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Low cost BMS

How do you store and manage your electricity?
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Renard
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Post by Renard »

Nevilleh wrote: Is that pitch for 100 AH cells?


Yes, for the CALB SE100, and for the SE60. The new grey series also uses this size for some types.
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Nevilleh
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Post by Nevilleh »

I think the way to go is for me to sell pcbs with the micro already mounted and programmed! That would solve the socket problem.

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Adverse Effects
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Post by Adverse Effects »

i dont know what chip your useing but dose this help?

Atmel Atmega Socket Firmware Flashing Tool $19.99

Image
Last edited by Adverse Effects on Fri, 14 Sep 2012, 19:50, edited 1 time in total.

Nevilleh
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Post by Nevilleh »

I used PIC micros in this project, just to see why the world raves about them. I could've used AVR and maybe should've, but there it is. Gives me some ideas though, thanks.
Last edited by Nevilleh on Sat, 15 Sep 2012, 02:53, edited 1 time in total.

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4Springs
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Post by 4Springs »

Nevilleh wrote: I think the way to go is for me to sell pcbs with the micro already mounted and programmed! That would solve the socket problem.

I wondered about that, but how would it go for the customer mounting the other components? It would be fine for hand soldering, but would it work for reflow? Would the micro move the second time through the oven?
And to get the micro in a state to program you'd have to connect a power supply - any other components required? You're raising your costs here, but you'd probably still be able to do it at an attractive price...

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Post by Nevilleh »

The micro wouldn't move if reflowed a second time, I'm fairly certain. The PICkit3 supplies the power so all you need do is hold the thing in place for the few seconds required for programming and verification.
I'd still rather not cook the micro twice however, so I'll continue to investigate the idea of a socket.

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Post by Richo »

You can order the pics pre-programmed.
https://www.microchipdirect.com/program ... aspx?mid=5
Then you don't have to program or solder.

Shame on you for not using an AVR Image
So the short answer is NO but the long answer is YES.
Help prevent road rage - get outta my way!

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Post by woody »

Nice find Richo!

Edit:
Looking further into it, it is 11c/chip for PIC12F1822 series.

Which seems worthwhile, assuming their prices ($1-$1.20) are OK for the chips.
Last edited by woody on Mon, 17 Sep 2012, 11:16, edited 1 time in total.
Planned EV: '63 Cortina using AC and LiFePO4 Battery Pack

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Post by Huub35 »

Dear Nevilleh,

looking at the User manual at ecomodder, I was a bit concerned reading about the full battery voltage that would be on the Dout connector of the last BMS.

How should I read that, is this related to ground, or the first BMS. Wouldn't this mean that also on the master board there would be the full battery voltage?

As I am thinking about an AC conversion, and truly believe in the Weber/Coulomb approach of only having the essential high voltage leads coming out of the battery boxes, I would be very much interested in learning the effort to make all "out-of-box" communications optical.

Would this be merely a sort of swap with the current optocouplers, or would there be more involved? I have no large background in electronics, so any advice would be very much appreciated.

Thanks a lot, regards,


Huub

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Post by Nevilleh »

Its the battery voltage with respect to the negative terminal (of the battery). If you have both positive and negative isolated from the chassis, as you should, then there is no problem. I put that warning in there so users would be aware that the comms cable is connected to the battery positive. It wouldn't be hard to opto-isolate that last connection if you were concerned about it. Probably would be a good idea in an ac powered vehicle where the battery voltage may be 400 or more, I'll make up an adapter.

I've now modified the cell module board to fit 81, 62 and 60 mm terminal spacings.
Last edited by Nevilleh on Tue, 18 Sep 2012, 06:40, edited 1 time in total.

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Post by Nevilleh »

Richo wrote: You can order the pics pre-programmed.
https://www.microchipdirect.com/program ... aspx?mid=5
Then you don't have to program or solder.

