LiFeP04 Batteries for Solar & BMS

[Sunking]

My brothers hybrid system is nearly ready to finish, there was some debate as to the type and brand of batteries and inverters and CC and BMS or, to BMS or not to BMS that is the question.....Going to get some photos and spec's together and when that is done we are starting a new Solar Hybrid section...

Pete what is boils down to is Top Balance is the world of DIY, Hobby, and Novices. It is off the shelf one-size fits all approach. It works just like Mid and Bottom Balancing but has disadvantage like Mid and Bottom Balancing. Top Balancing has two major disadvantages. 1 is taking cells to the Top or near the Top stresses the cells and shortens cycle life. 2 unless you use a BMS with an associated LVD control greatly increases your chances of over discharging a cell.

Mid and Bottom Balancing are the realm of professional and production products like an commercial EV, Laptop, Marine, Power Tools, and to a very small degree RE Solar. There are a couple of small companies like Elithion out of Boulder Co that make Solar Charge Controllers and package 24 and 48 volt Lithium Batteries. But do not rush out to buy it because it is all custom built to order.

Of the three methods being Top, Middle, and Bottom Middle Balance would be my first choice because that is what is used by Commercial EV manufactures. I did a hybrid approach mimicking Middle Balance. I use a Battery Monitor that monitors each cell that prevents any one cell for charging more then about 95% SOC, or discharging anyone cell more than 90% DOD. That does two things for me. 1. Extends battery cycle to maximum. 2 Eliminates any risk of over charging and over discharge. I sacrifice a bit of capacity which is not an issue because like EV manufactures I sized my battery to meet the requirement. What I cannot do for any reasonable amount of money is Distributive or Active Balancing like commercial EV manufactures do. Distributive or Active Balancing is taking power from higher energy cells or blocks, and transferring to weaker cells or blocks.

FWIW Top Balance using Vampire Boards is Dissipative aka Passive which means you bleed off energy as waste heat in a Resistor.

[Sunking]

http://www.teslamotorsclub.com/showt...l=1#post737775

he BMS Service Screen has 96 voltage measurements (one for each brick) and highlights the high bricks and low bricks. On the Roadster the high bricks are bled off during balancing - the Model S probably does the same, the pack architecture is very similar between the two (Roadster having 99 bricks of 69 cells).

Which is pretty much what I told you. What that author misstated is the power is not bled-off which was just a poor choice of words, it is transferred to weaker bricks. Balancing is either Active aka Distributive taking energy from higher cells or blocks to weaker cells or blocks. The opposite of that is Passive aka Dissipative (vampire or bleeder boards) that burn off power in stronger cells as waste heat. But take note they DO NOT MONITOR CELLS only groups of series/parallel cells called Bricks. They could not Top Balance if they tried, it would be a nightmare and never work with over 6800 cells. The dam thing would glow in the dark burning all that power off.

EV's don't waste energy, that is the job of novice DIY Solar users. Commercial EV's move energy around so it can be used to get that extra mile if needed. That is the heart and sole of Middle Balance systems. Middle Balance systems never touch the Top or Bottom while in operation. But they start initially at the Bottom when the batteries are assembled. You and I cannot do that. You chose the Top, I chose to mimic the Middle via Bottom. What I do is not pure Bottom Balance because I do use a Battery Monitor with control charge and discharge as a Backup Fail Safe. I have two lines of defense from over charging and discharging. .

[solar pete]

Pete what is boils down to is Top Balance is the world of DIY, Hobby, and Novices. It is off the shelf one-size fits all approach. It works just like Mid and Bottom Balancing but has disadvantage like Mid and Bottom Balancing. Top Balancing has two major disadvantages. 1 is taking cells to the Top or near the Top stresses the cells and shortens cycle life. 2 unless you use a BMS with an associated LVD control greatly increases your chances of over discharging a cell.

Mid and Bottom Balancing are the realm of professional and production products like an commercial EV, Laptop, Marine, Power Tools, and to a very small degree RE Solar. There are a couple of small companies like Elithion out of Boulder Co that make Solar Charge Controllers and package 24 and 48 volt Lithium Batteries. But do not rush out to buy it because it is all custom built to order.

