Here's my "boilerplate" summary for LFP care advice, mostly from marine electrics discussion forums involving long-term users and professionals, with special thanks to Maine Sail, Bruce @ OceanPlanet, tanglewood and nebster (see below).
Note that most people talking "lithium" or "lithium-ion" batteries, mean completely different chemistries, not nearly as safe or long-lived as LFP, and with completely different setpoints and behaviors. Especially those coming from EV or RC contexts.
_____
Let me state from the outset, I am committed to "the cause" of extending lifetime cycles far past manufacturer specs, while still drawing down to high DoD when needed, rather than doing so by oversizing the bank.
I at least want to **know** as much as possible about how to get optimum lifespan; I think well over 10,000 cycles is achievable, unless calendar life issues, like ambient temperature, start interfering with the life-cycling factors.
I may not actually want or be able in all use cases to **implement** the necessary requirements, but I want to "not-do" so consciously, not out of ignorance.
If manufacturer spec'd lifetime cycles is sufficient for you, then charge to their spec, for 4S Absorb voltage 14.6V, holding to endAmps of .005C, or whatever you think best.
In which case, there is also not much point in your engaging in these discussions further just to say "y'all are fussing too much" just because you don't share the goal of optimizing for longevity.
_____
Any and all feedback is welcome, especially if more "canonical" information, e.g. from the links cited below, conflict with this text.
______
Systems: OPE-Li3 (Lithionics/Ocean Planet), Victron, MasterVolt, Redarc (Oz specific?)
Bare cells: Winston/Thundersky/Voltronix, CALB, GBS, Sinopoly and A123 (now Lithium Werks / Valence / Super B)
Best to size your cells for two parallel strings for redundancy, unless you have a separate reserve/backup bank. Don't go past three such strings, or you may see balancing issues that affect long-term longevity, but if you really need to, maybe four strings in a pinch.
_______
Every cell vendor, also those selling ancillary hardware touted as "LFP ready", gives charge-termination voltage / endAmps profiles **way too high** for those striving for maximum longevity.
How has this come to be?
First off, recognize the cell manufacturers are huge billion-dollar companies, and all their focus is on their main customers, first the Chinese military, and secondly EV manufacturers. These have their own PhDs in battery science and DC power, and priorities are high-C-rate discharge for propulsion and range / per-use cycle capacity - expected lifespans are relatively short in those use cases, and optimizing for longevity is just not high on their list of priorities.
The purchase of LFP cells directly by consumers for use as "House bank" storage in a mobile context and for solar storage is at most .0001% of the cell manufacturers' sales, not even on their radar in economic terms, even as a niche sub-market, .
It is for these same reasons that industry and academic research is not targeted toward maximizing longevity, but oriented around those EV & other high C-rate use cases, so **very** different from our much gentler House bank cycling.
And thus no testing / different specs are developed by industry for the House bank use case; that has been left to the end users, and the more objective/technical vendors like MaineSail.
The specs laid out by cell manufacturer are **absolute maximum** ratings, the limits to which the cells can be subjected for short times, without causing immediate irreparable damage. These are "stress ratings" only, and not recommended for normal functional operating conditions. As with all electronics, regularly operating at these maximum rating conditions for extended periods may reduce longevity and reliability.
The auto industry could easily have developed utilitarian cars and parts supply chains, so that their customers could have the option of buying vehicles that routinely last 50-100 years.
The free market gives them, as rational players, no incentive to do so. Just as there is no incentive to study or teach customers how to double or triple the lifespan of their LFP banks.
Not talking any "tin foil hat" conspiracy here, just an understanding of basic economics.
So, following those vendor charge specs is fine **if** you only want the cycle lifetimes they advertise. You can however get **much** longer life by looking at the vendor's SoC vs Voltage chart, and "avoiding the shoulders" at both ends; stay well within the smooth, low sloped parts of the curve.