Shame on you for not using an AVR Image


I didn't know they offered that service, pretty reasonable at only 10 or 11 cents each. That is obviously the place to buy from! If anyone wants to do that, drop me a line and I'll send you the .hex file.

I've been using AVRs for years and I wondered why people keep on about PICs, so I thought this project would be a good opportunity to find out why. They are pretty easy to use, quite cheap, lots of peripherals and te support tools are excellent. Particularly the CCS C compiler. But the architectures is awful! Fortunately, the C compiler insulates you from that, but I would probably use AVR in future.
Last edited by Nevilleh on Tue, 18 Sep 2012, 04:22, edited 1 time in total.

Nevilleh
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Post by Nevilleh »

I've just bought 20 x 12F1822s from element14 at $NZ1.97 each plus gst, a total of $2.27 which works out at about $US1.80. But free shipping! Dearer than Microchip and you still have to program them. Convenient though and they arrived next day.
I'm going to fit my bms to my electric motorcycle, hence 20 cells.

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Post by Richo »

The shipping from Microchip is cheaper than Digi/Mouser.
Also usu Microchip send from Thailand so is usu 2-3 days v's Mouser/Digi 5-days.
The unit price is consistant at most suppliers in low volumes for PIC parts.
I believe this is some pricing structure Microchip enforce.
In high volumes Microchip direct work out better.
So the short answer is NO but the long answer is YES.
Help prevent road rage - get outta my way!

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Post by Nevilleh »

Go for it guys! Buy your programmed chips direct from Microchip.

If you want to purchase assembled, programmed and tested cell modules, the price is $NZ18 ($US14.75) each, plus gst if applicable. The master controller will cost you $NZ65 ($US53.30) also assembled in a small plastic case, programmed and tested. Plus gst if applicable. Plus postage!
Last edited by Nevilleh on Wed, 19 Sep 2012, 04:16, edited 1 time in total.

Nevilleh
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Post by Nevilleh »

Have a look at my BMW forum to see the latest version of the cell module pcb, extended to fit 81 mm terminal spacing.

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Post by cts_casemod »

Nevilleh wrote: Haven't been on here for a while, but I have been working on a BMS to replace my original 8-cell module one that proved to be awkward and clumsy to install, although it does work OK.

This one uses a single module per cell attached directly to the terminals. It has "daisy-chain" comms, opto-coupled from cell to cell and back to a master control unit. It was inspired by Weber and Coulomb's one, although built with PIC micros as they are the ones I have development tools for. It uses a thermistor for temperature sensing and has a shunt capable of bypassing 1/2 amp for cell balancing, if required.

The master has a 2x16 LCD that can show cell voltages and temperatures and also the max battery voltage. It has a low voltage alarm output and also an output to turn off the charger when a cell reaches its max voltage.

The cell modules cost about $5 and the master unit about $45 or so, depending on the LCD used.

It should be able to handle a couple of hundred cells OK, mostly limited by the RAM in the master.



Here's a picture of a cell module:

Image



Note that it is all SMDs and this one is hand soldered. The next ones will be done in the oven.



The master control unit is not finished yet, waiting on some pcbs to test it with.


Nevilleh I am looking for something just like this. Where have you got your PCB's? Any chance I could buy some of them?

Regards

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Post by Nevilleh »

cts_casemod wrote:
Nevilleh I am looking for something just like this. Where have you got your PCB's? Any chance I could buy some of them?

Regards


I don't have any available at present. I bought them from PCB-Cart who seem to do a reasonable job at a reasonable price. Drop me a PM with your email and I'll send you the DesignSpark files and you can place an order with them direct.

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Post by BigMouse »

If he orders them through you (and the design hasn't changed), he wont have to pay the tooling cost that PCBcart charges. It can be a significant savings.

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Post by Richo »

Export NZ to UK would be a factor.
So the short answer is NO but the long answer is YES.
Help prevent road rage - get outta my way!

