I'm curious to here others expert opinions on this. I'm paging
@Steve_S for his wisdom, I have found his information on cell voltages extremely helpful. (sorry to the other experts whose names I can't remember)
As long as you hit a full charge a few times a week, daily better, the coulomb counter in the BMS will reset.
In my opinion those voltages are rather aggressive and could cause more harm than good. There is no need to float them that high in the charge curve.
I'm running with a 55.2V bulk, holding for 30 minutes and floating at 54.8V.
I have a feeling you will be triggering over voltage protection with the recommended voltages, being user defined the BMS has no input to throttle charge current at xx.xV.
FIRST, I will say that I have not been following this thread but read a bit of the current talk. My response therefore will cover more stuff and some obvious Wives Tales / Postulation & Theorization.
Working LFP batteries have absolutely NO PROBLEM with sitting at 3.400 Volts per cell (Official 100% SOC) as LFP is designed to do just that. With ESS systems, the Batteries never actually sit at 100% for very long (during the daytime maybe when fully charged, till the sun goes down). Even if they were at 100% SOC (3.4Volts per cell) in a Part-Time home and not being used partially (small loads), they would sit happily without issues.
Next key point that FAR TOO MANY MISS !
LFP should NOT be charged too far above the Working Voltage Range (3.000-3.400 Volts Per Cell), yet people do it and THAT is NOT GOOD !
Bulk/Absorb should be set to 3.450Vpc (13.8 / 27.8 / 55.2) (Variable Voltage / Constant Current)
Float should be set to between 3.425-3.410 Vpc (13.7-13.6 / 27.4-27.2 / 54.8-54.4) (Constant Voltage / Variable Current)
Absorb can last up to 1 Hour - this allows for the internal balancing to be complete. It will stop as soon as EndAmps/TailCurrent is reached.
Transition to Float from Absorb MUST occur when the battery packs reach EndAmps/TailCurrent !
EndAmps / Tailcurrent is calculated as follows: 100AH Battery X 0.05 = 5A. 280AH X 0.05 = 14A.
Whether you have 1 or 5 100AH Batteries in Parallel the EndAmp value is still 5A.
* Not all equipment has an EndAmps/Tailcurrent setting. Vctron, Midnite Solar and other Tier-1 products do.
Low Voltage Disconnect ! (another area of confusion)
Simply PUT. Never EVER set the Disconnect too low ! It only takes 1 cell to be below the disconnect voltage point to prevent a BMS from starting up & taking a charge ! That low voltage (even above 2.50) is not healthy for the cells. Low Voltage Disconnect should never be below 2.650, so as to allow enough to remain keeping the BMS in Standby mode to receive charge. Additionally, IF the batteries are already low, you need to keep enough room if a heavy load is applied, the cells will sag for a few moments and you do not want an instant cutoff. The IDEAL Low Volt Disconnect should be set no lower than 2.700Vpc (10.8 / 21.6 / 43.2) I set my systems to 2.8Vpc LVD.
!! CHARGING NOTE !!
LFP has a Working Voltage Range of 3.000-3.400 while the Allowable Voltage range (that does not cause harm) is from 2.500-3.650.
Cells are tested & validated to deliver their specified Amp Hours from the WORKING RANGE ! (hence why 3.200V is Nominal for LFP)
When charging above 3.450, the cells can & often will "run" which means that 1 or more cells may reach higher voltage than their neighbours, this is often the cause for HVD/OVD Disconnects when any cell reach above the High Volt Disconnect setting in the BMS (usually 3.650). This can create a large internal imbalance and affect performance of the entire pack.
FLOAT NOTES ! (lots of BS on this, straight poop here - no theories/postulation)
Float DOES NOT HARM BATTERIES IN ANY WAY !!! The Constant Voltage is kept going to the batteries allowing for internal balancing & topping off. Batteries will NOT take more than their Internal Resistance will allow and you will see Float Energy going to batteries drop to 0A once the batteries are full. FLOAT will continue to provide solar power to service any loads the Inverter demands and will provide all it can without pulling from the battery bank WHILE Solar Power Input is available. IF there is more demand than it can provide, the balance gets pulled from the batteries, once the load is done, then Float will replace what was taken from the batteries provided there is enough solar input.
NOTE that even with Float,, the batteries will take a bit more over the Working Voltage range of 3.400 Vpc (why we set to 3.425-3.410 Vpc) because the moment Input power stops (sundown) the LFP will settle down (normal & natural for the chemistry) and the cells within the battery pack will drop to between 3.390-3.405 depending on the saturation.
CRITICAL INFO VENDORS DON'T MENTION !
Systems MUST be Calibrated & Voltage Corrected/Compensated.
There are ALWAYS losses on the wire from SCC / Inverter & the Battery Bank. A Simple FACT ! Every connection, lug, fuse/breaker etc ALL add resistance & voltage losses. While it may not seem like a lot and in the Lead Acid battery days it did not matter that much because FLA is "Brute Force" technology. ALL LITHIUM CHEMISTRIES and NOT Brute Force Tech, these ARE Millivolt & Milliohm sensitive and even a 0.100 Volt difference CAN muck stuff up (Think Hi / Lo Volt disconnects and more) as we are charging to a Fine Resolution. IE 54.8-54.4) which is a 0.400 difference ! Does not seem like a lot BUT for Lithium Cells it IS !
You MUST look at the Voltage at the Solar Controller Terminals, the Inverter Terminals and the Battery Terminals and note the difference (there will be some). The Battery Packs themselves are the BOSS when it comes to Voltage.
If the Battery pack(s) is 50.0V and SCC is charging and reads as 49.5V you have a 0.500 Volt differential to Correct for. Then look at the inverter and it may be reading 49.25V then it needs to be corrected for the 0.750V difference.
The GOTCHAS ! There is a BIAS issue here as well. When a SCC or Inverter/Charger are CHARGING the batteries, the voltages at the terminals of the SCC or Charger will read higher as that is input power. That is Charging Bias.
- When there is no solar power coming from the SCC the bias will make it read a lower voltage (normal) but you have to account for that when reading the displays/software.
- When the Inverter is Pulling from the batteries it will also be biased as well ad will read the voltages as a bit lower. THE GOTCHA is that it may disconnect before the actual voltage hits Low Volt Disconnect.
The Key, is to correct the SCC so it is 100% corrected WHEN IT IS CHARGING so that is never goes over the settings for the batteries.
The Inverter must be corrected so that it is reading the correct voltage during discharge, to prevent early / late Low Volt Disconnects.
Tier-1 Equipment do have OFFSET settings in their programming, some 2nd tier products do as well while Value grade generally do not.
I will not argue these points / issues there is no point... If you want to go ahead but I'm not interested... I have repeated this info NUMEROUS TIMES and tired of it. I am an ENGINEER (3 degrees) and doing fine electronics, data systems then into renewables for Many Years ! I build batteries and complete systems and have quite a bit out there and NO I do not mention my business ventures in here, I'm busy enough. I've written guides, docs for public consumption as well as a few BMS Manuals and work with BMS Manufacturers as we as Inverter/Solar systems manufacturers directly. It is also a bit of a bugger because I have several Non-Disclosure Agreements in place so I am restricted with the info I can provide in some cases.
Bottom Line
Use your Critical Thinking and apply conservative rationale and keep your system settings reasonable & polite to not suffer issues & problems.
Hope it helps, Good Luck.