Of the three methods being Top, Middle, and Bottom Middle Balance would be my first choice because that is what is used by Commercial EV manufactures. I did a hybrid approach mimicking Middle Balance. I use a Battery Monitor that monitors each cell that prevents any one cell for charging more then about 95% SOC, or discharging anyone cell more than 90% DOD. That does two things for me. 1. Extends battery cycle to maximum. 2 Eliminates any risk of over charging and over discharge. I sacrifice a bit of capacity which is not an issue because like EV manufactures I sized my battery to meet the requirement. What I cannot do for any reasonable amount of money is Distributive or Active Balancing like commercial EV manufactures do. Distributive or Active Balancing is taking power from higher energy cells or blocks, and transferring to weaker cells or blocks.

FWIW Top Balance using Vampire Boards is Dissipative aka Passive which means you bleed off energy as waste heat in a Resistor.

Thanks for the heads up SK, I will be getting our sparky thats designed and will install the system to read a few of your posts, some of it goes over my head. Pretty sure we have had an aussie company build us a custom BMS I will learn more about it in the next week or so, cheers

[Sunking]

Thanks for the heads up SK, I will be getting our sparky thats designed and will install the system to read a few of your posts, some of it goes over my head. Pretty sure we have had an aussie company build us a custom BMS I will learn more about it in the next week or so, cheers

Shoot me a PM or email. You know how to find me. I will be happy to help. Pretty easy really.

When you get the cells check the voltages. They should be around 3.2 volts and YOU MUST BALANCE THEM before assembly. You want to make sure all the cell voltages are within .05 volts. Then you connect them all in PARALLEL and walk away for a couple of hours. Then you decide which way to go. Down to the Bottom, or Up to the TOP. More latter if you inquire. Once you go Up or Down then you assemble them in series.

[tom rickard]

Sunking, how is it that bottom balancing needs to be done only once ?

If you bottom balance (parallel your cells and let them settle to 3.0V or whatever your chosen voltage for 0%SOC is), then assemble your pack and run many cycles with a CC/CV charger set to CV below the knee (ie 55V), the voltage of the cells when the pack is at 55V will drift.

If the voltage of the cells is now different at 55V to what it originally was (say after 3 years), how is it that when the pack is returned to the voltage it was set at for the 0%SOC reference the cells will all return to their original voltages ? That's the bit I don't understand.

I am kind of the opposite to you in that I actually started with a bottom balanced pack, then found that it was more practical to move to a "working voltage" balance, hence the use of programmable shunts, which cost me less than $400(AUD) including the PC interface and software.

One of the reasons I doubt what you say is that you continually say how the BMS system is so expensive, once you say something that is obvious BS, then it's hard to justify the correct things you are saying.

With the vampire boards, if you set them to the working voltage, during typical daily use, they will only bleed current for a few minutes.

There is more than one way to skin this cat. From what I can see, I totally agree with your charging regime, I just don't see that for less than $400AUD to have a ready made cell-monitoring system (will operate HVD and LVD contactors at cell or pack level) and monitor cell voltage / bypass current and temp.. why wouldn't I use it? Even if I wasn't using the shunt function (which you don't have to), i'd still want some way of monitoring cell voltage and a mechanism to activate the disconnects.

In summary:

I KNOW how to bottom balance, I started off doing just that. My cells drifted at the working voltage after around 9 months of operation, so I wanted to get some form of cell-monitoring to ensure the highest cell didn't drift into the knee.

At less than $400AUD with full customer support and monitoring software, they system I got wasn't much more expensive than a home-made system of cell-logger and unit to activate disconnects.

The shunts when used daily at the working voltage typically bleed less than .5A for less than 10mins to ensure that the cells are kept at the same voltage.

The pack never gets above 54.4V, the cells are pegged at 3.4V (this is programmable).

I have a lower cell voltage disconnect programmed into my BMS as a back up to the programmed inverter LVD.

What is the disadvantage with this method?

