Jump to content

Cheap LiFePO4 BMS?


jetzi

Featured Posts

7 minutes ago, nicknorman said:

Yes the wires from the cell terminals to the BMS are fused, except for the 0v one. I pass them through a small PCB (cut off from my original BMS board design) which has 4 surface mount fuses. So they are soldered in place. Obviously it will be a bit of a pain to replace a blown fuse but that should not happen unless I’m very careless, in which case serves me right!

 

Yes Im sure passing 200mA through the contacts will help a lot. I plan to have the MOSFETS and resistors remote from the BMS, with separate wires going to the cells. This was based on a presumption that for 600Ah of batteries I would need a fairly hefty balancing system capable of several amps. However the perceived wisdom on here seems to be that balancing is not much required so maybe I’ll scale that back a bit and follow the MP strategy of “little and often”.

Depending on exactly how that's laid out, I might feel happier having fuses next to the cell terminals,  rather then running unfused wires to a PCB somewhere remote. If the PCB is in the middle of the battery stack, that might be OK. If those wires ever rub through the insulation and short, they will act as their own fuses, but with rather unpredictable consequences for smoke and flame. I use the fuses as the isolation mechanism if I ever need to work on the BMS or its connections. Soldered in fuses don't work for that, but you clearly have a removable plug.

 

200mA is fine for trimming. If I was doing this again I'd got for bigger to make it practical to use as the method of getting a new pack in gross balance.

 

MP.

 

Link to comment
Share on other sites

16 minutes ago, nicknorman said:

Yes the wires from the cell terminals to the BMS are fused, except for the 0v one. I pass them through a small PCB (cut off from my original BMS board design) which has 4 surface mount fuses. So they are soldered in place. Obviously it will be a bit of a pain to replace a blown fuse but that should not happen unless I’m very careless, in which case serves me right!

 

Yes Im sure passing 200mA through the contacts will help a lot. I plan to have the MOSFETS and resistors remote from the BMS, with separate wires going to the cells. This was based on a presumption that for 600Ah of batteries I would need a fairly hefty balancing system capable of several amps. However the perceived wisdom on here seems to be that balancing is not much required so maybe I’ll scale that back a bit and follow the MP strategy of “little and often”.

It is an issue if I am to have an accurate assessment of SoC based on a hybridised parameter using Ah counting (the BMV or Mastershunt) and rested voltage vs SoC data. Gold plating may sound posh but the amount of gold is miniscule and in terms of the additional cost, it is a few 10s of pence for the gold plated connector vs the tin plated one. Choice of contact plating is about the best plating for the application, it is barely at all about cost.

Nick when I put together my 3 x 48 volts blocks for my electric car I just used cheap balance boards that had good reviews, a few months down the line the blocks are in good balance, they can move up to 5 amps around,  I have no idea if its true because after a run the batteries are still in balance, this either down to good cells or the balance boards and I can't be bothered to find out which! I suppose I could disconnect the boards and go on a run but can't arsed if I am honest I just accept that things work 

  • Greenie 1
Link to comment
Share on other sites

2 hours ago, MoominPapa said:

Depending on exactly how that's laid out, I might feel happier having fuses next to the cell terminals,  rather then running unfused wires to a PCB somewhere remote. If the PCB is in the middle of the battery stack, that might be OK. If those wires ever rub through the insulation and short, they will act as their own fuses, but with rather unpredictable consequences for smoke and flame. I use the fuses as the isolation mechanism if I ever need to work on the BMS or its connections. Soldered in fuses don't work for that, but you clearly have a removable plug.

 

200mA is fine for trimming. If I was doing this again I'd got for bigger to make it practical to use as the method of getting a new pack in gross balance.

 

MP.

 

Yes maybe I wasn’t clear, there is a small bit of PCB with fuses adjacent to the battery terminals, then a further run of wires to the actual BMS PCB. So the only unprotected wires are the few inches from cell terminals to fuse board within the battery box. I suppose I could have had an individual micro-PCB with SM fuse for each terminal, but worst case scenario is some fairly thin wire vaporising itself - not really the same as a dead short with 75mm^2 cable!

