LiFeP04 Batteries for Solar & BMS

[Sunking]

SK, In the "Bottom Balance" example how do we work out the voltage to charge to?

Depends on what the users is comfortable with and how much money you are willing to throw into Automation.

Most are minimalist. Initially perform a Bottom Balance by connecting all cells in parallel. and use something like a Hobby RC Charger or even just an old salvaged Power Resistors. Once they get down to 2.4 to 2.7 volt range, let the batteries set over night while still in parallel to get them all exactly equalized. Anything from 2.4 to 2.7 vpc is good enough as there is no energy left in that voltage range to drive current. It is just rested open circuit voltage.

From there connect the cells in series and charge. First initial charge you set the voltage set point to 90% SOC charge of xS. Example in a 48 volt system x = 16. You would set the voltage to 54.4 volts. If you are like me and most others will have a Coulomb Counter set to Amp Hours. Anyway you monitor cell voltages and Amp Hours. Once you get near set point voltage the weakest battery will have made itself known being the highest voltage of the batteries. Once the pack voltage reaches 54.4 volts and the batteries saturate, current will stop flowing which is a unique characteristic of lithium batteries. You measure the weak cell and see where you are in terms of Amp Hours and voltage. If rested voltage is a little high, you lower charge voltage a bit, and vice versa.

Only other thing you need to do is set your LVD voltage, and monitor things periodically with voltage checks of cell voltages. If something is going to go wrong, it is going to happen within the first month or two. Any battery manufacture defect is going to show up by then which does happen with Chi-Com cells. What you get is two usable calibrated duel gauges so to speak. Pack voltage actually means something and gives you a rough idea of SOC. Coulomb Counter's are pretty darn accurate and can be calibrated to about any unit you want. I use Amp Hours an don a 100 AH battery all it takes is a glance to know what I have in the tank at any time.

[Sunking]

How do you discharge to -5 Ah?

That is Reversal. Technically there is no Energy it is to demonstrate the stronger cells driving the depleted cell into Reverse Polarity. Lithium cells have a unique characteristic in that the further you discharge them, the lower their internal resistance goes. Batteries like lead acid do the exact opposite and not possible for stronger cells to drive a depleted battery into Reversal. You would not even know it happens until you either notice low voltage after you charge, or smoke and fire.

[karrak]

SK, In the "Bottom Balance" example how do we work out the voltage to charge to?

Simon

[catharsis]

How do you discharge to -5 Ah?

[Sunking]

Pictures Are Worth 1000 Words.

Perhaps this might enlighten some.

[Sunking]

What is the difference?

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.

[russ]

Repeat over and over - I smart fella, I fella smart

[karrak]

Winston is LFP aka LiFeP04.
When you use a Solar Panel with a conventional off the shelf charge controller, the charge controller never terminates the charge until darkness.

How did we get from the above statement 22 posts ago to the following statement with its clever play on words without you addressing the subject. The subject being that with many commercially available charge controllers you can program them to stop charging an LFP battery after the charging voltage has reached a programmed value and not put any more charge into the battery until the next day or until the battery voltage drops to a another programmable value. You are not a politician by any off-chance.

In context that is incorrect. The charger has not stopped providing power, it is still on and active like a wall socket in your home. The battery would be in higher energy state which only happens when you switch from Absorb to a lower Float voltage. That only happens with Pb batteries, what Charge Controller are designed to work with. As soon as the surface charge is absorbed or the load demands power the controller is then providing current. At no point does it ever turn off power except at night. Duh!

All commercial off the shelf Charge Controllers except for maybe GenSun, are made for Pb batteries.

There are a growing number of solar controllers on the market that can be programmed to charge either Pb batteries or LFP batteries.

Here is my explanation for how the terminology "Constant Current" has been adopted by some of the solar controller manufacturers to describe how their products charge a battery.

Before the advent of solar power most batteries were charged by current limited chargers connected to a generator, the power grid or whatever. This current limit was constant and so we get the term Constant Current Charging, this had been going on for over 50 years before solar controllers came along and everyone, both engineers and lay people got used to the term and it correctly described what was happening.

The solar controller manufacturers adopted the same terminology that had been used in the past that everyone understood, so they adopted the term Constant Current Charge even though from an engineering point of view it is not true, as the current from solar panels is not necessarily constant. For Lead batteries it probably doesn't matter whether or not the current is a fixed constant as any variation in the SOC achieved by the battery because of the differing charge current is balanced out by the Absorb phase of the charging. The term "Constant Current Charging" has stuck even though it is not strictly true.

