LiFeP04 Batteries for Solar & BMS

[Sunking]

Once the weakest (lowest capacity cell) has been located, is it possible / reliable to use it as a pilot cell to monitor the battery pack, and have it control LVD and 90% full charging ?

On paper yes Mike. But would you be willing to bet $2500 to $15,000 on it? Intially assuming you correctly dead nuts bottom balanced to 0% it would be the one weak cell driving the train.

Spending a couple hours for the initial set up, and a monthly verification that the chosen cell is still the weakest, would eliminate a rats nest of wires and fuses and a couple handfuls of questionable quality BMS sensor board, thereby increasing the overall system reliability ?

OK fair point, wrong picture in your minds eye. If you Bottom Balance, then you eliminate the boards on each cell unless you want to monitor temperature. For RE applications temperature is not needed because the charge and discharge currents are so low there would be no heating issues. EV is another story coupled with the elements.

Rats nest well depends on how you look at. For a 48 volt 16S pack all you need is a 16 wire ribbon cable running right across the top of the pack breaking one wire out at each cell. One wire connected between each cell to monitor voltage of each cell. In addition to that you would need a pair of small gauge power wire from the batteries to power the BMS. Keep in mind the inter-cell connections are not wire and lugs like you use on lead acid. LFP uses buss bars. So all you have is a neatly placed ribbon cable running across the tops of the batteries neatly secured and dressed in. IMO or my eye is a work of art, not a Rats Nest.

Does this look like a rats nest even with the Balance Boards?

[Mike90250]

Once the weakest (lowest capacity cell) has been located, is it possible / reliable to use it as a pilot cell to monitor the battery pack, and have it control LVD and 90% full charging ? Spending a couple hours for the initial set up, and a monthly verification that the chosen cell is still the weakest, would eliminate a rats nest of wires and fuses and a couple handfuls of questionable quality BMS sensor board, thereby increasing the overall system reliability ?

[Sunking]

Oh. Ok. I was thinking of an interface to the BMS, to program it and display parameters. It's possible that could be done, but you appear to be saying DIY BMS for our LFP bank, since we know what actions we want to do based on voltage (not really an accurate statement since we need to account for sags under load). I like your thinking. And there won't be any top balancing to disable.

One bit would be 0.005V, which works, assuming there is accuracy across channels. You could calibrate that with a table though, if necessary. Of the top, it sounds like a definite candidate. The challenge would be integrating with the rest of the system, I suppose.

One bit would

Accuracy as I understand it is completely dependent on the 5 volt supply. There is a Forum called Endless-Sphere a E-Bike enthusiast group. Over there there are two threads running. One is an Arduino BMS that sound slike a commercial product development, but anyone can get into the action and buy a test unit. If you can help with development they will sale it to you at cost. Sounds like a Chi-Com manufacture ripped them off already and beat them to market. Since it is open source not squat they can do about it. The have all the code and schematics right there to be downloaded so anyone can copy. Might be a bit more work that I am willing to put into it.

The other thread is using a CellLog charger. That is a RC Hobby Charger and can charge or do anything with up to 8 cells, so it would take two of them for a 16S pack. I have something similar but is called an iCharger.

There are lots of ways to skin this cat. Might even be a biz to be born for Bottom Balancess since no commercial product exist. Everything is built around Top Balance.

As you can tell I am kind of all over the place with it evolving. Perhaps we can work together on a common goal. You want RE, I want EV. Really makes no difference what the application is.

[Living Large]

Arduino is a family of circuit boards used by Robotis and Electronic Hobbyist to do about anything you want. They are a family micro controllers and what they call Shield boards. The controllers have up to 54 digital inputs and 20 Analog inputs. You right code to process what they do with the input information. The Shields are various I/O interfaces for WiFi, Bluetooth, USB and Display. I have used them to make a 8 channel RC remote control using a computer joystick. It is all Open Source Code and Circuit Designs. Literally hundreds of user groups to make anything you want. Think of it like Linux.