Edited by nicknorman
  • Greenie 3
Link to comment
Share on other sites

  • 3 weeks later...
6 minutes ago, nicknorman said:

Got my 4 layer boards back from China just before Christmas, pretty quick really considering it was the cheapest postage. Also done a bit of work on the software and sleep modes to minimise power consumption so now, once the display times out, it uses just under 2mA. And if it has to do emergency disconnect of the Tyco relay due to cell under-voltage it goes to sleep and only uses 0.2mA. Eat your heart out Raspberry Pi users! You wake it by pressing the button, which also makes it reconnect the relay (hopefully after you’ve put a bit of charge in).

 

Here’s a pretty pic to pass the time, also showing in the base of the plastic box it’s going to be installed in. Yes I know I haven’t finished soldering in a couple of chips, they are just the CANBUS, RS232 and battery temperature probe interfaces and I know that all works. And the buzzer, that works too. Yes that will increase the power consumption, but only by a few 10s of micro amps when they are sleeping. I managed to unsolder many of the components including the AD7280 (the tiny square chip) from an earlier prototype and resolder it onto the new board. Which was fun (not!). It must be quite resilient because it still works!

 

C92532B5-97DC-4B9E-9177-2A974C147C0B.jpeg.1d158c9f36bf7d3d033c3f0c74d4acc1.jpeg
 

yellow wires go to the Tyco relay, other connector is to the cells (via the little fuse board) and the other 2 small connectors are for the cell balancing and interface to the BMV712.

 

Cant remember if I mentioned it but it now looks like it might be possible to write a new value of SoC to the BMV using the VE.Direct Hex protocol, which would be good because it would allow a hybridised SoC based on Ah counting and cell voltages to be displayed on the BMV. I need to investigate that one further.

 

Where is the toothbrush controller wired in?  @Peanut34 wanted to know on the other thread ...

 

On 24/12/2020 at 01:08, Peanut34 said:

I could 'build' an alternator controller whilst I was brushing my teeth ?

 

  • Love 1
  • Haha 2
Link to comment
Share on other sites

21 minutes ago, TheBiscuits said:

 

Where is the toothbrush controller wired in?  @Peanut34 wanted to know on the other thread ...

 

 

Haha yes. But just for clarity, this is the BMS not the alternator controller. I decided early on in the project that they would be separate entities, not quite sure if that was the right decision as they share a lot of data, but it now is what it is!

  • Greenie 2
Link to comment
Share on other sites

13 hours ago, nicknorman said:

Got my 4 layer boards back from China just before Christmas, pretty quick really considering it was the cheapest postage. Also done a bit of work on the software and sleep modes to minimise power consumption so now, once the display times out, it uses just under 2mA. And if it has to do emergency disconnect of the Tyco relay due to cell under-voltage it goes to sleep and only uses 0.2mA. Eat your heart out Raspberry Pi users! You wake it by pressing the button, which also makes it reconnect the relay (hopefully after you’ve put a bit of charge in).

 

Here’s a pretty pic to pass the time, also showing in the base of the plastic box it’s going to be installed in. Yes I know I haven’t finished soldering in a couple of chips, they are just the CANBUS, RS232 and battery temperature probe interfaces and I know that all works. And the buzzer, that works too. Yes that will increase the power consumption, but only by a few 10s of micro amps when they are sleeping. I managed to unsolder many of the components including the AD7280 (the tiny square chip) from an earlier prototype and resolder it onto the new board. Which was fun (not!). It must be quite resilient because it still works!

 

C92532B5-97DC-4B9E-9177-2A974C147C0B.jpeg.1d158c9f36bf7d3d033c3f0c74d4acc1.jpeg
 

yellow wires go to the Tyco relay, other connector is to the cells (via the little fuse board) and the other 2 small connectors (not yet installed) are for the cell balancing and interface to the BMV712.

 

Cant remember if I mentioned it but it now looks like it might be possible to write a new value of SoC to the BMV using the VE.Direct Hex protocol, which would be good because it would allow a hybridised SoC based on Ah counting and cell voltages to be displayed on the BMV. I need to investigate that one further.

