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Why not build a no-frills bulletproof BMS?

I have a design for a no-frills BMS I’d like to bring to market and would love to hear what you think about it.

We have inexpensive “dumb” and “smart” BMS’. But that low cost often means lower reliability and can lead to us being the beta-testers for new models with the manufacturer saying they’ll fix things in the next batch.

We have very capable, fully configurable “smart” BMS’. But that flexibility can sometimes mean dealing with confusing software/app installation, firmware upgrades, cabling issues, incorrect settings, and inadequate or confusing documentation.

I think there is room in the market for another type of BMS. A no-frills, high-reliability* device that does one job and does it very well. No settings, no telemetry, and no apps or software. It just works.

In my opinion a big reason why we want our BMS’ to be smart is because we don’t trust them. Sure, we’re DIY’ers and like seeing all the details but would we care about cell voltages and balancing if we knew the BMS did its job? We don’t care about the voltages of the cells in our other devices.

Why can’t we have this for esk8 too? Why can’t we just install a pack with a fully integrated BMS and trust it to do its job? Why do we accept having to treat our packs like something could go wrong at any moment?

You might feel that the BMS needs to be outside of the pack and easily replaceable because they can be unreliable. Why do we accept having to do this?

We build packs that make it a tough to replace a cell because we trust decent cells to have a long life. Why not have that kind of trust in our BMS’?

We should never have to worry about needing to remove the BMS. We should never have to deal with flimsy connectors, crimping connector pins onto wires, or having a pile of balancing wires outside the pack (this is insanity IMO). We should never need to upgrade the BMS’ firmware or deal with buggy apps. Heck, we should never even need to read an instruction manual!

I want to create a high-reliability BMS that “disappears”, that can be embedded into the pack. The only wires leaving the pack are the main charging and discharging leads. No settings, nothing for the user to deal with, just some balancing status LEDs to watch if desired.

An integrated BMS is standard practice for so many products and I think we should have a good option to do that in esk8 too. It’s not the perfect option for everyone but in my opinion it could be a great choice for many. Here’s why…

  • No balancing wires outside of the pack.
  • No dealing with apps, programs, firmware upgrades, or programming cables.
  • Reduced user confusion, no incorrect settings.
  • No assembling of connectors and crimping of wires to terminals.
  • Easier, more sensible layout of balancing wires within the pack.
  • Easy swapping of packs, no delicate balancing connectors to deal with.
  • Shorter development time, comes to market faster.
  • More time spent on creating good hardware rather than telemetry and apps.

Features

  • This is a “dumb” BMS, no Bluetooth, no comm ports, no display.
  • No firmware upgrades (no compatibility issues, no version number confusion).
  • Two charge-only models: 9S-15S and 12S-20S
  • 24S possible but only if the sales volume is there…100pc minimum.
  • True continuous 20A protected charge rating with 20A protected discharge rating (for powering buck converters for accessories).
  • 50mA balancing, begins at 3.8V and only during charging.
  • 4.25V over-voltage/2.8V under-voltage protection, triggers after one second.
  • No LFP (LiFePO4) chemistry support for this first version….sorry.
  • Two levels of time-based over-current protection. Charge and discharge overcurrent is stopped after about 200mS, short circuits are stopped in about 1mS.
  • 50°C charge/70°C discharge over-temperature protection.
  • 0°C charge/-20°C discharge under-temperature protection.
  • LEDs to check balancing status of each p-group.
  • On-board 12V and/or 5V buck converter can be included but adds to the cost, size, heat, and development time.
  • Design goal is 35mm x 105mm x 5mm maximum for the 12S-20S version. The 9S-15S version will be smaller.
  • Connector types are unknown as of now. Strain-relief-protected solder pad balancing wire connections are being considered to create a very low-profile BMS. No connectors. This can be made reliable, it does not have to be a short-circuit danger.
  • The balancing status LEDs can be viewed through the shrink wrap.
  • A diagnostics port can be added to allow easy checking of all the cell voltages.

I do not present this BMS design as the perfect solution for everyone. Like all of the BMS’ we can choose from there are compromises. My BMS makes different compromises though and is a simple unit that does one thing only and does it very well.

You’re wondering what it will cost…I don’t know. It will not be cheap, we cannot expect it to compete with higher sales volume China BMS’. It might not even be considered affordable by many as a very reliable design has a much higher parts cost and months of testing will be done (which is hugely expensive since other revenue-creating projects cannot be worked on). But this thing will be close to bulletproof.

So, is it worth bringing to market?
Your thoughts and feedback are invaluable and would be very much appreciated. Thank you for your patience in reading this long post!

I do have a “smart” BMS design in the works too. But that could easily take another year to develop, if not longer.

*Since 1992 only two of the hundreds of devices I have designed and built have failed. One was dropped from a height of about 70ft but still worked when I replaced the battery connector. The other was partial loss of function due to, I believe, static (ESD) damage. Since then all connections and ports on my devices are ESD-protected. There have been no reported failures for over eleven years now, for any of the devices I have built.

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Yes, because sometimes a p-group can die due to a bad cell.

Protection against short circuits sounds very interesting though. Also small size sounds attractive. Otherwise, I don’t see much reason to go with this over a Daly, which are reliable enough.