[Sunking]

Sunking, how is it that bottom balancing needs to be done only once ?

Exact same way any manufacture that makes 12 volt LFP batteries for cars, trucks, RV's, boats, ect.... Six months in and I do not have any drift. In addition I know dozens of DIY EV owners with 2 to 3 years in service with no drift.

One of the reasons I doubt what you say is that you continually say how the BMS system is so expensive, once you say something that is obvious BS, then it's hard to justify the correct things you are saying.

BMS is expensive. At a minimum using passive Vampire Boards is $10 to $15 per cell x 16 or how many ever cells you use. With Vampire Boards that you can program and use to monitor cell voltages and temps are even higher per unit cost, plus another another $400 to 600 for a BMS control unit. How is that BS?

Bottom Balance does not require any of that. Just a charger and use your Inverter LVD to the disconnect. You can certainly add a battery monitor with control like I did, but not required. I use an Orion Jr which is a full BMS, I just do not use the Top Balance function. Originally I intended to use it as Top Balance, but ended up using it just as a monitor and secondary fail safe. Correction I do use it to shut off the charger, but that is only because I use Fast Charge. I can just as easily lower the charger voltage and get the same result, it just takes longer operating in CV mode.

With the vampire boards, if you set them to the working voltage, during typical daily use, they will only bleed current for a few minutes.

There is more than one way to skin this cat. From what I can see, I totally agree with your charging regime, I just don't see that for less than $400AUD to have a ready made cell-monitoring system (will operate HVD and LVD contactors at cell or pack level) and monitor cell voltage / bypass current and temp.. why wouldn't I use it? Even if I wasn't using the shunt function (which you don't have to), i'd still want some way of monitoring cell voltage and a mechanism to activate the disconnects.

In summary:

I KNOW how to bottom balance, I started off doing just that. My cells drifted at the working voltage after around 9 months of operation, so I wanted to get some form of cell-monitoring to ensure the highest cell didn't drift into the knee.

At less than $400AUD with full customer support and monitoring software, they system I got wasn't much more expensive than a home-made system of cell-logger and unit to activate disconnects.

The shunts when used daily at the working voltage typically bleed less than .5A for less than 10mins to ensure that the cells are kept at the same voltage.

The pack never gets above 54.4V, the cells are pegged at 3.4V (this is programmable).

I have a lower cell voltage disconnect programmed into my BMS as a back up to the programmed inverter LVD.

What is the disadvantage with this method?

Good for you, keep it.

My set up if you want to get technical is Mid Balance like commercial EV's use to maximize battery cycle life. Since it is almost impossible for DIY to have a true Mid Balance system, I figured out how to mimic Mid Balance from the Bottom. That is what the Orion Jr is for plus it gives me a Coulomb Counter gas gauge and power gauge (current charge and discharge) when going down the road. It gives me the ability to glance at the display and determine instantly system sanity.

[karrak]

Which is pretty much what I told you. What that author misstated is the power is not bled-off which was just a poor choice of words, it is transferred to weaker bricks. Balancing is either Active aka Distributive taking energy from higher cells or blocks to weaker cells or blocks. The opposite of that is Passive aka Dissipative (vampire or bleeder boards) that burn off power in stronger cells as waste heat. But take note they DO NOT MONITOR CELLS only groups of series/parallel cells called Bricks. They could not Top Balance if they tried, it would be a nightmare and never work with over 6800 cells. The dam thing would glow in the dark burning all that power off.

Wrong, wrong, wrong, wrong wrong. Do us and yourself a favour and read the thread http://www.teslamotorsclub.com/showt...e-battery-pack at least up to page 40 and see if you change your mind.

EV's don't waste energy, that is the job of novice DIY Solar users. Commercial EV's move energy around so it can be used to get that extra mile if needed. That is the heart and sole of Middle Balance systems. .

The burning off of energy will only occur when you are charging so isn't going to make any difference to the range. You could move energy around from one set of cells to another while charging but I would think that Tesla has decided not to bother due to the extra complexity and cost for the very good reason that balanced cells do not drift very far so the amount of energy needed to be moved around is negligible and not worth worrying about. This agrees with my experience that with a period of 11 months between balanaces that the amount of power 'wasted' to do the rebalance was less than 10Wh. Hardly enough to get excited about.

