Low Cost BMS for Home Storage Batteries

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4Springs
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Low Cost BMS for Home Storage Batteries

Post by 4Springs » Sun, 24 Jun 2018, 06:02

Looks like I will be getting some of the ex-AVASS cells (as per the the forum topic here: viewtopic.php?f=16&t=5574).
The "Low Cost BMS" is a project I used in my car, and should be ideal for these - low cost but just fancy enough to do all that is required.
As is the tendency for these sorts of projects, there are multiple versions out there. People tend to grab the source files and modify it to fit their own circumstances.
This is Nevilleh's original AEVA forum on the topic: viewtopic.php?f=17&t=2753&hilit=low+cost+BMS
The topic went on a few tangents over the years...

This is a version that I made for my car: https://sourceforge.net/p/low-cost-bms/wiki/Home/
I think this will be fine for home storage.

The design has a single master unit, with a display and two buttons.
Then there is a circuit board on each cell. The celltop module fits onto the terminals, so each type of cell needs a specific board made.

So.....
Despite having too many projects on the go at the moment, I think I'll make a version of the Low Cost BMS for these cells. I'll get one of the circuit board manufacturing companies to make the boards for me, and it will be cheaper per unit to get a larger quantity done.
If you also have these cells, and would like to use the Low Cost BMS, the process would be:
  • Buy the components (I'll be making a list of what to get and from where)
  • Buy a programmer thingi (Again, I'll let you know which one and from where)
  • Buy the circuit boards (Let me know how many you want and I'll get them made - then buy them from me)
  • Assemble the components on the boards
  • Programme the microcontrollers (master board and each module board) using the programmer thing connected to your computer
  • Hook them together and test
This should be possible for someone with no electronics or coding experience, although it would be a bit of a learning curve. The components are surface mount, so they are fiddly to put on the boards, but shouldn't be too bad with only 16 celltop modules (assuming 16 cells in a home storage unit). I bought myself a little oven (cost $100), or you could try doing it with a soldering iron.

Let me know if you are interested.
Especially if you are interested in doing some of the work! (re-design the celltop module for instance, or source the components).
Post a reply here, or send me a private message through this forum, or email me at: secretary@tas.aeva.asn.au

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Re: Low Cost BMS for Home Storage Batteries

Post by rhills » Tue, 26 Jun 2018, 12:14

Hi Christopher,

This looks like a great project. I'd like to help, but the soldering puts me off, I've never been much good at it. I followed your link to the BMS you made for your car and read in the Wiki that another option could be to get the whole board manufactured. Would this add much to the cost? I'd certainly be willing/able to help out with any coding, though I suspect that's all pretty well sorted.
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Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs » Tue, 26 Jun 2018, 17:57

rhills wrote:
Tue, 26 Jun 2018, 12:14
Hi Christopher,

This looks like a great project. I'd like to help, but the soldering puts me off, I've never been much good at it. I followed your link to the BMS you made for your car and read in the Wiki that another option could be to get the whole board manufactured. Would this add much to the cost? I'd certainly be willing/able to help out with any coding, though I suspect that's all pretty well sorted.
Someone else has suggested getting boards populated, he knows a bloke who does it in Launceston. I think we would get the boards manufactured overseas, then take them to this local company and get them to populate them with components. Definitely worth investigating.
Apparently you can get the microcontollers pre-programmed if you order them from certain suppliers. This would mean that people wouldn't have to buy the programmer if they were happy with the program as-is.

There are a few jobs to do:
- Figure out how to power the BMS Master. Currently runs on 12VDC, because that is what is in a car. Not sure if it will run on 48VDC, it might need a component change.
- We need to figure out what the BMS Master needs to do. How does one interface with an inverter? We may need different input or output options, so we may need to modify the circuit board.
- The coding will need some modification, depending on the outcomes of the previous question. This may be different for different people.
- The parts will need to be sourced. One of the files in the project is a list of parts and suppliers that I used once - well out of date now, but probably a good starting point.
- The celltop board will need to be modified to fit the cells. Hopefully both cell types have the same terminal dimensions.

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Re: Low Cost BMS for Home Storage Batteries

Post by jonescg » Sat, 30 Jun 2018, 20:22

4Springs wrote:
Tue, 26 Jun 2018, 17:57

There are a few jobs to do:
- Figure out how to power the BMS Master. Currently runs on 12VDC, because that is what is in a car. Not sure if it will run on 48VDC, it might need a component change.
I've found some really nice little DC/DC converters that will put out over half an amp - https://www.digikey.com/product-detail/ ... ND/4477405
Probably good enough to run a BMS but you can't hold contactors open with it.

