your thesis included the application of a modular battery to multiple usage applications (voltages).

As someone who only has only a general understanding of building battery packs (no practical experience though), wouldnā€™t 1S parallel packs (i.e 1S4P) pose greater flexibility for electric skateboard applications? Theoretically at 1S, each unit could charged without balancing with something like this. They could be wired in greater voltage configurations, arguably reducing wiring complexity, as each parallel group would essentially be a self contained unit, which could be swapped out if faulty.

Thoughts?

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Yes but no. Let me explain better.

The BM-1 modules while also having an xt30 direct ouput, have asb-c Power Delivery Board that can ouput up to 60W in cuts of 5v 9v 12v 15v 20v, and can charge the pack independently also at 60W. In order to power such a board is required a 4S configuration which then led me to a this final cell configuration and system.

About the 1S4P. Yes, it is way more convinient on paper and is a consideration i made in the past. Unfortunately this does not reflect well in practice. The BMS you have rightly provided a link for, will be absolutely futile (please donā€™t take me wrong, I mean literally, I dont want to sound obnoxious lol). On the other hand though it would be possible to integrate a bigger smart bms in the connection piece which could still work. Unfortunately this would mean having to seat 12/10/8 modules at minimum which, in addition to be very big, size wise, I think also removes a bit the ā€œmagicā€ of modularity which with 6 module you have.

Tldr; the biggest problem is that in order to make it modular, you need a shell. Said shells for 12 modules are very big size wise and would fit almost no board.

If I did not explain myselft right i can share a render of said system.

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So if I want a 12s system I need 3 modules. And if I want more range I need to add packs in groups of 3?

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Exacly! Same for 8S

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Basically I started working on the V2 of the BM-1. Guess the name? BM-2 obv.

The new version will be made specifically to be used with 21700 cells only. This choice has been led by the lithium market itself, which in 2021 is gradually shifting away from 18650 cells, which have become more rare than good humour on Reddit.

The BM-2 will feature a much better BMS ( whose features I will list in the next post ) that is able to handle 50A continuos discharge and 70A peaks for 10 seconds, Ideally this should be suitable for ev but donā€™t quote me on that as I had not had the time to test it yet.

Iā€™m planning to make a single version of the module this time, as 21700 cells are marginally better, capacity and output wise, and should cover the majority of applications.

Iā€™m now testing molicelā€™s P42A cells, rated at 45A discharge ( I strongly doubt it ) and 4.2 Ah. A single module with the use of these cells should provide 63Wh of capacity ( BM-1 had 49Wh).

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  • Overcharge protection voltage:4.25V+/-0.025V
  • Overcharge release voltage:4.15V+/-0.05V
  • Overcharge protection delay:1S+/-200mS
  • Over-discharge protection voltage:2.7V+/-0.05V
  • Over discharge release voltage:3.0V+/-0.1V
  • Over-discharge protection delay:1S+/-200mS
  • Discharge overcurrent protection:70A
  • Discharge overcurrent protection delay:100ms+/-50mS
  • Short circuit protection:Yes
  • Short circuit protection delay:250uS+/-50uS
  • Short circuit protection recovery method:Disconnect load recovery output
  • Discharge current:50A
  • Charging current:50A
  • Balanced function:Yes
  • Balanced turn-on voltage:4.18+/-0.025V
  • Balanced current:55mA
  • Wire break protection:Yes
  • Temperature protection:Yes(User need install 10K NTC sensor additional)
  • Charging low temperature protection:-7ā„ƒ
  • Charging high temperature protection:50ā„ƒ
  • High temperature protection:70ā„ƒ
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They have a genuine 45A rating but we have to take into account the intended market for the cell since the ratings will be tailored for its large commercial/industrial buyers. Typical uses include power tools and electric vacuum cleaners where non-continuous use is standard so the 45A rating is for non-continuous use too.

The datasheet mentions that use at 45A will reduce the cycle life and lists a max surface temp of 85Ā°C with a recommended max of 75Ā°C at high discharge rates.

Bottom line, the 45A rating is genuine but itā€™s a temperature-limited rating that can reduce cycle life. But for some applications reduced cycle life is not a problem for the company since it creates more product sales.

Using that 85Ā°C max temp spec I rated the P42A at 30A continuous down to 2.5V.

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Mooch YOU ARE THE REAL MVP.

Iā€™ll keep you all updated with the next tests!

Ps Iā€™m planning to replace the XT30 connector with 4mm bullet connectors, they should carry 45A with ease right?

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Depends on what that 45A really means. :grin:

45A continuous is too much IMO. Good ones wonā€™t melt but theyā€™ll get warm to toasty (depending on air flow, nearby stuff, etc) and you will have some losses in the connection. That might not be a concern though as long as high temp rated wire insulation is used.

45A ā€œpulsedā€, which can mean just about anything, should be okay with 4mm bullets. You only have to worry, or not, about any voltage drop across the connection as long as good bullets are used.

Just a side note for anyone else reading thisā€¦
The gauge of the wire can have a big effect on connector heating too. Using 16AWG at 45A will mean hot connectors (and wire!) but using 10AWG or 8AWG will help bring down the connector temperature since the wire can sink away heat.

Your power losses are lower too with larger gauge wires but most people will trade off those minor power losses for smaller and easier to run wires. Just so long as they donā€™t get hot enough to soften the insulation (if using PVC insulated wire).

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