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Out in the boonies, need LiFePO4 settings

HRTKD

Boondocker
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I'm out in the boonies in my RV and I think that my Victron components aren't setup right. The BMV-712 is thinking that 13.2 is 100% SOC. It used to be synchronized after I did the top balance, so I don't know why it's off now. The 100/50 SCC keeps resetting the "Battery preset" to user defined instead of LiFePO4. When I change it to LiFePO4 it then has an "Absorption voltage" of 14.20v and "Float voltage" of 13.50v. I'm using the app on my phone for all these settings.

The forecast says I'm going to get snow tomorrow and really cold temps a couple days after. So I need to make sure my batteries fully charged.

I had to drive halfway into town to post this. It's a really weak signal. All my specs sheets for my EVE 3.2v 280Ah cells and the Victron equipment are on my desktop PC at home. I'm trying to download the manuals but not making much progress. The forum requires far less bandwidth.

Thanks in advance for any assistance you can provide.
 
Had the occasional issue with it not accepting changes. Resolved either by turning off "Charger enabled" while making the changes, or by a power cycle (charger disabled + panel disconnect + battery disconnect):

1603383645125.png


Make sure your BMV is set to 0.2V below your absorption voltage and not your float voltage.
 
Actually, I think you want user defined, don't you? If you change to LFP, you're set for their defaults. If you change them, it reverts to user defined.

IMHO, 14.6V and 13.6V.

BMV set to 14.4V and 5%
 
Damn... with the victron connect update, now I have to remotely update my charge controller... skeered...
 

Charging an LFP Battery​


Most regular solar charge controllers have no trouble charging lithium-ion batteries. The Voltages needed are very similar to those used for AGM batteries (a type of sealed lead-acid battery). The BMS helps too, in making sure the battery cells see the right Voltage, do not get overcharged, or overly-discharged, it balances the cells, and ensures the cell temperature is within reason while they are being charged.
The graph below shows a typical profile of a LiFePO4 battery getting charged. To make it easier to read the Voltages have been converted to what a 12 Volt LFP battery pack would see (4x the single-cell Voltage).
LiFePO4 Charge Voltage vs. SOC

