Low cost BMS

[4Springs]

Originally Posted by mora

Did you include your version of master board with TCcharger control circuit in the package?

No, I didn't put the TC Charger control circuit on the Master board. The reason was just that I made the Master board before I knew what charger I was going to use. I ended up making a small extra board for the charger PWM control. The circuit I used was straight out of the TC Charger user manual.

Originally Posted by mora

This got me interested. I could try and build it with for my 12V battery (4x 90Ah LFP) which currently doesn't have any BMS. Has been so for years already. And same for other 12V pack which I use as trolling motor battery.

Yes, this BMS is cheap enough to use even on small batteries!
Having said that, it sounds like BMSs are less important on smaller batteries. The more cells you have in series the more likely they are to get out of balance. Can you measure the voltage on individual cells in your batteries? If so you can get an idea of how unbalanced they are now...

[SpmP]

Thanks astro123, foursprings for putting the infos on Sourceforge.

My friend has the car I believe these were developed for, and we have found a few hardware mods necessary... Mostly the resistors are inadequate and need to be 1W+ and off the board 8) (probably add an LED too)

Anyway, I am going to need 28+spares of these soon, and looking at getting a digikey order over the threshold, but before I do, I figure I should ask if anyone is needing any boards fabbed, or has experience with who is good for doing it. Maybe a bulk order is better.
Also any feedback as to which parts are better etc, or maybe has an old order in csv that may speed things up 8)
I would much rather work with AVR due to my experience, for those who have played with the code does it look very PIC specific? Would it be a hard port to AVR C?

Does anyone have an approximate cost breakdown for master and cell

My friend apparently has the source for both top and bottom balancing if this is of any use.
Sorry if there are RTFM questions here 8)

[4Springs]

Originally Posted by SpmP

My friend has the car I believe these were developed for, and we have found a few hardware mods necessary... Mostly the resistors are inadequate and need to be 1W+ and off the board 8) (probably add an LED too)

Interesting!
I actually reduced the value of the resistors (300mW) and it has been working very well. But this may be because I reduce the current from the charger as soon as one cell starts to balance. If your charger can't do that then it changes things. Still, have you tried the software options? - turn on balancing resisters at a lower voltage and then keep balancing once the charger has turned off (I can't remember if harlequin's code did that or not?).

I'll post my views on your other questions (prices, sourcing etc.) in another post. Good to hear from you!

[4Springs]

Originally Posted by SpmP

Anyway, I am going to need 28+spares of these soon, and looking at getting a digikey order over the threshold, but before I do, I figure I should ask if anyone is needing any boards fabbed, or has experience with who is good for doing it. Maybe a bulk order is better.
Also any feedback as to which parts are better etc, or maybe has an old order in csv that may speed things up 8)

I have posted details of my component purchases on the Sourceforge website (look under Files -> Components). Modules were about $4 each, Master about $50. These prices are parts only without the board itself and without the PIC microprocessors.
Have a look at the AEVA forum thread (low cost BMS) if you haven't already - there is a lot of activity over there. In particular Astro has found that the Master board can use the 16F1847 PIC microprocessor chip. This is a drop-in replacement for the 16F1827 listed in the bill of materials, but it has more memory if you think you might need it.

I had my boards made by PCB Train in the UK. They were very fast, knowledgeable and patient (I had never done this before and they helped me figure out what file formats worked best). For some reason I can't find an invoice right now to see how much they cost...
I will be wanting some Master boards soon, but I'm not sure if the economics would work out in our favor considering that we don't live in the same country. It may cost more to send mine over to me than it would have to get them done separately! Worth looking into though.

Originally Posted by SpmP

I would much rather work with AVR due to my experience, for those who have played with the code does it look very PIC specific? Would it be a hard port to AVR C?

I have not used AVR but I can make some general statements...
There are two versions of the code now, both written for the PICs. The original is for the Hi-Tech compiler and the later versions are for the XC8 compiler. My understanding is that both compilers are/were available from Microchip, but that XC8 is their current version. For some reason XC8 is quite different from Hi-Tech though - Hi-Tech has lots of useful library functions that are not included in XC8. XC8 has a free version, Hi-Tech does not. Hence the port to XC8!

It took a fair bit of effort to convert the code from Hi-Tech to XC8. It meant that I had to learn PIC in great detail before I could work out exactly what the Hi-Tech code was doing. I would suggest that you would have the same problem porting to AVR? You may need to learn PIC to work out what the code is doing, so might as well have used PIC anyway! Having said that, I did try to write the code in a portable way, but I am not the best coder in the world (or street), so there are probably lots of PIC specific things scattered around.

[Astro]

Originally Posted by 4Springs

I have posted details of my component purchases on the Sourceforge website (look under Files -> Components). Modules were about $4 each, Master about $50. These prices are parts only without the board itself and without the PIC microprocessors.

I managed to do my cell top modules for AUD$3.66 each (including PCB, components, PIC micro and all shipping to Australia).
I used Diygsm they made very nice boards and shipped via DHL with a tracking number. They double checked the files i sent them and let me know when something wasn't quite right.

I don't have the figures for my master boards but i would estimate less than AUD$10 each. The masters PIC chip was USD$1.72 compared to the cell tops AUD$1.32 PIC chip.
The master PCBs were AUD$2.40 compared to cell tops AUD$0.98 per module.
The only other significant difference would be the voltage conversion chip and the LCD.
I can't remember what the voltage conversion chip cost but it wasn't much and the 16x2 line LCD was AUD$2.25 from eBay. I upgraded my LCD to the 20x4 line version which costs AUD$7.23.

