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All,
as I have been toying with the idea of simple and cheap BMS I realize that it would be best to make it an open source project. I'm pretty good at reading schematics and making electronics, but certainly not as smart and/or experienced as some folks on this board. I'd like this to be super technical thread with input from anyone who can contribute specific details, we can keep general discussion at the other threads.
I have researched various BMS types and designs for many months, I have contributed a little to Brian's Volt Blochers project, I made my own version of VBs, which is currently installed in my EV and works great, I have a couple of satisfied customers using my modules as well. Since VBs aren't available anymore and there seems to be a market for cheap, simple and reliable BMS ( see poll results in the New BMS poll thread ), I figured why not take the best ideas and try to implement it all in one universal product.
I especially liked the idea of simple BMS proposed by Johnny on Australian forum, so I have taken his schematic and applied few changes based on input from other BMS discussions. I also done some research on key components and their prices at Digikey, to make sure we keep total parts cost around $5 per cell.
In past few days I prototyped this schematic on a bread board using basic Radio Shack parts and it appears to work great. I really like how comparators allow precise and fast switching when voltage gets to trigger levels. It switched from on to off with 0.01V precision and very little hysteresis.
I'd like some input from pros out there, my goal is to minimize number of parts, improve anything that may need improvement, pick best resistor values and minimize different values so there is less different resistors to buy. For example, I think we can collapse R6 and R7 if we find LED with similar voltage drop as in optorelay, so they can share same current.
I'd like to use Clare CPC1014 optorelay for NC signal loop, since its the only suitable low on-resistance relay I found which can be purchased for 80 cents in reasonable quantity.
http://www.clare.com/home/pdfs.nsf/www/CPC1014N_R01.pdf/$file/CPC1014N_R01.pdf
Using 4 comparators may seem complicated at first, but they are all integrated in one IC chip which only costs 22 cents in quantities
I'd like to use these trimpots, which can be bought for 85 cents, yet very reliable for automotive environment.
Since there are so many needs for various HLVC levels, I think trimpots are pretty much a necessity, and they reduce number of resistors a little.
http://www.bourns.com/data/global/PDFs/3361.pdf
Later in this thread I will post initial schemaitic for head end controller, where I will also need some input from the pros.
According to the poll, many people still want shunting function in their BMS, despite recent ideas against top balancing. I'd like to keep this as an option on the PCB and simply skip these components if one doesn't want them. I'd like to keep shunting current low, like 0.5A, to avoid heat sink on the transistor and use 5 Watt resistor, which is reasonably small to fit on PCB.
I will be designing a modular PCB for this, which will allow a number of modules on one PCB, yet be able to cut it in pieces if one wants per cell installation. This will reduce PCB cost and assembly labor. However, for this and other reasons I abandon an idea of PCB under terminal bolt, so I will just have holes for either wires or rigid bars as in my current BMS modules.
I am planning on SMD components to minimize PCB size and further reduce costs.
Once we have working design I can produce them for DIY community at best price possible. This will probably take some time, so if someone needs BMS today, you might want to explore other options.
So, please, look at the schematic and question every component, every connection, everything...
as I have been toying with the idea of simple and cheap BMS I realize that it would be best to make it an open source project. I'm pretty good at reading schematics and making electronics, but certainly not as smart and/or experienced as some folks on this board. I'd like this to be super technical thread with input from anyone who can contribute specific details, we can keep general discussion at the other threads.
I have researched various BMS types and designs for many months, I have contributed a little to Brian's Volt Blochers project, I made my own version of VBs, which is currently installed in my EV and works great, I have a couple of satisfied customers using my modules as well. Since VBs aren't available anymore and there seems to be a market for cheap, simple and reliable BMS ( see poll results in the New BMS poll thread ), I figured why not take the best ideas and try to implement it all in one universal product.
I especially liked the idea of simple BMS proposed by Johnny on Australian forum, so I have taken his schematic and applied few changes based on input from other BMS discussions. I also done some research on key components and their prices at Digikey, to make sure we keep total parts cost around $5 per cell.
In past few days I prototyped this schematic on a bread board using basic Radio Shack parts and it appears to work great. I really like how comparators allow precise and fast switching when voltage gets to trigger levels. It switched from on to off with 0.01V precision and very little hysteresis.
I'd like some input from pros out there, my goal is to minimize number of parts, improve anything that may need improvement, pick best resistor values and minimize different values so there is less different resistors to buy. For example, I think we can collapse R6 and R7 if we find LED with similar voltage drop as in optorelay, so they can share same current.
I'd like to use Clare CPC1014 optorelay for NC signal loop, since its the only suitable low on-resistance relay I found which can be purchased for 80 cents in reasonable quantity.
http://www.clare.com/home/pdfs.nsf/www/CPC1014N_R01.pdf/$file/CPC1014N_R01.pdf
Using 4 comparators may seem complicated at first, but they are all integrated in one IC chip which only costs 22 cents in quantities
I'd like to use these trimpots, which can be bought for 85 cents, yet very reliable for automotive environment.
Since there are so many needs for various HLVC levels, I think trimpots are pretty much a necessity, and they reduce number of resistors a little.
http://www.bourns.com/data/global/PDFs/3361.pdf
Later in this thread I will post initial schemaitic for head end controller, where I will also need some input from the pros.
According to the poll, many people still want shunting function in their BMS, despite recent ideas against top balancing. I'd like to keep this as an option on the PCB and simply skip these components if one doesn't want them. I'd like to keep shunting current low, like 0.5A, to avoid heat sink on the transistor and use 5 Watt resistor, which is reasonably small to fit on PCB.
I will be designing a modular PCB for this, which will allow a number of modules on one PCB, yet be able to cut it in pieces if one wants per cell installation. This will reduce PCB cost and assembly labor. However, for this and other reasons I abandon an idea of PCB under terminal bolt, so I will just have holes for either wires or rigid bars as in my current BMS modules.
I am planning on SMD components to minimize PCB size and further reduce costs.
Once we have working design I can produce them for DIY community at best price possible. This will probably take some time, so if someone needs BMS today, you might want to explore other options.
So, please, look at the schematic and question every component, every connection, everything...
