I think with the more and more widespread usage of lithium batteries, where a daily full recharge will shorten the lifespan of the battery, there should be an opion to prevent this case.
I personally would appreciate such an option very much as my system is offgrid and used just a few times a year or even just once every two years. So at the moment to prolong my battery lifespan, each time i arrive or leave, i have to disconnect or connect the controller manually. And just turn of the charge function doesnt solve my problem as the controller still draws current, yes very little but it might be enough to run my battery empty after 2 years from a SoC of approx 60%, like a battery should be stored. I use smartsolar mppt 75/15.
I can imagine doing having a solution for my case in these ways in the settings of the charge controller:
on/off option for daily re-bulk. The battery then would float as long as the re-bulk offset voltage will be reached and fully recharge again.
re-bulk timer in days. For example set the timer to 180 days, so the re-bulk starts 180 days after the last charge.
a combination of both. So re-bulking as soon as voltage drops to re-bulk voltage offset, or if the last charge cycle exceeded 180days, whatever comes first. (Like the solution in absorbtion with combination of fixed time and tail current, whatever happens first).
For those who go into detail: yes with lifepo4 the voltage is generally a bad indicator for the SoC, but at least at SoC about 20% and down it is an easily accessible useful information for the SoC to prevent a battery from deep discharge.
There are already some other posts like mine, not many, but i can imagine that the requests for this feature will rise as there will be more & more lithium batteries in such systems.
Hope to find this feature in the near future on my victron app to slightly improove an already great product!
If the Battery voltage parameters are correctly set, this should not be needed.
With lithium batteries, the float voltage to maintain capacity should be set close below the absorption voltage - typically just 100mV or so. If the battery will be normally at maximum charge for extended periods, then the absorption voltage should be trimmed down - typically reducing this to 3.45 - 3.5V per cell from the max of 3.65V per cell for LiFePO4.
Could you please elaborate a little on the above statement?
Because I specifically asked this at Pylontech and Pylontech technical department is saying that there is no problem whatsoever recharging their batteries daily to 100% with a 52.5V voltage. That’s 3.5V per cell.
If you set the voltage for Pylontechs to 52.5V, the batteries are charging to 100% and once they hit that voltage and after balancing, they enter idle mode drawing no more than 30-40mA, the current needed for BMS electronics.
And considering that 3.65V is absolute maximum for LiFePO4, the 3.5V margin is for sure that safety margin that all are talking about when they are saying to not charge fully. So no need for additional safety margins.
I assume you have a GX?
With node red you could just set it to on or off based on a time or date.
Cap the voltage in DVCC so it doesn’t fully recharge. Or set the rebulk offset to way low down. So it only triggers rebulk later? Then set the bulk/abs to not full voltage.
What you might find though the bms will have its protection trigger and switch off with no or low load
Hi Alex, thanks for your input. I could send you many links with that hint to not fully charge a lifepo4 to 100% daily, but i think in these days ai-based (like ms copilot) feedbacks sum i up pretty well:
You
Sent by you:
is it bad to charge a lifepo4 daily to 100%
Copilot
Sent by Copilot:
Charging a LiFePO4 battery to 100% daily isn’t necessarily bad, but it’s not the best practice for maximizing the battery’s lifespan. Here are a few key points to consider:
Battery Lifespan: Regularly charging to 100% can lead to faster degradation of the battery. [It’s generally recommended to keep the charge level between 20% and 90% for everyday use]
[Cell Balancing: Occasionally charging to 100% can be beneficial for cell balancing, ensuring all cells within the battery pack are at the same voltage level]
[Thermal Management: Overcharging can cause excessive heat buildup, which can be detrimental to the battery’s health and safety]
[Battery Management Systems (BMS): Modern LiFePO4 batteries come with sophisticated BMS that manage the charging process efficiently, but frequent full charges can still impact the overall health of the battery over time]
For optimal performance and longevity, it’s best to maintain a charge range of 20% to 90% and only charge to 100% occasionally for cell balancing.
Furthermore the optimal soc for storing lifepo4 is around 50%. In my case it comes close to this storing use case because i use my battery just a few times a year.
You
Sent by you:
at what soc should i store my lifepo4
Copilot
Sent by Copilot:
[For optimal storage of your LiFePO4 battery, it’s recommended to maintain a state of charge (SOC) of around 50%]
[If you plan to store the battery for an extended period, it’s a good idea to cycle the battery (charge and discharge) every six months to maintain its health]
I’ve already set my absorbion voltage to 3.5v per cell, probably i’ll go down to 3.45v as this seems the only option so far to maintain the battery well in my case…
Hi Mike, see my answer to alex’s input. I guess it’s not very common yet to not rebulk every day, but still what i know so far it seems benefical to do so with lifepo4…
The thing is most bms don’t allow the cells to be charged the whole way. So that is already managed in the correct range. No need for an end user to do so.