LiFePO4 Charge Voltage vs. SOC
Shown in the graph is a charge rate of 0.5C, or half of the Ah capacity, in other words for a 100Ah battery this would be a charge rate of 50 Amp. The charge Voltage (in red) will not really change much for higher or lower charge rates (in blue), LFP batteries have a very flat Voltage curve.
Lithium-ion batteries are charged in two stages: First the current is kept constant, or with solar PV that generally means that we try and send as much current into the batteries as available from the sun. The Voltage will slowly rise during this time, until it reaches the ‘absorb’ Voltage, 14.6V in the graph above. Once absorb is reached the battery is about 90% full, and to fill it the rest of the way the Voltage is kept constant while the current slowly tapers off. Once the current drops to around 5% – 10% of the Ah rating of the battery it is at 100% State-Of-Charge.
In many ways a lithium-ion battery is easier to charge than a lead-acid battery: As long as the charge Voltage is high enough to move ions it charges. Lithium-ion batteries do not care if they are not fully 100% charged, in fact they last longer if they are not. There is no sulphating, there is no equalizing, the absorb time does not really matter, you cannot really overcharge the battery, and the BMS takes care of keeping things within reasonable boundaries.
Charge Voltage Needed
So what Voltage is enough to get those ions moving? A little experimenting shows that 13.6 Volt (3.4V per cell) is the cut-off point; below that very little happens, while above that the battery will get at least 95% full given enough time. At 14.0 Volt (3.5V per cell) the battery easily charges up to 95+ percent with a few hours absorb time and for all intents and purposes there is little difference in charging between 14.0 or higher Voltages, things just happen a little faster at 14.2 Volt and above.
Lithium-ion cell structure
Bulk/Absorb Voltage
To summarize this, a bulk/absorb setting between 14.2 and 14.6 Volt will work great for LiFePO4! Lower is possible too, down to about 14.0 Volt, with the help of some absorb time. Slightly higher Voltages are possible, the BMS for most batteries will allow around 14.8 – 15.0 Volt before disconnecting the battery. There is no benefit to a higher Voltage though, and more risk of getting cut of by the BMS, and possibly damage.
Float Voltage
LFP batteries do not need to be floated. Charge controllers have this because lead-acid batteries have such a high rate of self-discharge that it makes sense to keep trickling in more charge to keep them happy. For lithium-ion batteries it is not great if the battery constantly sits at a high State-Of-Charge, so if your charge controller cannot disable float, just set it to a low enough Voltage that no actual charging will happen. Any Voltage of 13.6 Volt or less will do.
Equalize Voltage
With charge Voltages over 14.6 Volt actively discouraged, it should be clear that no equalize should be done to a lithium-ion battery! If equalize cannot be disabled, set it to 14.6V or less, so it becomes just a regular absorb charge cycle.
Absorb Time
There is a lot to be said for simply setting the absorb Voltage to 14.4V or 14.6V, and then just stop charging once the battery reaches that Voltage! In short, zero (or a short) absorb time. At that point your battery will be around 90% full. LiFePO4 batteries will be happier in the long run when they do not sit at 100% SOC for too long, so this practice will extend battery life. If you absolutely have to have 100% SOC in your battery then absorb will do that! Officially this is reached when the charge current drops to 5% – 10% of the Ah rating of the battery, so 5 – 10 Amp for a 100Ah battery. If you cannot stop absorb based on current, then set absorb time to about 2 hours and call it a day.
Temperature Compensation
LiFePO4 batteries do not need temperature compensation! Please switch this off in your charge controller, or your charge Voltage will be wildly off when it is very warm or cold.
Be sure to check your charge controller Voltage settings against those actually measured with a good quality digital multi-meter! Small changes in Voltage can have a big impact when charging a lithium-ion battery! Change the charge settings accordingly!

fROM: https://www.mobile-solarpower.com/diy-lifepo4-solar-battery.html

My Recommended Charge Profile Parameters
That Will Work With All LiFePO4 Batteries (Highly Recommended)​

My Updated Charge Profile for LiFePO4:
  • Charge Limit Voltage (Absorption):
    14.2 volts for 12 volt battery, 28.4 volts for 24 volt battery
  • Float Voltage:
    13.5 volts for 12 volt battery, 27 volts for 24 volt battery
  • Set Equalize Time to: Disabled or 0
  • Set Temperature Compensation Coefficient to: 0 (usually its -3)
  • Low temp cut-off: 5 degrees C
On the right is what your settings should look like if you are using a victron charge controller with a temp sensor.

Recommended Discharge Profile:
  • Use a battery protect by Victron Energy and set it to option "8", which will disconnect at your loads at 12 volts for 12v batteries, and 24 volts for 24v batteries. That's it! Be sure to bottom balance your cells at the cut off voltage so that the battery protect can do its job. If you do not bottom balance and you use a battery protect, one of the cells may be out of balance and damage itself.

If you are using your LiFePO4 battery for solar application only, and the cells came from a good seller, you will not need to balance for years. But you need to keep an eye on the balance. If the cells charge and discharge together nicely, charging from 0%-100% is possible. I like to stay on the safe side though, and charge between 10%-90% (and use an active balancer).

If you have used/mismatched cells that go out of balance:

When cells are not matched by internal resistance and capacity (which should be done by the seller at time of purchase), the cells will go out of balance easily. If you are using used cells that are not matched by the seller, what you need to do is charge and discharge between 20% and 80% and use an active balancer/BMS. This will prevent over charging or discharging individual cells on accident. It will be important to watch the cell voltage at high and low states of charge to ensure that none of the cells are different than the others.



