LiFeP04 Batteries for Solar & BMS

[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.

[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]

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.

[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]

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. .

[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.

[karrak]

Tesla and Bosch do not use Chi-Com LFP batteries. No commercial manufacture uses Chi-Com cells. They do not use any form of Top Balance either. Tesla uses Panasonic LiCo cells in the Roadster all 6831 of them. They do not monitor any cell Voltages, they monitor 11 Sheets made from 9 bricks. Tesla battery is 11S 9B 69P. Similar design concept in the Nissan Leaf, Toyota Prius, and Chevy Volt, just fewer cells and modules to monitor. No commercial manufacture uses TOP BALANCE.

http://www.teslamotorsclub.com/showthread.php/34934-Pics-Info-Inside-the-battery-pack?p=737775&viewfull=1#post737775

The 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).

For those interested around page 30 of this thread we get pictures of the balancing circuitry (the dreaded Vampire circuitry) and monitoring boards that Tesla call BMB boards that are connected via CAN bus to the overall BMS controller, and some more technical descriptions of it. 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.

Actually I wasn't talking about EVs. I was talking about home power systems Bosch, Sony and Tesla are making.

Simon

[solar pete]

[ BMS is the Spandex Pants of the Lithium battery world where one size fits all and ugly as sin. If you do not have the knowledge to work with Lithium batteries, you use a Top Balanced BMS and take your chances. Otherwise it is not needed. That is the secret.

Four years ago, I was pro BMS. Then I got educated. End of story.

Now it is your turn. What are you going to do? Stick with status quo, or break free? Either wayy makes me no difference, what ever floats your boat.[/QUOTE]

Hehehehe,

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...

[karrak]

I think that the bottom balance may drift if the cell current leakage is different between cells but will have to think further about this.

Here is a quick thought experiment
What if we have two identical cells connected in series with a capacity of say 100Ah, one has a cell leakage of 5% per year, the other 2.5% per year. We charge them both to 100%SOC, We then leave them for one year. At the end of the year one has 97.5Ah of charge stored, the other has 95.0Ah of charge stored. If we try to draw 97.5Ah from this battery one cell will be at -2.5Ah. Are they still balanced at the bottom end?

Simon

[Sunking]

You set the Float voltage, Float time and the silent period takes over ( charger shuts off ) and set the re-float voltage. This is a feature on every Outback product made for the last 10 years, of course you already knew that.

Yes I do know, but the point is Controllers for lithium batteries have no set points or algorithms. They are pure FLOAT CV controllers with no interface. Your Outback is optimized for Lead Acid using a 3-stage algorithm which is useless for solar. But that is another subject.

Go look at the Genasun MPPT Lithium Charge Controller Specification page. It can only handle up to 120 to 160 watts depending on which Lithium Chemistry you are operating. There is nothing for you to set or adjust. No BMS. Just Panel Input and Battery Output. You tell them what battery configuration you are using, and they program the firmware. The algorithm is FLOAT only.

Lipo 3S = 12.5 volts
LiFePO4 3S = 14.2 Volts
LiPo 4S = 16.7 volts

They are made to use on 12 volt lithium batteries you can buy from any Marine or RV Lithium battery manufacture. They are sealed battlers with no access to the individual cells or a BMS inside. At most a LVD relay to prevent accidental over discharge.

Willy honestly what is what is so hard for you to understand? No BMS is required if you follow a couple of simple rules.

That same Genasun MPPT charge controller is the exact same one they use for Pb batteries. What is different? The charging algorithm. The Pb version uses 3-stage algorithm (Bulk - Absorb-Float with temp compensation). Look at the specs Willy and see for yourself.

Note where some of your confusion may stem from is natural. There is no definition for BMS. When the term is used for with Lithium Batteries can mean two different methods.

1. Passive. Which is Top Balance using Vampire Boards attached to each cell. They are dumb boards with only two states of on or off. They turn on when the battery voltage reaches 3.55 to 3.6 volts and shunt or bleed a small amount of current around the battery when charging. They turn off when the battery voltage has bleed down to 3.5 volts or 100% SOC. Some models will add voltage and temperature monitor capabilities and interconnect with a small Ribbon Cable connected in series to communicate with a monitor controller. Each of those Vampire boards have a dip switch you set to tell it which number cell it is. Otherwise it is just a dumb passive board. Those use the cell power to communicate and are a parasitic load.