@rhills - the WA shipment will probably be here in a few weeks time. I think the best thing to do is put them all in parallel and leaver them like that for a week while the all reach the same state of charge. Should make the balancing job a bit easier.
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Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs » Sun, 01 Jul 2018, 06:31

jonescg wrote:
Sat, 30 Jun 2018, 20:22
I think the best thing to do is put them all in parallel and leaver them like that for a week while the all reach the same state of charge. Should make the balancing job a bit easier.
I did this with the LFP cells for my car. 45 of them. The way I did it was to loosely put bolts in the terminals, line them all up in a row, and run from terminal to terminal with electric fence tape.
This really didn't do much. I worked out later that it was because the LFP chemistry produces such a minimal voltage change over a huge amount of its state of charge.
For instance, say we had a difference between cells of 1kWh. If this is in the middle of the capacity of the battery it is in the flat bit of the charging curve. So the voltage difference might be (perhaps) 0.001V between cells. Let's say our wire is 1 ohm of resistance. I=V/R, so the current that flows would be 0.001A. W=VI, so we have 0.000001W flowing.
1000Wh/0.000001W is approximately 114 thousand years to balance.

So doing this passively is not really going to do much. If you can parallel them all up and charge them, that is a different proposition. Once they get almost full the voltages should balance themselves out.

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Re: Low Cost BMS for Home Storage Batteries

Post by jonescg » Sun, 01 Jul 2018, 08:53

This is true - but assuming they were way out of balance it would help. I did that for my scooter batteries however they were lower capacity and I did leave them for a good 3 weeks. Probably the better option is to charge them in this state as a giant 3.2 V cell. That way any individual cell differences will start to make a difference at the upper end of the curve.
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Re: Low Cost BMS for Home Storage Batteries

Post by Nevilleh » Sun, 12 Aug 2018, 12:42

Hi Chris,
I haven't looked on here for years, been busy with other stuff, but i am wondering if this project ever got off the ground? Nice to know my old BMS stuff still has relevance!
Neville

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Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs » Sun, 12 Aug 2018, 15:01

Nevilleh wrote:
Sun, 12 Aug 2018, 12:42
Hi Chris,
I haven't looked on here for years, been busy with other stuff, but i am wondering if this project ever got off the ground? Nice to know my old BMS stuff still has relevance!
Neville
Hi Neville, good to hear from you!
Every couple of months I'm at a car show somewhere, showing someone the BMS modules and explaining that I made them from a design by a bloke in New Zealand...

I'm still intending to modify this BMS for these new cells. Haven't been inundated by people wanting to hop on the bandwagon, but I may be a bit slow for some.

From what I've learned so far, it looks like I might want a contact closure as an output. Something to say to an inverter "stop discharging the battery". Or just hook it up to a contactor so it drops the connection. Plus the existing 0-5V output that controls the charger.

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Re: Low Cost BMS for Home Storage Batteries

Post by Nevilleh » Tue, 14 Aug 2018, 14:45

Oh well,the hard part is usually working out what you want to do rather than doing it. I'm sure you'll get it going once you do that. If you want any help, let me know. I still have all the original files.
Neville

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Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs » Mon, 20 Aug 2018, 19:08

I've done some work on the BMS module board to fit the Brighsun cells. Here is the version I made for my car:
CellTopBMS-SE130AHA_1.jpg
CellTopBMS-SE130AHA_1.jpg (46.65 KiB) Viewed 270 times
I had some notes on this version:
1. The board was a bit wide at the terminals. Some boards had to be trimmed a bit narrower to sit on top of a certain type of battery connecting lug.
2. The isolated input from Din (Data In) was quite close to the other circuitry. The isolation would benefit from moving things away further.
Here is my updated design for the Brighsun cells:
CellTopBMS-Brighsun_220AH_1.jpg
CellTopBMS-Brighsun_220AH_1.jpg (59.55 KiB) Viewed 270 times
It is 22mm wide instead of 30mm, plus shorter to suit the Brighsun cells.
I've re-arranged the components somewhat to make some more room. Gained a few vias and probably made a mistake or two.

Here is the circuit diagram - unchanged from Neville's original:
CellTopBMS_1.jpg
CellTopBMS_1.jpg (41.4 KiB) Viewed 270 times
I did toy with the idea of a differently shaped board to go around the central vent. Here is a cardboard version I made, next to one of the existing BMS boards:
IMG_20180819_080240195.jpg
IMG_20180819_080240195.jpg (100.89 KiB) Viewed 270 times
This didn't look great, so I had a look at raising the board up over the vent:
IMG_20180819_074737193.jpg
IMG_20180819_074737193.jpg (44.09 KiB) Viewed 270 times
This looks fine with a spacer like the nut I've put in. The bolts are available in 20, 25 & 30mm lengths - this one is a 25mm one. The one on the right is screwed all the way in to the bottom. There are spacers available, but they are zinc plated rather than stainless steel like the bolts and washers. I guess that wouldn't matter in this application.

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