LiFePO4 State Of Charge, Voltage Chart​

lifepo4 has a strange charge/discharge curve, and it can be very difficult to estimate what the current capacity is based on voltage alone.

Understand that if you wish to use the voltage chart to the right, you need to disconnect all loads/chargers that are connected to the battery for an accurate estimation of capacity. You may need to let the cells rest for a few hours and then check the voltage to determine the state of charge.

If you need a battery capacity monitor, check out what I recommend by clicking here
Picture
 
@snoobler, when I calculate down to 12v from your 48v, I get an absorption value of 14.82. That can't be right. I wouldn't expect it to be any higher than 14.6 for a 4s battery. Maybe you have a different battery setup?

To be on the safe side, I'm going to take the Victron defaults of 14.2 absorb and 13.5 float. Once I see the batteries at 14.2, I'll set the BMV to synchronize the SOC to 100%.

With clear skies (maybe a little haze from forest fire smoke), I'm generating 550 watts (portable ground, tilted) and 350 watts (rooftop, flat). I'm charging my laptop and running a 500 watt space heater. Life is good.
 
If their is voltage drop between the charge source and the battery one solution is to factor in the offset on the charge controller. I thought the victron gear would make that unnecessary though.
 
@snoobler, when I calculate down to 12v from your 48v, I get an absorption value of 14.82. That can't be right. I wouldn't expect it to be any higher than 14.6 for a 4s battery. Maybe you have a different battery setup?

To be on the safe side, I'm going to take the Victron defaults of 14.2 absorb and 13.5 float. Once I see the batteries at 14.2, I'll set the BMV to synchronize the SOC to 100%.

With clear skies (maybe a little haze from forest fire smoke), I'm generating 550 watts (portable ground, tilted) and 350 watts (rooftop, flat). I'm charging my laptop and running a 500 watt space heater. Life is good.

Again, 14.6 and 13.6. Sorry for the confusion.

I wouldn't set 100% sync to full until absorption ends.

Your tail current is important to both the SCC and the BMV. In the BMV, it's expressed as a % of capacity. 5% in the case of LFP is fully charged at 3.65V.

In the SCC, you need to set the actual current in Expert Mode. For 280Ah 12V, the tail current should be 14A, i.e., when the battery is at 14.6V and at or below 14A, it is truly charged and float mode should be entered.
 
smoothJoey, the distance between the charge controller and the batteries isn't much. Maybe 25" of total wire, one way. 6 gauge from the charge controller to the circuit breaker and then to the bus bar, 2/0 from the bus bar to the BMS (with steps in between for shunts, fuse and switch) and then three 8 gauge wires from the BMS to the battery.

I don't have access to the solar charge controller's output ports to put a voltmeter on. The victron app shows a voltage that is within .01 of the voltage that the BMS is reporting.

snoobler, thanks for the clarification. I'm going to stick with 14.2 on the charge controller for now. I have things to do and it's a safe setting. Later I may split the difference and bump it to 14.4.
 
The goal was for max capacity, so that's 14.6V. 14.2V or 3.55V/cell is close to full charge, but you probably want to lower your tail current to 3A to ensure max charge at that voltage.
 
I've been using 14.1v and at times 14.2v since 2016 never seen a reason to go any higher, unless maybe the bms needs it for balancing.
 
The settings I used worked fine. The whole system worked great! After a few days, I stopped checking the charge level. Having gobs of power is such a different experience.
Agree, very nice to have the extra power.

Greg
 
I turned on a 500 watt space heater at about 10 am to keep the trailer warm without using the propane furnace. I didn't get back to the trailer until well after dark. Even then, the state of charge only went down to 70%. If I had done that to my Trojan FLA battery bank it would be dead, dead, dead.
 
There is no comparison to dead lead. Once you get the feel for the system it will bring a smile to the face everyday. The recharging time is also a nice benifit and if it doesn't get recharged full no big deal, welcome to the living in a psoc life. Without to much fear of the system shutting down. :cool:
 
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