And no, don’t take AI generated stuff as content to rely on. There is alot of old internet content that is more myth than good practice. Thinking and tech have changed alot.
Pylontec are the ones that i know of and personally used, who have a recipie for storage. They want it stored at 50% in a certain temperature, charged to at least 90% once every 6 months. They have a known self discharge amount as well.
You also get calendar ageing as well. So not using it has known degradation as well.
If the battery is passively balanced chances are it needs to hit it’s charged voltage to do so.
Set the rebulk offset so it allows a voltage drop before starting a new cycle if you don’t want a rebulk everyday.
The only other option (besides adding a pi or a GX) is to completely disconnect it and leave it in storage at a certain percentage. Factor in the self discharge.
storage of LiFePO4: this is usually between 25% and 50% soc, internal discharge is very low, cells can be stored for > 1 year in open circuit condition.
To re-bulk every day?
If you look at the cell voltage vs SOC curve for LiFePO4 batteries, the curve is essentially very flat from about 10% soc to 90% soc, then curves steeply. As the batteries age, the flat portion of the curve acquires more slope. However, at the high end, as the voltage starts to increase rapidly, so does the internal corrosion rate of the cell. Keeping the battery at high voltage for extended periods does shorten the life. Similarly, at low voltages (<2.5V) real nasty things happen to the cells, and they become inoperative. I’ve lost more cells from low voltage then the occasional overcharge.(I use a lot of 3.6Ah cells). In my view, terminating the charge at 3.55V and floating at this value for 3 -4 hours a day is unlikely to adversely affect the life of the cells. My packs (10kWh total) cycle about 50% capacity daily, and are now between 1500 and 2000 cycles.
3.55V per cell is 53.25V per pack → Exactly what BMS is broadcasting, CVL: 53.2V
And another observation on my system:
If CVL is set to 52.5V (3.50V), it takes about 25 minutes from raise from 98% to 100%.
If CVL is set to 52.8V (3.52V), it takes about 5 minutes from raise from 98% to 100%.
So this I believe is proof enough that the 52.5V is near the lowest voltage the battery likes.
Exactly what Pylontech says in their docs: charge with voltages from 52.5V (3.50V) to 53.5V (3.57V).
PS: I keep saying the voltage per cell, because I don’t know if the OP has an 15S or 16S.
I actually own an old lfp battery from the year 2015, didnt got any specs beside max charge voltage, max charge amps, max disc amps. But i intend to buy a new battery with cells i mentioned above and probably jk bms.
“The thing is most bms don’t allow the cells to be charged the whole way. So that is already managed in the correct range. No need for an end user to do so”
I dont really understand how this would happen with a jk bms or any other basic bms (pylontech probably excluded in this case…). What i see on the charge controller is what i also expect to happen to cells itself. I cannot imagine that these kinds of bms just “burn” some energy because it was configured like that on the bms. Unless its an overvoltage-case where bms would disconnect the cells, but then i would also see that on the charge controller to immediately stop charging. Or do you have another opinion?
So in conclusion i still think a simple solution to configure daily bulk (like mentioned in my first post) is important in combination with lfp batteries… do you agree?
Thanks a lot for that input. My actual old lfp battery from 2015 has exactly that rather steep curve compared to what to expect from lfp. So that explains a lot
@alexpescaru yea i’ve seena post where they compare charging time with different charge voltage, from 3.45, 3.55, 3.65 volts.
I dont remember in detail what else to consider when lowering the charge voltage to a deep level like 3.45v. but in your case with pylontech batteries i personally would stay in the range of manufacturers recommendation to prevent warranty issues. Eve doesnt provide min charge voltage so there is more space for playing around…
And in my case it is not really about absorbtion voltage but more about daily bulk, so dont wanna dig deeper in that topic…
In your case, leaving the battery alone for 1 or 2 years, it’s quite a particular case…
Question:
It will be OK for you if the battery voltage, after charging, will remain to a certain value, the set bulk voltage?
In this way the voltage will not drop and the rebulk will not be performed.
But would you be satisfied with this?
The simplest is to change the bulk voltage maybe and the rebulk offset.
But a battery that is cycling with a load on e it (all be it slowly) is not in storage. Definition of storage is actually not connected.
The JK comes with the caveat if you don’t know what you are doing its your responsibility/problem… A pre programmed battery is a manufacturer responsibility.
The SOC will be tuned by a careful choice of voltage. For example 50.4V for 15S or 53.8V for 16S. About 3.36V.
That way the battery will not reach 100% and the charging current CCL will never drop to 0 and the system will remain in that “unfinished” state indefinitively, keep trying to charge to full.
I know, it’s not an elegant way, but will solve your situation. Just need to fine tune a little that voltage.
And set bulk, absorption, float, all to the same value.
two Alex’s here.