I am seriously impressed (seriously)

 

As a professional electronic designer about thirty five years ago I was drawing up plans for the Foreign Office for a micro-controlled HF antenna tuning unit (unskilled user abandons Embassy, clips crocodile clips to a random barbed wire fence, switch on and talk to London...) - As surface mount devices became increasingly common and those nice little SMD processors, I/O and RAM became commonplace I remember thinking "that's the end of the hobby market; veroboard has had its day and there is nothing to keep the 'tinkerer' going"

 

I'm very glad to be proved wrong.

Link to comment
Share on other sites

38 minutes ago, 1st ade said:

I am seriously impressed (seriously)

 

As a professional electronic designer about thirty five years ago I was drawing up plans for the Foreign Office for a micro-controlled HF antenna tuning unit (unskilled user abandons Embassy, clips crocodile clips to a random barbed wire fence, switch on and talk to London...) - As surface mount devices became increasingly common and those nice little SMD processors, I/O and RAM became commonplace I remember thinking "that's the end of the hobby market; veroboard has had its day and there is nothing to keep the 'tinkerer' going"

 

I'm very glad to be proved wrong.

Thanks. I think it’s like a lot of such things, you have to move with the times but you can still DIY stuff. And cheaply - free PCB design software (I use RS Design Spark), free software development environment with Assembler and C compiler from Microchip. I just needed to fork out £60 for the dongle that plugs into PC USB port at one end, and the in-circuit programming port of the microprocessor at the other end, which allows programming and debugging. And I had to buy a hot air rework gun for soldering the surface mount components - about £35 from ebay.

 

And these modern micros are very easy to use, no external components required - built in oscillators of various flavours, built in power on reset, lots of flash programme memory, RAM and EEPROM. And fully loaded with every peripheral under the sun. And all for under £2.

With the ability to get 4 layer PCBs made up for under a tenner, it is a cheap hobby. But of course there is a lot of investment in time, to use the PCB design software and software development suite. And the datasheet for this particular micro is 840 pages long! And in my case, learning to programme in C. But that is what hobbies are about surely - a way to occupy time and get a sense of achievement at the end.

  • Greenie 2
Link to comment
Share on other sites

8 minutes ago, nicknorman said:

And in my case, learning to programme in C.

I think (but it's a long time ago) we programmed in raw assembler. We had a bit of paper which said "Highest observed Frequency = 01FC (2 bytes)" and turned this into LDL 01FC, LDH 01FD as we typed the code in. And we had two (very early) IBM PC's (between 14 people) so you had to kick someone off who was writing a test spec in Wordstar to get even the assembler entered and downloaded. Happy days...

Link to comment
Share on other sites

1 hour ago, 1st ade said:

I am seriously impressed (seriously)

 

As a professional electronic designer about thirty five years ago I was drawing up plans for the Foreign Office for a micro-controlled HF antenna tuning unit (unskilled user abandons Embassy, clips crocodile clips to a random barbed wire fence, switch on and talk to London...) - As surface mount devices became increasingly common and those nice little SMD processors, I/O and RAM became commonplace I remember thinking "that's the end of the hobby market; veroboard has had its day and there is nothing to keep the 'tinkerer' going"

 

I'm very glad to be proved wrong.

Swings and roundabouts. Being able to buy packs of 2000 assorted resistors for not much cash on Ebay sure beats recycling them from old tellys.

 

MP.

  • Greenie 2
Link to comment
Share on other sites

18 minutes ago, 1st ade said:

I think (but it's a long time ago) we programmed in raw assembler. We had a bit of paper which said "Highest observed Frequency = 01FC (2 bytes)" and turned this into LDL 01FC, LDH 01FD as we typed the code in. And we had two (very early) IBM PC's (between 14 people) so you had to kick someone off who was writing a test spec in Wordstar to get even the assembler entered and downloaded. Happy days...