For voltage you just use a A/D converter for 0 to 5 volts and it converts it to 10 bit number from 0 = 0 volts and 1023 = 5 volts and input it into one of the digital inputs. Do that 16 times for each cell. Then write the code to do what you want to do with with it like switch a output when the voltage goes to 2.7 volts for more than 15 seconds is you LVD control. Another code line to through another output if any input goes to3.7 volts your charger disconnect. Then the code to drive a Display anything you want based on the inputs to a LCD screen.

Custom BMS to your likes at half the cost of a commercial unit.

Oh. Ok. I was thinking of an interface to the BMS, to program it and display parameters. It's possible that could be done, but you appear to be saying DIY BMS for our LFP bank, since we know what actions we want to do based on voltage (not really an accurate statement since we need to account for sags under load). I like your thinking. And there won't be any top balancing to disable.

One bit would be 0.005V, which works, assuming there is accuracy across channels. You could calibrate that with a table though, if necessary. Of the top, it sounds like a definite candidate. The challenge would be integrating with the rest of the system, I suppose.

One bit would

[Sunking]

The thought occurred to me that it may be possible to disable top balancing. Since you aren't working with dumb boards, that makes sense. Great.

I have seen references to Arduino, and haven't looked into it. Seems the time has come. I think Jack mentioned it in the video. Clue me in - how would Arduino fit in here? Would it be an interface to the BMS, or something else? I'm seeing RS-232 - would that be the communication with the BMS?

Arduino is a family of circuit boards used by Robotis and Electronic Hobbyist to do about anything you want. They are a family micro controllers and what they call Shield boards. The controllers have up to 54 digital inputs and 20 Analog inputs. You right code to process what they do with the input information. The Shields are various I/O interfaces for WiFi, Bluetooth, USB and Display. I have used them to make a 8 channel RC remote control using a computer joystick. It is all Open Source Code and Circuit Designs. Literally hundreds of user groups to make anything you want. Think of it like Linux.

For voltage you just use a A/D converter for 0 to 5 volts and it converts it to 10 bit number from 0 = 0 volts and 1023 = 5 volts and input it into one of the digital inputs. Do that 16 times for each cell. Then write the code to do what you want to do with with it like switch a output when the voltage goes to 2.7 volts for more than 15 seconds is you LVD control. Another code line to through another output if any input goes to3.7 volts your charger disconnect. Then the code to drive a Display anything you want based on the inputs to a LCD screen.

Custom BMS to your likes at half the cost of a commercial unit.

[Living Large]

Unfortunately all the BMS systems are intended to Top Balance. However they do everything required for Bottom Balancing. Trick is you just program Top Balancing out or defeat it.

Spoke with a few user groups who Bottom Balance on the Robot side of the house. Are you open to Arduino? Real simple to do and most of it has already been done so no circuit design or code writing. Takes three circuit boards, a display, and some fabrication. Then dump the code in. Dirt cheap!

The thought occurred to me that it may be possible to disable top balancing. Since you aren't working with dumb boards, that makes sense. Great.

I have seen references to Arduino, and haven't looked into it. Seems the time has come. I think Jack mentioned it in the video. Clue me in - how would Arduino fit in here? Would it be an interface to the BMS, or something else? I'm seeing RS-232 as part of Aduino, but I saw no other interface at a glance - would that be the communication with one of these BMSs?

[Sunking]

Hey, like wow. Neat. One thing that pops out is "passive cell balancing". I wonder how that works and if it is desirable, viz a vie the previous discussion about top balancing. From the look of the device, it isn't clear it involves shunting.

I'll take a further look later today. Thank you for this research - promising.

EDIT I also looked at the one you mentioned above. Question - if the minimum and maximum voltages at the cell level are programmable, it seems like we could eliminate the concern about shunting, even if the BMS includes it. The only thing that pops to mind is if shunting occurs earlier when you decrease the max voltage threshold. You'd possibly be fighting the BMS' attempt to top balance your batteries? Just musing. I any event, these are a great start.

Unfortunately all the BMS systems are intended to Top Balance. However they do everything required for Bottom Balancing. Trick is you just program Top Balancing out or defeat it.