There are other expenses like solder paste that i haven't included.
Mainly because i made over 50 cell top modules and 3 master modules and still have heaps of paste left in the syringe. So it will be a shared expense over many projects.

So i think the BMS really does live up to the threads title of "Low cost BMS".
When i was shopping around for a BMS system i was seeing commercial cell top prices around $14 with the master module with display around $250.

Commercial 48 cell system = $922
Low cost BMS 48 cell systems = $185

It all depends on how you value your time, if you are comfortable with DIY and how much satisfaction you get from knowing you did it better and cheaper yourself.

[arber333]

Hi all!
My friend and i bought each a set of LiPo cells to put in our cars. He keeps them as primary movers but i use them as range supplement in parallel with my LiFe pack.
We messed about with voltage sensing ADC and came out with reliable software for use with LiPos. Those modules now go up to 4.5V and balancing starts at 4.15V.

LiPos voltage is so tough they doesnt move unless you apply substantial load to them. They are also very nearly spaced... 4R7 5W resistors are enough for eventual balancing though. It gets hot under the hood there - 61°C!!!

I will put the code here when i pack it together. Both modules have to have constants changed a bit, but it works...

https://mazdamx3ev.wordpress.com/201...daljsan-domet/

On the other note i managed to burn one of my modules trough my skin! Yes i happen to be removing the last module on + side and i creased the - wire (i just jumped) and comms didnt want to work anymore. i removed the module and the rest were fine. This kind of thing can happen when servicing a pack. So i will try to make one module with totaly separated output lines. Those modules will reside on output side of individual boxes. I just have to make some space for opto...

[arber333]

This topic hadnt had an update for some time.

Last couple of months i was thinking of designing a new concept BMS for my LiPos that are packed 8S. Well not one DIY BMS todate has ability to monitor 8S packs in one PCB. There is always a limit like 32V Vmax for controlling chip etc...
So i thought why not join Neville modules and make them 8S with 9 wire connection. That makes for a very simple assembly

I started first model PCB with full +/- connection. Mistake! After balancing when more and more modules started to balance cell value began to climb!!! I got 4.10V when highest cell was 4V! The best explanation was that current was seeping trough wires back and forth and creating residual voltage that showed itself on neighbouring cells.
When i saw that even those cells were balancing that werent trough balancing treshold initially, this confirmed my suspicion.


I need a new design.
New PCBs have default neighbouring connection that require only 9 wires. Certainly i can also connect only 6S or 4S cells.
I dont need resistors anymore. I use TIP122 darlington and i open it with weak base 10K. It can give 2A shunting there. Best thing about this is i can use car chassis as heasink. I just have to use good mica sheets to separate transistors from metal heatsink.
I also planned on using small 8A fuses just in case. However system is designed to monitor cells even if transistors burn out. I would get plenty of notice.


Also i designed new master board that has optical separation built in. There is also separate serial port connection that is allways transmitting.
By all effects this system is still distributed, i just packaged it as a 8S system for difficult to connect cells. Also due to optical separation BMS can be chained for 16S cell packages etc...

I will post new build files later when i try the whole system, but with one 8S PCB working good i am optimistic.

[thingstodo]

Originally Posted by arber333

However system is designed to monitor cells even if transistors burn out. I would get plenty of notice.

This sounds like a good feature. Monitoring only.

I'm a fan of bottom balancing, but I would feel better if the cell voltages were monitored.

I am not confident enough in my DIY fabrication skills to put a board on the batteries. I would likely INCREASE the chance of a battery fire instead of DECREASE.

Instead of replaceable fuses, would *REALLY* thin wire be better protection? It would take some experimenting to determine how small it would need to be. AWG #40 is the smallest that is listed in WIKIPEDIA here https://en.wikipedia.org/wiki/American_wire_gauge

That way if the insulation rubs off, or anything goes wrong in a vibrating, moving vehicle, the thin wire just melts and the short circuit clears. A small amount of damage but no battery fire! AWG #40 lists 8.5 amps for 32 ms ... I assume longer would melt it - which is the idea.

[arber333]

Originally Posted by thingstodo

This sounds like a good feature. Monitoring only.

I'm a fan of bottom balancing, but I would feel better if the cell voltages were monitored.

I am not confident enough in my DIY fabrication skills to put a board on the batteries. I would likely INCREASE the chance of a battery fire instead of DECREASE.

Instead of replaceable fuses, would *REALLY* thin wire be better protection? It would take some experimenting to determine how small it would need to be. AWG #40 is the smallest that is listed in WIKIPEDIA here https://en.wikipedia.org/wiki/American_wire_gauge

That way if the insulation rubs off, or anything goes wrong in a vibrating, moving vehicle, the thin wire just melts and the short circuit clears. A small amount of damage but no battery fire! AWG #40 lists 8.5 amps for 32 ms ... I assume longer would melt it - which is the idea.

Well system WAS designed with bottom balancing in mind. But on my insistence Neville changed code for top balancing - hence balancing switch. So if you dont want you can leave balancing off and charger will detect if one cell goes over threshold and stop charger.
I even have the old code if you are interested. BMS would discharge cells down to 2.8V and then you would charge them together up to 3.5V.
If you want to use only detection you leave transistor pins unpopulated. All sensor stuff functions good without it.
What kind of cells you use? I still have spare PCBs for LiFe.

Check here for new design files https://mazdamx3ev.wordpress.com/tag/bms/

[thingstodo]

Originally Posted by arber333

What kind of cells you use? I still have spare PCBs for LiFe.

I'm using Leaf cells - lithium polymer - so the boards likely won't fit without some modifications.