Yes I certainly started out in Assembler in the 70s, it’s all there was when I briefly worked as a design engineer. In fact now I come to think of it, my degree project was done in machine code since there was no assembler (it was pretty simple though. Later, as a hobbyist,  I did quite a bit with 8051 series in the 80s, that was a microprogrammed device with a relatively powerful instruction set. I then moved on to high level languages like VBA. C has been around since the 90s but never got into it. One “issue” with the PIC micros I now use is that they are RISC and so not that easy to programme in assembler, hence my move to C which, now that I have more or less got the hang of it, is great. Not intuitive at first though!

Edited by nicknorman
  • Greenie 1
Link to comment
Share on other sites

On 27/12/2020 at 19:17, nicknorman said:

Cant remember if I mentioned it but it now looks like it might be possible to write a new value of SoC to the BMV using the VE.Direct Hex protocol, which would be good because it would allow a hybridised SoC based on Ah counting and cell voltages to be displayed on the BMV. I need to investigate that one further.

 

Update: Yes you can write a new value of SoC to the BMV712 using the VE.Direct Hex protocol. Which is great, for the above reasons.

The micro has 2 built in UARTs (serial interfaces) so I can keep one for sending logging data to a laptop etc and the other for bidirectional communications with the BMV712.

 

 Except that I didn’t make provision in my board for sending data to the BMV, only receiving data from it.?

 

So another order for PCBs off to China, and another £10 spent. ?

 

Actually I tweaked the design for the alternator controller too, and sent that off in the same order. 5 2 layer boards for the alternator controller = £1.48, 5 4 layer boards for the battery monitor = £5.17, shipping both lots = £4.09 making a grand total of £10.75

Edited by nicknorman
  • Greenie 2
Link to comment
Share on other sites

  • 2 weeks later...
On 03/11/2020 at 20:36, Craig Shelley said:

We ended up using these heated mats. (See photo) They are available on eBay, search for "camper van heating mat" They are rated at 220w/m2. They can be cut down lengthways to fit the size of your enclosure.

We have 3 wired in series which, from memory, draws about 1 amp at 12V. In our setup this provides enough heat to maintain the cells above 5 deg C when the ambient temperature is at about -10 deg C.

We've fused both the +ve and -ve supply feed to the battery heater circuit so that a single fault e.g short to +ve battery terminal could not cause a meltdown.

--

Craig

20190120_180444.jpg

I got this heating film, thanks. How did you attach the wires? I'm thinking cutting a hole for a bolt through the copper strip either side with washers making contact with the copper.

 

I also got a cheap thermostat and will wire it to a circuit breaker in the 12V board plus a fuse on both terminals.

 

And did you place the film directly under your batteries? or between the cells?

Link to comment
Share on other sites

On 10/01/2021 at 14:14, jetzi said:

I got this heating film, thanks. How did you attach the wires? I'm thinking cutting a hole for a bolt through the copper strip either side with washers making contact with the copper.

After digging back through the photos.. (memory not as good as it used to be) I can vaguely remember cutting through the transparent surface film with a scalpel. I can also remember it being possible to thread the wire in from the ends alongside the copper foil. I think the copper foil is only adhered to one side of the transparent film. I remember cutting a hole in the film part way along, to allow the wire to be soldered, either to the foil or back onto itself.

You can see on the photo that I did a staggered cut of the sheet so that the connection point wasn't located directly beneath the batteries.

The batteries sit directly on top of the heating film, with insulation beneath that. The sides of the batteries are also insulated, as are the lids of the enclosures.

 

Until recently we didn't have much in the form of battery temperature monitoring, we just used to use the read-out on the solar charge controller, which had a single sensor on a long lead.

In my previous post, I described the fact that we had wired all 3 heaters in series to limit the maximum power and make the system "inherently safe" I recently determined that this winter the series heater configuration would not be sufficient to heat the batteries in a sensible time period, and heating the batteries over a long period of time wasn't very efficient due thermal loss from our enclosures.

We now have the heaters wired in a parallel configuration, with fuses rated at 7.5A on each of +ve and -ve as before.

 

Each heater measures as approx 4.9 Ohms.