2. Active. This type really opens up a can of worms. They can be used to Top Balance, Middle Balance, and Bottom Balanced. Commercial EV manufactures use Active BMS in the Middle Balance topology. They do not monitor at the cell level as that would be way to complex basket of eggs. They monitor modules or a group of parallel and series cells. Tesla calls them Sheets. A "sheet" consist of 9 Bricks in series. Tesla only monitors and charges the battery at the Sheet level. not cell level. But it is not Top Balance, it is Middle Active Balance. That means they take power for a Stronger Sheet and transfers it to a weaker sheet if needed. Sorry to go off on that but the point is no manufactured system uses Top Balance.

Not even your lithium power tools use Top Balance. There is no need to and only increases the risk of damage by using top balance. Not to mention getting deeper into your pockets.

Top Balance comes from RC Hobby and DIY/Custom EV markets. In those markets the users use a wide mix of off the shelf parts and pieces that follow no industry standard. There is no way a BMS manufacture can make a product to fit every possible configuration a user can come up with. So they created a niche product to get into the consumers pockets. That is the purpose of any business. BMS is the Spandex Pants of the Lithium battery world where one size fits all and ugly as sin. If you do not have the knowledge to work with Lithium batteries, you use a Top Balanced BMS and take your chances. Otherwise it is not needed. That is the secret.

Four years ago, I was pro BMS. Then I got educated. End of story.

Now it is your turn. What are you going to do? Stick with status quo, or break free? Either wayy makes me no difference, what ever floats your boat.

[Willy T]

If you do have cell monitoring, how the heck are you going to tell your Solar Charge Controller to switch to Float and lower the voltage? You cannot because they are not designed to charge lithium batteries and communicate with a BMS. You have to work around it.

You set the Float voltage, Float time and the silent period takes over ( charger shuts off ) and set the re-float voltage. This is a feature on every Outback product made for the last 10 years, of course you already knew that.

[Sunking]

The right answer is: There is NO difference of charge you can use. The weakest cell determines the charge of the battery whether the cells are top or bottom pre-balanced. But, if the cells are not pre-balanced, you can't get it all.

I agree. When you receive your cells from the distributor, they are sent at Storage Voltage of 40 to 60%. When they arrive you have to pick one method or the other and balance the cells before assembly. Bottom Balance is precision, Top Balance is only a 100% SOC Voltage Reference Point and does not indicate any capacity. Bottom Balance is both a 0% SOC reference and also a capacity reference point

Perhaps you didn't notice the used cell voltage in my example was between 3.04 and 3.5 V. The TOP pre-balanced cells will reach their high charging voltage (3.5V) at the same time and the high voltage can be monitored and cut-off by the battery voltage. Discharging will require cell-level monitoring to cut-off and to prevent over discharging.

Perhaps I am missing something because that would be impossible. A LFP cell resting at 3.04 volts is essentially 0% SOC, and 3.5 is just a bit over 100% SOC

The BOTTOM pre-balanced cells will reach the low limit voltage at the same time. So this can be monitored as a pack (maybe just by noticing the lack of power).

Absolutely correct. There is no requirement need for any cell level voltage monitoring. However you do not have to really even to monitor the pack voltage for it to work. The danger is cell reversal, which requires cells in a series string to have capacity left in them to drive the depleted cell(s) into reversal. If all cells are depleted and you apply a load, the voltage collapses and no current flows. No current flow, no damage.

By the addition of say your Inverter default Low Voltage Disconnect either programmable or not, gives you two fail safe modes. Default voltages are 11, 22, and 44 volts which is above 2.5 vpc limit of a LFP cell. 2.5 vpc is what all the manufactures specify low limit to CYA, but all LFP being the same chemistry is really 2 volts. So lots of room to spare if the Inverter should fail.

If you have an Inverter with Programmable LVD, you can set the voltage to trip off at 10% SOC or 3.1 vpc which equates to set points of 12.4, 24.8, and 49.6 volts. Due to Ri of the batteries and voltage drop under load 12, 24, and 48 volts is a really good LVD set point. Lead acid batteries are spent at 12, 24, and 48 volts under load.

But, charging will call for cell monitoring to not overcharge the weakest cell. If you just use "safe" battery voltage the battery is never full (which can be good for the battery life but limits the amount of charge).

No Sir, cell voltage level monitoring is NOT REQUIRED. Initial charge, you set the charger voltage to 3.3 to 3.4 vpc. Example 52.8 to 54.4 volts on a 16S battery. You monitor cell voltages during the first charge. Once the batteries begin to reach the charger voltage, current will start to taper down and you find the cell with the highest voltage and make note of it. Allow the charge to complete, terminate the charge (disconnect), and allow the pack to rest for a few hours. After resting measure the weakest cell voltage. You are looking for about 3.4 volts at rest. If it is to high, lower charger voltage for next round. That is how it is dome on an EV.