Lead acid batteries should be charged in three stages, which are [1] constant-current charge, [2] topping charge and [3] float charge. The constant-current charge applies the bulk of the charge and takes up roughly half of the required charge time; the topping charge continues at a lower charge current and provides saturation, and the float charge compensates for the loss caused by self-discharge.

The above statement from Battery University has no reference to Solar Charging, I would think the context here is standard current limited chargers which are constant current.

The BULK stage involves about 80% of the recharge, wherein the charger current is held constant (in a constant current charger).

I interpret this as saying that the charge current is constant, but only with a constant current charger.

Bulk MPPT: This stage of the Classic means; that the Classic will be putting out as much current as it can trying to charge the batteries to the absorb voltage set point. This is also known as constant current mode

I do not see that this statement from the Classic 150 Owners Manual says the charge current is constant. To me this part of the sentence "the Classic will be putting out as much current as it can" is saying that the Classic will be putting out as much current as the solar panels will provide, it does not say that it is constant. The statement "This is also known as constant current mode" Is saying that it may generally be known by the target audience as the term "constant current mode". It is not saying that the controller is charging the battery with a constant current.

Simon

[catharsis]

Some graphs for the bottom/top balance debat

What is the difference?

Here the red cell is 95 Ah, the green cell 100 Ah and the blue cell 105 Ah. For a pre-balanced pack the charge you can use is always depending on the weakest cell.

LiFePo Top Balanced web.png

LiFePo Bottom Balanced web.png

LiFePo Not Balanced web.png

Just don't start without pre-balancing.

[Sunking]

You are right that Absorb, Float and EQ are Constant Voltage modes. If the battery voltage is greater than the voltages set for the same charging phase the controller will supply no current to the battery, no current means no power, i.e it will stop supplying power to the battery.

In context that is incorrect. The charger has not stopped providing power, it is still on and active like a wall socket in your home. The battery would be in higher energy state which only happens when you switch from Absorb to a lower Float voltage. That only happens with Pb batteries, what Charge Controller are designed to work with. As soon as the surface charge is absorbed or the load demands power the controller is then providing current. At no point does it ever turn off power except at night. Duh!

Bulk charge is not Constant Current, it is maximum charge that the panels will supply.

You need to look up the definition of Bulk Charge, it is a Constant Current mode.

You have just contradicted yourself, I thought you said earlier that Charge Controllers never stop supplying power.

No contradiction just your lack of understanding electronic and electrical principles. Lithium batteries are the only batteries with such a low self discharge that charge current can stop when both supply and battery voltage are equal. It takes a difference of potential for current to flow. First day electrical student knows this will when he/she learns Ohm's Law. In your analogy a battery does not supply power either, the power is there, you just have to use apply a load to make the current flow. If you have an inverter connected and turned on, current is flowing and some if not all of it is coming the panels assuming sun shine.

Go educate yourself and read up on 3-stage Pb Battery Charging using this link. All commercial off the shelf Charge Controllers except for maybe GenSun, are made for Pb batteries. It starts real clearly describing the 3-Stages every Charge Controller goes through and what each stage does.

Lead acid batteries should be charged in three stages, which are [1] constant-current charge, [2] topping charge and [3] float charge. The constant-current charge applies the bulk of the charge and takes up roughly half of the required charge time; the topping charge continues at a lower charge current and provides saturation, and the float charge compensates for the loss caused by self-discharge.

Don't like that link, try this one. It is from Midnite Solar Classic 150 Owners Manual. Do you think the manufactures do not know what they are talking about and you are the only one who knows better? Go to page 31 bottom of the page CHARGE STAGES AN MEANINGS, MPPT BULK it reads and I directly quote:

Bulk MPPT: This stage of the Classic means; that the Classic will be putting out as much current as it can trying to charge the batteries to the absorb voltage set point. This is also known as constant current mode.

Still not happy, well try this link another Tutorial.

The BULK stage involves about 80% of the recharge, wherein the charger current is held constant (in a constant current charger).

[karrak]

Simon you have proven again you do not understand Charge Controller or Lithium batteries. Charge controller never stop supplying power with sun light striking the panels. Absorb. Float, and EQ are all Constant Voltage Modes. Bulk is Constant Current. Look it up. You do not understand what they mean or do.

You are right that Absorb, Float and EQ are Constant Voltage modes. If the battery voltage is greater than the voltages set for the same charging phase the controller will supply no current to the battery, no current means no power, i.e it will stop supplying power to the battery. Bulk charge is not Constant Current, it is maximum charge that the panels will supply.

To charge a LFP battery as fast as possible with a off the shelf Charge Controller you set Bulk = Absorb = Float = Target SOC%. This forces the controller to operate in CC/CV mode. The CC will pump as much current into the batteries as the panels can generate. This is the Constant Current phase to charge as fast as possible. When the voltage reaches Set Point the CC then switches to Constant Voltage and the current will Taper Off to Zero Amps when battery voltage = charger controller voltage Lithium is the only battery that will stop charging when its battery voltage = supply voltage. All others have self discharge and thus never stop charging.