Spoke with a few user groups who Bottom Balance on the Robot side of the house. Are you open to Arduino? Real simple to do and most of it has already been done so no circuit design or code writing. Takes three circuit boards, a display, and some fabrication. Then dump the code in. Dirt cheap!

[Living Large]

OK LL I found one that is come to the top, the Orion Jr. It is a 1 to 16 channel device.

Hey, like wow. Neat. One thing that pops out is "passive cell balancing". I wonder how that works and if it is desirable, viz a vie the previous discussion about top balancing. From the look of the device, it isn't clear it involves shunting.

I'll take a further look later today. Thank you for this research - promising.

EDIT I also looked at the one you mentioned above. Question - if the minimum and maximum voltages at the cell level are programmable, it seems like we could eliminate the concern about shunting, even if the BMS includes it. The only thing that pops to mind is if shunting occurs earlier when you decrease the max voltage threshold. You'd possibly be fighting the BMS' attempt to top balance your batteries? Just musing. I any event, these are a great start.

[Sunking]

OK LL I found one that is come to the top, the Orion Jr. It is a 1 to 16 channel device.

[Sunking]

Hey LL found this nice little SELECTOR TOOL for BMS. Check the boxes and it spits out all compatible models. They have 62 models in their database. When I enter 16 cells, 3.6 volts, and In Production gives 40 of 62. THIS ONE might work.

[Sunking]

I don't believe I stated what meant clearly - I was speaking about triggering when the lowest cell hits the threshold, but not knowing which one it was. Like the dumb BMS setup with a make/break loop. I am guessing we would want this threshold to be user-selectable, to account for manufacturer differences and/or user preference.

Reasonable and doable expectations. Anything I come up with will monitor every cell. It would be useless without that IMO. That is where Jack caught criticism because at the time he was only monitoring PACK VOLTAGE not cell voltages. To only monitor PACK VOLTAGE then Bottom Balance is the only way to go. What we are discussing is Bottom Balance with at least cell monitoring with CONTROLS and Variable Set Points. In my book that is BMS, just not mainstream Top Balance BMS. Bottom Balance has Passive over discharge protection built in without a need for LVD. I advocate Bottom Balance with LVD because I do not want to flirt with that last 10%. In the event the LVD fails, I have the passive design inherent as a Fail Safe to CYA. In my book is smart design having redundancy, one Active and Passive as backup. Like a Molten Salt Reactor, if it gets to hot the Salt Plug melts and drains the fuel if the cooling systems fail for whatever reason.

[Living Large]

This is where I part ways with Jack and so does the community. He lost cells because he monitored Pack voltage, Not Cell voltages. The stronger cells destroyed the weaker cells by reverse polarity. He finally admitted this oversight latter. That does not invalidate Bottom Balance. It just means whatever method you use you need to montoor all cell voltages and LVD if any cell falls below set limit.

I don't believe I stated what meant clearly - I was speaking about triggering when the lowest cell hits the threshold, but not knowing which one it was. Like the dumb BMS setup with a make/break loop. I am guessing we would want this threshold to be user-selectable, to account for manufacturer differences and/or user preference.

I think I confused things by alluding to what I thought might be included with a "BCU" in my dreams - which would be to monitor each individual cell voltage and display it - as a nice to have feature, but not needed for the daily charge/recharge cycling we are discussing. My thinking is simply that long term monitoring would be easy at a glance. But this detracts from the real discussion - how to implement protective monitoring, and then tackle integrating the result with the charger/inverter equipment. One step at a time. Thanks for looking into this - I really appreciate it and it should be informative for anyone considering LFP.

[Sunking]

It is of note that Jack's criticism of BMS is what happens on the low end of the curve as a result of top balancing, not that shunting itself causes damage (unless your board shorts out, as he demonstrated). Overall, I found it a convincing argument. It would very interesting if there is a rebuttal out there - I would almost expect it.

There is here, some very heated rebuttals. In the end Jack wins by admitting his oversight which I addressed. Its a learning curve thing. I know Jack personally and is one of the two I mentioned.

[Sunking]

I just finished the video. Great find. I note that Jack specifically states the control system doesn't need to know which cell is low, which is correct.