Powering from 13V, with approx 0.4 Ohms of wiring resistance, we are able to dump about 22W of heating power into each battery. Approx 6.4A total.

The attached graph shows the the temperature rise. The heaters were run for 2 hours, then turned off for 1 hour before charging the batteries using the alternator. The latter stage of charging was done at a lower engine RPM and the lower charging current this caused is evident in the graph.

The graph shows the self-heating of the batteries during charging at a similar rate to what the heaters achieve.

 

The thermal resistance per enclosure (leakage) was determined to be about 1.16 °C per W, and the heat capacity per enclosure was determined to be about 35.5kJ per °C.

From these values it is possible to predict how much heating time is needed to raise the temperature by a given amount, and to determine the maximum achievable temperature etc...

 

Depending on the design of your enclosures, wiring the heaters in parallel like this might not be inherently safe i.e. it might be possible to heat the batteries beyond their specification. I'd recommend installing an additional protection system such as thermal fuses, clockwork timer unit etc.. With our system, it is theoretically possible to heat the batteries to about 19 °C above the engine room temperature. We'll certainly be removing the fuses once the winter is behind us.

--

Craig

Screenshot from 2021-01-12 21-36-39.jpg

Screenshot from 2021-01-12 21-30-04.png

Screenshot from 2021-01-12 21-53-40.jpg

  • Greenie 1
Link to comment
Share on other sites

Boards now back from China. 18 days turnaround time using the cheapest shipping, pretty good I think.

 

Anyway, on the subject of the MP patent filter for trimming SoC from an Ah counting device, using SoC vs zero current voltage, Ive done a bit of testing on the voltage characteristics of Li at zero current. It seems that following even a modest charge or discharge followed by disconnection, there is a very long period of settling. I logged some data for 16 hours but it’s still debatable whether the voltage had fully settled. Having fully charged the batteries, then discharged to about 40%, I added 10% at 5A into a 200Ah bank (so pretty slowly) and then disconnected the charger. So the graph relates to 50% SoC..

 

A7D01E33-4569-429D-943F-D62C936F4765.jpeg.be291a6d8202da3a7cef1a87f3793dff.jpeg
 

This is probably not relevant to MP because he is live aboard and his batteries are either constantly discharging (at various rates) or being recharged. I don’t think his would spend a long period (days or weeks) at zero current. When we are on our boat, it will be similar, but when the boat is in the marina on shore power, weeks (or at the moment, months) could go by with zero current. I therefore think that in the algorithm to tweak SoC according to (short term) zero current, I will have to add a state that modifies the voltage vs SoC relationship once current has been zero for many hours.

 

im going to repeat the test going the other way (discharge) but suspect it will be similar.

Edited by nicknorman
Link to comment
Share on other sites

20 hours ago, nicknorman said:

 

This is probably not relevant to MP because he is live aboard and his batteries are either constantly discharging (at various rates) or being recharged. I don’t think his would spend a long period (days or weeks) at zero current. When we are on our boat, it will be similar, but when the boat is in the marina on shore power, weeks (or at the moment, months) could go by with zero current. I therefore think that in the algorithm to tweak SoC according to (short term) zero current, I will have to add a state that modifies the voltage vs SoC relationship once current has been zero for many hours.

They sound like brilliant bits of precision kit Nick, very impressive. I haven't lived with my batteries for long enough yet to judge if my use of my setup is as easy (fit and forget) as others have found theirs. I do like to moor up for a few days and I run engine daily for hot water.  The 60amp B to B will show its limitations re fast charging on moored up days. Time will tell if it's satisfactory.

I left my Valences just prior to November lockdown @ 13.25 ish via BMV (rested for a few hours) Haven't been back since. Had I known they would be left for months I would have checked voltages via Valence software so I could do a proper comparison when I get back. They do have the internal bms so expect some draw

Link to comment
Share on other sites

  • 4 weeks later...
12 minutes ago, George and Dragon said:

Probably neither cheap nor for LiFePO4 but interesting. I must admit I don't understand how cells are balanced...