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Post by BigMouse »

PCBCart will let you had orders sent to wherever you choose. You'd basically have it drop shipped. You could have him send you the payment, place the order, and have it shipped directly to him.

That'd be the most cost effective way to do it at least.

Either way, I think it's great that you're offering the design for free.

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Post by cts_casemod »

PM Sent, You should make this a sticky topic or something.

There is a huge lack in the EV World of a proper BMS for those who plan to use a High Voltage battery bank. This is by far the best I have seen.

Do you have any idea if your design will be able to monitor 192Cells?

I am also curious of what would happen if I take some cells from the pack. Let me explain:

Lets say I have 4*48Cells and one of these packs gets empty.

The BMS shuts down the whole array.
But what happens if I remove this pack that its depleted and work with only 3*48Cells?

Since the dead cells are not in the arrray any more, would the BMS resume operation with the remaining pack?

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Post by Nevilleh »

The number of cells that can be monitored is limited by the available RAM in the PIC16F1827 and is a max of 150 with that micro. I haven't had a look to see if there are other devices in that family with more RAM, but with Microchip being what they are, its more than likely.
The way it works in the daisy chain is that each cell just keeps adding its data to the string which is terminated by 0xFF and the control unit just stores the data in an array, incrementing until it finds that 0xFF, so you can store any number of cells up to the RAM limit. At present I hard code the max number of cells which is used to allocate array space - I couldn't figure out how to use malloc or something to dynamically allocate array space.
This means that every poll can be a different number of cells so if you take a few out it still works.
I've sent you the "User Manual" which explains everything!

Edit: I've just had a look at Microchip's web site and it would appear that the PIC16F1847 is the same as the PIC16F1827 except for having twice as much Flash and 1024 bytes of RAM instead of 384. A straight swap to this chip would allow about 500 cells! Its only a few cents more expensive.
Last edited by Nevilleh on Thu, 15 Nov 2012, 00:58, edited 1 time in total.

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Post by cts_casemod »

Thats fine, as long as the thing does what is supposed to do, otherwise I will be happy to use two master units. {Read this as the master wont freeze with noise from the larger data string}

It was important to me to that I could have a "limp mode" with only 96 Cells, if needed as my packs may have different soc (removable).

I understand the BMS would not disable the pack (Only an alarm), but would it still monitor the lower cells? Because if I took a string of cells out, the master would now read the lowest as 0, instead of the actual working cells with 2.xx or 3.xx

I would also like to keep a LVC just in case someone other than me was using the car and decided it was a good idea to keep driving with a few cells going negative! A good idea was 30 seconds after the alarm was triggered at 2.2V if the driver failed to stop the car.

Do you have any long term experience with the BMS?
The batteries will be the last thing to be purchased and the most expensive on the project.

Thanks

Last edited by cts_casemod on Thu, 15 Nov 2012, 02:55, edited 1 time in total.

Nevilleh
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Post by Nevilleh »

Sorry, don't understand what you mean by "took a sting of cells out, the master would now read the lowest as 0". It wouldn't! Try and understand how the daisy chain comms works and you will see why.
I've only been using it for a bit over a year - would you call that long term?

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Post by cts_casemod »

Yes, I thought if the space was alocated it would always display something, even if 0, but I got it now.

Well I see the circuit as battery monitor, not as a BMS, unless I can have an extra set of outputs dedicated for LVC and HVC.

I will not be the only one using my car and you wouldnt be happy if some untrained driver would ignore the alarm and destroied a number of cells. I have worked a few years as a mechanic, you wouldnt believe how distracted people can be in regards to simple things like oil pressure/temperature warnings etc.

I am also thinking in a weay to use a multiplexer to read the actual cells to make the circuit a little bit more simple for a large array of cells.

It would also be nice to have more than one daisy chain imput to connect more than one pack. Lets suppose you have a main pack and an auxiliary pack and you want to isolate them.

My idea is to have a removable pack, but later I would also put a fixed pack where the fuel tank is now.

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