This is where I part ways with Jack and so does the community. He lost cells because he monitored Pack voltage, Not Cell voltages. The stronger cells destroyed the weaker cells by reverse polarity. He finally admitted this oversight latter. That does not invalidate Bottom Balance. It just means whatever method you use you need to montoor all cell voltages and LVD if any cell falls below set limit.

What it boils down to is a battery pack (all the cells) capacity is limited to the weakest cell in the pack. Makes no difference if you top or bottom balance. If you top balance it is very possible to destroy a cell or cells by the stronger cells destroying the weaker cells. This happens because the weaker cells reach 0% SOC before the stronger cells and will be driven to reverse polarity and that instantly destroys the cell. Of course you can avoid that with active cell monitoring and a LVD.

But here is the Magic. LFP batteries have a very unique characteristic no other other battery has. The higher the SOC, the higher the Ri is. It has a negative Ri coefficient vs SOC. It is the only battery that does that and is a HUGE ADVANTAGE. If we change the reference to Bottom SOC we still have the same capacity governed by the weakest cell. but they all reach ZERO at the same time and no cell can be eaten by th estronger cells in reverse polarity. It is passive protection requiring nothing but an initial Bottom Balance. NOTE I DO NOT advocate that. Use cell voltages and LVD if any cell reaches threshold. I like 10% SOC as it is safe and extends battery life.

Additionally By not taking the cells to 100% SOC extends cycle life. Add it all up and IMO Bottom Balance makes more sense. What it boils down to is a battery pack (all the cells) capacity is limited to the weakest cell in the pack. Makes no difference if you top or bottom balance. If you top balance it is very possible to destroy a cell or cells by the stronger cells destroying the weaker cells. This happens because the weaker cells reach 0% SOC before the stronger cells and will be driven to reverse polarity and that instantly destroys the cell. Of course you can avoid that with active cell monitoring and a LVD.

[Living Large]

Correct if you are Bottom Balancing there is no need for Top Balance Boards. Just remember there methodologies of Top and Bottom Balancing. Today Top Balancing is the direction Manufactures are going. This comes from the commercial EV market. Well most of the commercial EV market. Example the Leaf uses top balancing, and they are having problems which have made the news. Chevy Volt on the other hand uses a different methodology of not allowing their batteries to ever be charged to 100% SOC. Its a whole new field with Lithium and breaking Pb ways of doing things.


Yes. Depending on manufacture can be called anything like EMS (Energy Management System) Like I said there is no definition of BMS.



I am new too to some extent. My LFP experience is with EV's and I am still evolving. There is a Forum devoted to DIY EV and many of the members are the engineers and designers of commercial EV's. That got me started down the Top Balance road. However their are a few Contrarians in that group with considerable experience of 7 to 10 years I have come to be friends with. They are ole dogs like me that take their experience and have learned to refine and evolve their approach from hard lessons of the past.



Give me a day to do some research. I know of a couple but there is more out there. Some do use individual circuit boards to monitor cell temperatures which i simportant for EV's, not so much for stationary uses like RE. Hum this gives me an idea of a biz. This would be real easy to use something like Arduino controller with 16 DAC's.

I just finished the video. Great find. I note that Jack specifically states the control system doesn't need to know which cell is low, which is correct. I can manually check voltages for my own information. However, I would need a BCU with the right thresholds.

Interesting that this video was made 5+ years ago.

I had come across a video a month ago of a guy who was experimenting in his house with cycling LFP's. He had a large array of 200Ah cells. At one point, there was a pile of discarded BMS boards in a closet. I think he said "these ruined my batteries", and he gestured to a few dead cells. That is consistent with what Jack is saying, but it wasn't clear what the failure mechanism was. The BMS that shorted in Jack's video, and ruined a battery, was striking. He made a great point that a cheap device, that needs to be cheap, ruined a very expensive battery. Also the heat/wasted energy aspect.

It is of note that Jack's criticism of BMS is what happens on the low end of the curve as a result of top balancing, not that shunting itself causes damage (unless your board shorts out, as he demonstrated). Overall, I found it a convincing argument. It would very interesting if there is a rebuttal out there - I would almost expect it.