Solar is pretty much the same method, except you want the Float voltage to be 3.4 volts after charge while holding voltage in Float during the day. Once you find the correct charger voltage set point, you are done. No cell level monitoring or control is needed. Not to say you cannot do that if you wish. but it is added expense.

If you do have cell monitoring, how the heck are you going to tell your Solar Charge Controller to switch to Float and lower the voltage? You cannot because they are not designed to charge lithium batteries and communicate with a BMS. You have to work around it.

Stop and think about something for a minute. You can go out and buy yourself a 12, 24, or 48 volt LiFePO4 battery for your car/truck starting battery, RV house battery, golf cart battery, Wheel Chair or whatever gizmo you got. They are drop in replacements. Nothing external needed. None have any BMS. Some have a built-in electronic LVD inside so you cannot over discharge them and they call that a BMS in Marketing terms as a sales point, but LVD is not really a BMS. So how can they do that and give 2 to 5 year warranties?

Have you ever thought about that or questioned yourself?

What do the manufactures know that you do not? No commercial EV manufacture uses Top Balance. No Laptop manufacture uses Top Balance. So why do you use Top Balance? It is a fair question. Could it be you do not know any other way? That it is just the way we always have been told and always done it that way?

So how do manufactures get around it with packaged 12, 24, and 48 volt batteries? Real simple they buy hundreds of LFP cells, match the capacities. Bottom Balance the cells, put them in a box with a +/- post for you to connect to, charge them up to storage voltage, and sell them to you.

So when you get your 12 volt 200 AH Lithium RV House Battery the instructions tell you in great big print to limit Charge Voltage to no higher than 14.2 volts, and never discharge them below 10 volts. Do that and your new LFP batteries will give you years of reliable service. No expensive monitoring or BMS needed.

Now if you want to use Top Balance BMS. You go right ahead and do it. In fact you should because that is all you know how to do.

[Sunking]

Yes, maybe with the way you treat LFP batteries it is far fetched. How about Tesla's 10 year warranty with 10 year extension, not sure what the fine print is here. I believe Bosch and Sony are talking about 10 years with their home LFP batteries.

You have no clue what you are talking about.

Tesla and Bosch do not use Chi-Com LFP batteries. No commercial manufacture uses Chi-Com cells. They do not use any form of Top Balance either. Tesla uses Panasonic LiCo cells in the Roadster all 6831 of them. They do not monitor any cell Voltages, they monitor 11 Sheets made from 9 bricks. Tesla battery is 11S 9B 69P. Similar design concept in the Nissan Leaf, Toyota Prius, and Chevy Volt, just fewer cells and modules to monitor. No commercial manufacture uses TOP BALANCE.

I understand why you are stuck on Top Balance. You are from Australia, you guys do not know any other way. Time to catch up to modern technology.

[Sunking]

So you are talking about keeping the weak cell at 3.4 volts for a long period of time, isn't high cell voltage bad for LFP batteries. No wonder you give a lifespan of only 5 years.

You really do not understand any thing about LFP batteries. 3.4 volts at rest is 85 to 90% SOC. Stress only occurs at 100% SOC of 3.47 volts and up at rest.

[catharsis]

There is a huge difference as you charts are showing you. In a Bottom Balanced system all batteries arrive at 0% capacity at the same time making it IMPOSSIBLE to over discharge any one cell. Look at the Lower Right Discharge Curves. That is what you are not catching or understanding.

The right answer is: There is NO difference of charge you can use. The weakest cell determines the charge of the battery whether the cells are top or bottom pre-balanced. But, if the cells are not pre-balanced, you can't get it all.

Perhaps you didn't notice the used cell voltage in my example was between 3.04 and 3.5 V. The TOP pre-balanced cells will reach their high charging voltage (3.5V) at the same time and the high voltage can be monitored and cut-off by the battery voltage. Discharging will require cell-level monitoring to cut-off and to prevent over discharging.

The BOTTOM pre-balanced cells will reach the low limit voltage at the same time. So this can be monitored as a pack (maybe just by noticing the lack of power). But, charging will call for cell monitoring to not overcharge the weakest cell. If you just use "safe" battery voltage the battery is never full (which can be good for the battery life but limits the amount of charge).

And above has nothing to do with balance boards, BMS or Golf cart.

ps. Actually, the top pre-balanced battery can hold slightly more energy in it because the average voltage is a bit higher.