Once equalized as long as there is no load applied current will stop flowing, but the charge controller is still active siting in a FLOAT or Constant Voltage mode. If a load demands current, the panels will supply that power via Charge Controller assuming the load demand does not exceed what the panels can deliver. Works like any Float Service. The charge controller never shuts off until the sunset period end of story. The only question is did your batteries get fully charged up before sunsets

Sorry but you just clearly demonstrated you do not know what you are talking about. You do not know the basic principles of voltage sources, current sources, or even basic fundamentals electrical principles. You are not fooling anyone except yourself.

You have just contradicted yourself, I thought you said earlier that Charge Controllers never stop supplying power. I say again, yet again, Bulk charge is not Constant Current. I don't understand what you mean by the term "All others have self discharge and thus never stop charging."

Simon

[karrak]

You just lost all credibility here. Even a first year electrical student knows SOLAR PANELS ARE CURRENT SOURCES, not Voltage Sources like a battery or utility. Look it up. Hint: Isc/Imp and Bulk mode charge phase.

Well you should know then that the current output from a solar panel is proportional to the number of photons hitting it with the correct energy to push the electrons across the PN junctions in the solar cells. I am saying that the current from the panel will be dependent on the amount of sunlight hitting it which is not constant.

Solar panels are Constant Current Sources operating between Vmp and Isc meaning Constant Current at any operating point on the IV curve. Solar Panels are Voltage Sources operating between Vmp and Voc.

You got it wrong again, solar panels are current sources operating between 0 volts and Voc (Open Circuit voltage) with a maximum current at 0 volts of Isc (short circuit current) reducing slowly and nearly linearly to near Vmp (maximum power Voltage) then tapering off rapidly to 0 amps at Voc.

Simon

[Sunking]

That wasn't the context, he was answering you about Constant Current battery charging from solar panels.

Willy every charge controller made uses Constant Current Charging. That is the BULK mode. It is Constant, not fixed like a voltage source can supply.

Off the shelf controllers are not designed to charge Lithium Batteries. Some of them can be set up to work with them if you can set Bulk, Absorb, and Float to any value yyou want and set the to all be equal. That forces them to behave like a Float Charger with Current Limit. Set the voltage to something less than 100% SOC and the controller will be in a Constant Current Mode until the Controller Voltage and Battery Voltage are equal and all current stops flowing assuming there is no load. The Controller will sit there in Float mode acting as a Voltage Source so if a load demands current the power comes from the panels rather than the battery.

To take a Lithium battery to 100% SOC safely, there are two methods.

1. The charger and battery must be matched perfectly. You set the voltage to 3.64 vpc and current limit to as low as C/10 and as high as the manufacture sets limit to like 1C. So for a 12 volt battery like you have would be 3.65 volts x 4 = 14.6 volts. You apply that charge and initially the current is going to be whatever limit is set either from the panel wattage or regulator in the conventional charger say 50 amps on a 100 AH battery. 50 amps will until the battery voltage reaches say 14.5 volts and begin to taper off. You terminate when the current reaches .03C or C/33

2. The second method is a bit easier but much slower. Using a simple CC/CV Float Charger power supply. You set it up to 3.6 volts per cell. Any charge rate can be used within manufactures spec. So for you that would be 14.4 volts. Apply charge until current stops and terminate. Terminate means turn the dang charge off. If you let it float you will damage the batteries. There is the point you can work with using a Solar Charger. You set all the voltage to one setting below 100% like 3.4 volts per cell or 13.6 volts. When the batteries reach 13.6 volts charge current stops, but the Controller is now floating and will supply power to the load when the sun is shinning. Batteries will not supply power to load until the load demands exceeds panels capabilities or after sunset. Exactly what you want to happen. It is not that hard to understand if you know basic electrical fundamentals. If you do not understand you are hopelessly lost and only fooling yourself.

Now get out of my thread or I will nuke it.

[Willy T]

Solar panels are Constant Current Sources operating between Vmp and Isc meaning Constant Current at any operating point on the IV curve. Solar Panels are Voltage Sources operating between Vmp and Voc.

That wasn't the context, he was answering you about Constant Current battery charging from solar panels.

[Sunking]

Don't get all excited and wet your pants, he said they weren't Constant current sources.

Solar panels are Constant Current Sources operating between Vmp and Isc meaning Constant Current at any operating point on the IV curve. Solar Panels are Voltage Sources operating between Vmp and Voc.