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The BMS would shut off current (charge current and any current to accessories hooked up to the BMS) though once the bad p-group drifted too far, alerting you that there is a problem.

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Then I’m sold :smiley:

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A diagnostics connection could be added to make reading cell voltages easier. You can measure at the BMS or run wires to a multi-pin connector somewhere. Though those wires would ruin a lot of the advantages an integrated BMS offers.

I think if a user needs to access the diagnostics port due to whatever reason, the board is usually not in use and enclosure has been taken off.

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Sounds like a dream to my ears! Definitely go for it. You have my support, specially with the specs you posted :muscle:t4:

Agreed but I believe folks here treat their boards like vehicles and not just devices, and don’t have the support of oem manufacturer.

In these cases where you build and assemble everything, you do want to know what’s going on under the hood just like you’d do with a custom car build :+1:t4:

Welp this doesn’t apply to everyone, sure the main consensus is spot welding cells which make a pack hard to disassemble / service ; in my humble experience I have no spot welder and don’t need one for my battery packs, solderless assembly.

There are other users who do NESE packs for exemple so it’s not always hard to isolate cells and just take down the bad dud

This too, but from your previous post you have this covered :slightly_smiling_face:

Looking forward to the project!

What dimensions would you expect for a single unit?

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Or a led that lights up in a few colors

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Some years ago nobody did really mind.
Just used the bestech d140 and all was fine.
Than quality changed and bms started to drain p-groups and that’s really fucked.
That’s why I personally went the smart bms way, but lets be honest, we still can’t be 100% sure the displayed voltage in the app is really what’s on the p-group.

I really like your idea and would like to see a small reliable bms. I think charge only would even be enough for most of us.
I would like to see an indicator in case one or more of the p-groups start to get out of balance for more than 0.2V for example.
An easy red led starting to blink or something like that would be already enough.

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My post mentioned the dimensions for the 12S-20S board…max of 35mm x 105mm x 5mm but I’m hoping smaller. It’s not about fitting components that’s driving the size. It’s spreading the heat from the MOSFETs and balancing circuitry enough to keep temps down. This is an issue the really small BMS’ have to deal with since the heating goes on for quite a while during charging.

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throw the whole project away

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Actually, I felt self-banishment to a deserted isle for even considering this travesty of a BMS concept might be appropriate.

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but for real, great idea. I think a little speaker that made beeps when something goes wrong would be cool.

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I like the idea of it cutting out if a P group tolerance is too high. I’m running D140 BMS’s in both my builds and it would be nice to have some smarts like this built in. I don’t want to have to monitor cells.

So long as the BMS throws a warning light, I’d be happy to diagnose and trouble shoot any cell or P group issue.

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Well if your skilled and knowledgeable enough to spec and build a battry then the numbers mean some thing to you and are helpful and you keep a eye on them. Maybe a better user friendly interface is needed.

Different cells and p group configurations want different settings if you treat them in accordance to manufactures spec. How do you overcome this with out settings?

I can see it been popular for people wanting to buy a custom board pre made but not the skilled DIY enthusiast who understands what there building.

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I don’t wanna have to open my enclosure to see lights. I rather my board scream at me.

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External warning light FTW! Although power being cut / not charging is also a good indicator.

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I can see how this concept is appealing for some. It’s not a bad idea. But… from my personal experience there’s a huge value in being able to keep an eye on the cell voltages. I want to know if something looks wrong.

It has happened to me several times with the NESE modules that I had a cell with a bad contact in its P-group causing that P-group to randomly increase and decrease capacity as the cell was sometimes connected and sometimes not. With a dumb BMS, this would be very difficult to even detect. And it’s quite risky. Consider the following scenario. You have a P-group with 10 cells. The group is brought low, say to 3.2V and then one of the cells looses contact. You charge to full and will end up with 9 cells at 4.2V and 1 cell at 3.2V. Then you hit a bump while riding and suddenly the disconnected cell reconnects. Voltages start evening out at which point you’ll have significant current flowing from the 9 charged cells into the 1 discharged cell. That can cause a much higher charging current than the cell can handle. Bad things can happen. No BMS will save you at that point. You can only save the day by noticing that something weird is going on and take action on your own.

I’m definitely keeping my smart BMS.

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I respectfully disagree. I certainly don’t want to know the numbers and I’m sure there are others who would would like to not have to worry or even think about cell voltages. They don’t want that for their other battery powered devices.

We need to step back and think about why we feel the numbers would be helpful. What are we keeping an eye on? If the BMS is doing its job then we don’t need to worry about cell voltages.

Of course, some will definitely want all the data they can get and this BMS would be a terrible choice for them. :slightly_smiling_face:

A better UI is definitely possible for a smart version.

What voltage settings would have to be changed for different cells or p-group sizes? There’s no LFP support so “standard” NCA/NMC cells are the only ones being used.

The BMS current ratings are fixed for any BMS and the overcurrent and short-circuit settings would work for a wide range of pack configurations. Not every possible pack, of course, but most of them.

Like any BMS, you pick the one that matches your pack configuration.

Thank you for your thoughts and feedback!

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Great reason to not use anything but spot welds to make contact with cells is how I read this reply.

That shit sounds scary

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