Rebulk setting and no absorb - LiFePO4

[Cniemand]

@ ZONE

Exactly! That sunset is a failsafe of sorts. A cosmic reset. If we were running on hydro I probably wouldn't bother to have my FLOAT above the resting Cell voltage. LFP do not care whether they remain fully charged or at 50% all the time. With hydro always putting in something 24/7 It would make more sense to just leave FLOAT to 3.34vpc. With the occasional manually started REBUlK just to reset everything (SOC - WHizbang jr).

Also. If my system was one that sat unused for long periods of time I would either do a full battery disconnect to leave it or adjust the voltage setpoints lower than 3.34vpc so it never gradually creeps up as the CC would be a small draw on it anyway.

When you have access to the bank on a daily basis while utilizing the battery you do not come to a point where it ever starts to creep up in over-charging scenarios.

- Cloud

[lox]

I didn't manage to get it rebulk by putting a high load that drains the voltage lower than the rebulk voltage. It only triggers the FLOAT MPPT mode. The voltage has to be lower than the rebulk voltage for a certain amount of time?

But all in all it seems all good with float set to 27.2. With that setting the current to the battery (measured by the Whiz) stays between 0.1A, 0 and -0.1A. I guess that means that my 27.2V float setting is all right ?

[Cniemand]

@ Lox

I am not sure why the quote above says that most LFP users disable FLOAT. I certainly don't If you did disable it then you would end up in a position where instead of having a 100%SOC bank at sunset, it would be a bit lower depending upon your chosen REBULK setting. I want my pack to be fully charged so overnight I have access to 80% of it. Not just 60-70%.

If your float is showing around 0amps on the local app from the whizbang and your voltages are not creeping up then i'd say you have it about right. I am curious though. Your voltages have it a 3.41v each. TO me that seems like a slight charge. the resting voltage not under load is 3.34v. I have my FLOAT at 3.375. You use a BMS? (I do not) Those little boards to each cell? Is it possible that they are actually shunting around some energy to dissipate and leave them at 3.41? I thought some of those boards start shunting at either 3.4 or 3.6 depending on the board you chose?

[Westbranch]

I didn't manage to get it rebulk by putting a high load that drains the voltage lower than the rebulk voltage. It only triggers the FLOAT MPPT mode. The voltage has to be lower than the rebulk voltage for a certain amount of time?

Was that voltage measured with the load still attached or without the load?

I am thinking that the load was attached, and if that is correct, the batteries real voltage ,without load , would be higher and when the charger kicked in the panels were able to meet both the load and also to raise the battery voltage, so, naturally there would not be a ReBulk , only Float or Float Mppt..

Battery voltages that we talk about are not under load voltages but, unloaded & at rest voltages.
At rest means after 3 or more hours of no charging or load applied... ie a stable voltage
hth

[lox]

I have a BMS. I bought my battery pack from a Chinese factory and they made the BMS. I have one board for each row of cells (2 rows of 8 cells in series, then the two rows are in parallel)

One of the equalizing PCB failed so I disconnected one row until I solve the issue with the manufacturer. Those cells are now resting at 3.41V ...
But yes I can confirm that setting my FLOAT voltage at 27.2V, the current to the battery, under normal load, is never more that 0.1A and never less than -0.1A once it stabilizes (takes a few minutes to stabilize) and SOC stays at 100%.

Thanks for your worthy advices buds.

I didn't plug the battery temp sensor. Should I on LFP batteries ?

About working without a BMS, do you have to manually re-balance your cells often ? How do you bottom balance them ? I mean, how to discharge them to the same voltage. Is any special equipment required ?

[ClassicCrazy]

I am getting some LiFePo4 soon and I am also wondering about temperature compensation for them .

Larry

[Cniemand]

@ Larry and LOX

You do not need to Temperature compensate LFP batteries. I recently plugged in the BTS only for the purpose of having an ambient temperature reading on the Local App of sorts. It really doesn't modify the charging parameters at all.

@ LOX

Sorry to hear your PCB failed. Paying money for something... it should work. I hope they resolve the issue. One of my beliefs in regards to active BMS units is that they have a high probability of failure and when they do you take out half the pack.

"Bottom balancing" is pretty straight forward.

1. Pull each cell down to 2.75v (I used a giant resistor and heatsink in order to pull out energy at a 30amp rate for the 100aH cells)
2. After pulling a cell down to 2.75v the first time, let them recover over night.
3. They bounce back up in voltage. Pull them back down to 2.75v. let them rest for 30 minutes to recover.
4. repeat and let recover 30 minutes until they stabilize at around 2.75 (within a few millivolts of each other.)
5. When each cell is at 2.75 (within a few mV of each other. They are 100% discharged so balanced amongst each other) String the pack in series and charge up to 3.5v each ( 3.5v *16 cells = 56v)
6. When your CC hits the 5 ENDAMPS they are all full and balances with themselves. They may be off a few mV of each other. They settle down in rest.

I have a video explaining more about it and showing how I did it. Its connected to the little world icon under my screen name.

[zoneblue]

Rebulk on solar is pretty much pointless. On hydro it would make more sense, because there is no "day", and rebulk would allow you to create one.  As to what would happen if you set Vfloat to 10V on a 24V bank, well id be surprised if that worked, really i would, as the controller needs a voltage to regulate the loads at. Sure couldnt do that at 10V. AND, theres been a thread here recently where the controller does some pretty interesting things if the battery is held at a higher voltage than the controller. The controller seems to dump power into the PV for a period until it drops to sleep mode. ON this matter i beleive that 2079 alters the timing of that so exactly what occurs would need to be compared to the actual firmware you are using. Bob might chime in and clarify there, but for now your approach to choosnig a float voltage appears to have done the job, and with Tcomp disabled you should be away laughing.

[lox]

I ended up buying an awesome little and cheap BMS, it is called MiniBMS (the HousePower version). It took me so long to find such a great product that I wanted to share it with you guys. That BMS really is cheap and very clever, which makes it safe and efficient.

So I tuned my Midnite Classic to an absorb setting of 28.5V so the top balancing of the cells is properly done. It has been running on my LiFEPO4 cells for months and not single issue so far (I produce/use between 3 and 4Kwh daily)