 

https://www.electronicdesign.com/markets/automotive/article/21152534/electronic-design-wireless-bms-for-evs-reduce-weight-simplify-design-maximize-performance

 

The cells are balanced in the conventional way. What is different is that in a very large array of batteries, the data communications between the local BMSs attached to each section of battery, is done wirlelessly as opposed to by some wired data communication.

However IMO the benefits are somewhat exaggerated since whilst they talk of 75lbs to 200lbs of cables,  clearly that can't be the weight of some data communication cables, that might be the weight of all the cables needed to interconnect the batteries to themselves and the motors. The weight of the data communications cables, which could be something like CANBUS, will be a miniscule portion of that. And they also say that in order to protect the wireless system from interference/ hacking etc, the whole shebang has to be enveloped in a faraday cage. No mention of how much that might weigh! Whilst I realise that the automotive industry is chasing very small reductions in weight to reduce energy consumption, this does seem to be adding a lot of complexity and cost for what I suspect is a tiny gain.

 

Anyway, not something your average narrowboater needs to think about.

  • Greenie 3
Link to comment
Share on other sites

On 09/02/2021 at 23:15, nicknorman said:

What is different is that in a very large array of batteries, the data communications between the local BMSs attached to each section of battery, is done wirlelessly as opposed to by some wired data communication.

I feel like the benefits of a wireless communication BMS could largely be in simplifying the installation, so something that would actually help DIY narrowboaters more than automobile manufacturers. I imagine it would be quite nice to have all of the monitoring equipment and switches working wirelessly!

 

On 09/02/2021 at 23:15, nicknorman said:

they talk of 75lbs to 200lbs of cables,  clearly that can't be the weight of some data communication cables

Indeed when they said that I realised that the author probably didn't really understand what "wireless" meant. I also put a big question mark over whether wireless could ever be considered more reliable than wired!

 

Wouldn't all that wireless transmission take a bit more energy than wired? As you say, the weight savings would be miniscule, so there wouldn't need to be much inefficiency to make it pointless.

 

On 09/02/2021 at 23:15, nicknorman said:

the whole shebang has to be enveloped in a faraday cage

Wouldn't the car body count as most of the faraday cage?

 

On 09/02/2021 at 22:55, George and Dragon said:

Probably neither cheap nor for LiFePO4 but interesting. I must admit I don't understand how cells are balanced...

The article does say that it is designed for use with LiFePO4 (AKA LFP): "Both the ADI and TI wireless networks...support safe and sustainable zero-cobalt battery chemistries, such as lithium iron phosphate (LFP)."

 

Balancing is just a case of taking some of the power out of a cell that's highly-charged and putting it into a cell that is less-highly-charged. Unless they are using tiny little high-power microwaves or something to transfer this energy wirelessly, this balancing would be done through (relatively thin) wires connecting the cells.

Link to comment
Share on other sites

Well on the subject of balancing, I finally got around to getting a balancer PCB made up, they came back from China today. 4 x FETs switching 4x 1.5ohm resistors (on the heatsinks). So each cell can be made to dump 2A or so. Controlled by the BMS via the small connector, or manually by the DIP switches. Works fine, the only thing is that having played with the batteries I get the impression they will hardly ever need balancing once initially balanced. Maybe I’ll leave it disconnected until it’s needed (once a year or something). So now I have the alternator controller, the BMS, the BMV712 and the balancer board with everything talking to everything else. And of course the cells. All I need now is to be able to actually visit the boat to fit it all!


B792F721-4E44-42ED-8639-0CD33217F649.jpeg.e081a70775035f00bd160f1bffe8e6d8.jpeg
 

edit: oooh just noticed that the DIP switches are marked 1 to 4, but the silk screen on the PCB says 4 to 1. That’s going to be confusing!

Edited by nicknorman
  • Greenie 1
Link to comment
Share on other sites

2 hours ago, TheBiscuits said:

 

You've mounted the switch the wrong way round - rotate it 180 degrees ...

But then the writing on the switch would be upside down!

 

 

But yes, that is probably the best idea.

Edited by nicknorman
  • Haha 1
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.