Simon

[Sunking]

Wrong, wrong, wrong, wrong wrong. Do us and yourself a favour and read the thread http://www.teslamotorsclub.com/showt...e-battery-pack at least up to page 40 and see if you change your mind.

Yawn!

Do yourself a favor and read. You are really getting quite boring. If you read through no one can find any power resistors to do bleeding. Post 82

Aside from the passive switched-resistor method, active circuits can use switched capacitors, switched inductors, etc. to "shuttle" charge between the different series-connected cell groups as needed. Or even use small DC-DC converters to shuttle charge back/forth with the auxiliary battery. Many many ways to get the job done.

I'm not at all surprised that there are no power-dissipating resistors to be found -- wasting energy isn't really Tesla's style

Then in post 135 we have:

The Tesla pack as I understand it has a BMS to do centralized monitoring, but the actually balancing is done brick only brick to brick by using a rail capacitor to shuttle charge. The max potential the circuit between bricks "sees" is only one brick to the next brick in series. By shuttling you can move charge to any brick in the pack eventually. This variable impedance balancer can act totally independent from the BMS and is a very simple circuit. It doesn't need a sleep or off mode because if it is not shuttling it is using almost no power

So quit your sqauking. You lost the debate.

[karrak]

Pretty sure we have had an aussie company build us a custom BMS I will learn more about it in the next week or so, cheers

I wonder if this is the engineer you are using? He is from Australia.

http://www.teslamotorsclub.com/showthread.php/34934-Pics-Info-Inside-the-battery-pack?p=789299&viewfull=1#post789299

Obsevations from an Engineer

I've just completed a BMS for a commercial product and thought I would offer some observations.

Balancing.
Shunt balancing is the same method I'm using. Once a pack has been top balanced there is usually very little else to do. Total pack capacity is limited by the lowest capacity of a paralleled cell group.
There are ways to shuttle power around using switched capacitor, and or inductive (dc-dc) methods, but these are costly and add a whole lot of complexity to working out your failure mode analysis.
In fact, once initially balanced, future balancing has nothing to do with different cell capacities, but different internal cell resistances and leakage rates.

Reliability.
There would have been some noisy debates in the engineering team about this one. The tradeoffs of using higher capacity prismatic cells vs low capacity cylindrical cells is a hard problem.
eg. If one cell out of 7000 fails open circuit (or blows the protection fuse with an internal short circuit), the capacity of the entire pack is reduced by the number of seriesed cells. So for 16S system if one 5Wh cell fails, it reduces the pack by 16 x, so 80Wh.
Worst case is if one cell became leaky. This prematurely discharges a group (requiring constant re balancing) but with no hope of isolation the faulty cell.

There was a suggestion of having an ASIC per cell. This could disconnect the faulty cell, but on a 7000 cell pack this would add a lot more failure modes and significant cost that would be worse than the occasional cell failing.
The ASIC idea might have legs for a prismatic high cell capacity system, but not for cylindrical cells.

Effort is better spent on making high quality cells, rather than trying to work around them the complex electronics.

Future
If I had a crystal ball of what the future would be. I think the anode/cathode materials will improve to reduce impedance (so less heat in high charge/discharge), increase cycle & calendar life, and improve combustion safety to a point where where larger prismatic cells will be more viable. Having a prismatic with 100 times the capacity of a cylidrical makes it more economic to manufacture, maintain, and with more opportunity for electronics to intervene.

Wearing my systems engineering hat, the current Tesla solution 'feels right' given the current state of tech.
I think the genius of Musk is having both a good understanding of engineering AND the ability organize capital & labour. It is rare to find both skills in one person.

Simon

[Living Large]

Yawn!

Do yourself a favor and read. You are really getting quite boring. If you read through no one can find any power resistors to do bleeding. Post 82

So quit your sqauking. You lost the debate.

God bless you for having the strength to go browse a 51 page thread. Thank you for putting your reply here, and not sending me elsewhere to try to figure out what you're talking about.

I'm here in this thread for solar power in a house, and how LFP and BMS may be used with it. I believe that is the purpose of the thread. Your scheme of what you call "middle balancing from the bottom" + monitoriing + LVD and HVD makes perfect sense to me, though as you have said and recognized there are "many ways to skin a cat." Maybe you've brainwashed me, but I am an EE and I don't think so.

I am today putting an offer on the off grid house that brought me to this site in the first place - so I may go to the dark side with solar and LFP after all. Thanks for giving me the neon rope to hang myself, Dereck!

[karrak]

Yawn!

Do yourself a favor and read. You are really getting quite boring. If you read through no one can find any power resistors to do bleeding. Post 82

Keep reading you haven't gone far enough

Then in post 135 we have:

The Tesla pack as I understand it ....

As I understand it are the operative words

So quit your sqauking. You lost the debate.

How about post 245 http://www.teslamotorsclub.com/showt...l=1#post754273 with detailed picture of the bleed resistors and the following text

Great pictures!

Below are some enlargements of one of the repeating pattern of 6 bleed circuits, one for each group of paralleled cells; one enlargement for each side of the board. Don't get fooled by the bottom set; all the components are there, just rearranged a little for board layout.

Simon

[Mike90250]

...Thanks for giving me the neon rope to hang myself, Dereck!

That's an LED neon rope

[tom rickard]

BMS is NOT expensive, my system cost less than $400AUD excluding the contactors (which are required regardless of top or bottom balancing). The main reason i am posting in this thread is to make sure people are aware of that!

When you have run your bottom balanced system long enough for drift to occur (which will happen), three things will happen:
1: you will risk the highest voltage cell going into the knee when you are at CV charging at 55V
2: you will have lost useable capacity of your batteries
3: you will no longer have any idea (without returning to 48V) if your pack is still bottom balanced!

Kind of defeats the purpose of balancing in the first place doesn't it..

If you balance your cells each cycle at their working voltage (eg 3.4V), you have none of these disadvantages, and no other disadvantage that i can think of especially if you never plan to run your pack close to 0%SOC.

Basically Sunking it comes to this:

1: you insist a BMS that is programmable to balance at working voltage is expensive, i have the receipt to prove to anyone that is interested that it cost me less than $400AUD
2: you insist that your cells will never drift in voltage. My experience shows this is not true (i can only speak for my system).

If your cells never drift in voltage, then just balance them at any voltage within either knee one time and be done with it. Why would bottom balance be any more valid than top balance for any other reason than to get a more accurate representation of 0%SOC (largely irrelevant in an off-grid system)

[karrak]

Now maybe I am missing something and have made a horrible mistake as I have only skimmed the thread and only got to page 38 but it looks to me like top balancing using resistors to bleed off power.
Simon

Oops, I have made a mistake, using the terminology that SK uses, Tesla use 'Passive Middle Balance'. All this means is that they keep the charging voltage of all the cells in the battery within a narrow range of each other for the whole charging cycle. Fully charging a lithium battery using this technique is equivalent to top balancing a battery.

There is a good description of the different balancing techniques here.

http://liionbms.com/php/wp_passive_active_balancing.php

Simon

[Sunking]

All this means is that they keep the charging voltage of all the cells in the battery within a narrow range of each other for the whole charging cycle. Fully charging a lithium battery using this technique is equivalent to top balancing a battery.

There is a good description of the different balancing techniques here.

http://liionbms.com/php/wp_passive_active_balancing.php

Simon

I know the author quite well. His name is Davide Andrea. He owns Elithion who makes BMS, assemble LFP batteries, solar systems, and makes custom EV's. His company is in Boulder Colorado and hangs out on a couple of EV forums. I have done contract work for him, and was offered a position as Executive Engineer at his company 2 yyears ago before I moved to Panama. Known him for 7 years now and learned a lot from like BOTTOM BALANCE. He even has two Integrated Circuits he designed and made for TI to use in his BMS systems.