Their "theoretical 100%" SoC may be defined as, for example,
holding 3.65V until a taper as low as 0.02C (2A per 100Ah)
________
My usual end-charge voltage setting for charging LFP is 3.45Vpc, which for 4S "12V" packs = 13.8V max. Note that is usually at an amps rate of .2 - .3C (.3C = 30A charge current per 100AH bank size). At higher rates, to shorten ICE run-times, it is safe to go to 3.5Vpc / 14.0V.
Note even at the "low" max charge voltage, letting the charge source continue to "push" even low currents long **past** the endAmps point is over-charging, in the sense of reducing potential lifecycles.
At low charge rates, as with many solar setups, under say 0.2C, I reduce termination voltage down to 3.40Vpc / 13.6V.
If your charge rate is **very** low, below say 0.05C, or very variable above and below that ballpark, then you are in real danger of overcharging - even at that seemingly low voltage of 3.40Vpc / 13.6V. In fact, monitoring voltage while charging will not even be meaningful wrt SoC.
Some may think to use an AH counter to control charge termination in that situation, but for me that shows too much trust in what is usually a too-inaccurate tool.
IMO the best approach if this low-current scenario can't be remedied, is to periodically stop charging and let the bank rest isolated. Then, with a very accurate DMM, measure the bank's open-circuit cell voltage. When that reaches 3.34 - 3.35Vpc, the bank is Full, there is no point in going past that.
For daily use cycling, best and simplest is to use a 0.2 - 0.3C rate, and "just stop" charging when your end-point voltage is reached. A long Absorb stage is holdover "lead thinking".
Summary:
Stop at 3.45Vpc / 13.8V for 4S, for amps rate of .2 - .3C.
At higher rates, to shorten ICE run-times, it is safe to go to 3.50Vpc / 14.0V.
At **very** low charge rates, as with many solar setups, back off to 3.40Vpc / 13.6V and do not hold Absorb / CV at all.
---------
For precise benchmarking of your definition of 100% SoC, an endAmps spec of .03C (3A per 100AH) or even .05C is fine, but otherwise Absorb time only gets you another few percent of actual SoC capacity, mostly surface charge or dissipated as heat, and at charge voltages over 3.5Vpc will reduce longevity if held too long.
Note that "stop charging" may simply mean isolating the LFP bank, if you want your charge source to carry ongoing loads rather than discharging your LFP bank.
But if you can't do that or just prefer to Float, then at least set the Float voltage well below your bank's resting Full voltage point, maybe say 13.25V. But that is a compromise, and *may* still shorten life cycles.
With LFP, you don't **need** to ever fill up all the way, as far as the cells are concerned. In fact, it is **bad for them to sit** at Full for any extended length of time.
Therefore the optimum for longevity is to only "fill up" if consumer loads are present, ready to start discharging within the near term.
Charging at .5C (or even higher) is no problem **if** you want to minimize ICE runtime and don't mind sacrificing some longevity - just how much will depend on temperature. As long as your wiring and other infrastructure is robust enough to handle it safely; vendors may spec much lower rates out of legal caution. But do understand, .3C or lower charge rates are **much** better for longevity.
And of course, we're always talking about gentle "partial C" House bank discharge rates. Size the bank appropriately, and be careful feeding heavy loads like aircon, high-gph watermaker, a winch or windlass, heat-producing appliances, etc.
_______
Many sources point out that there is a "memory effect" from keeping charge voltage and ending point exactly the same every time if well below manufacturer specs as I advise; that can apparently over time lead to seemingly lower capacity.
There is no consensus as to just how serious this problem is, and IMO as long as you both
"go higher, into the shoulder" every so often, similar to "conditioning" a FLA bank monthly, and
vary your end-charge setpoints a bit, sometimes go a point or two higher or lower, put in some Absorb time, etc.
then there is no permanent loss of capacity from this phenomenon.
_______
Store at a low SoC. "Not over 50-60%" say most, to compensate for self-discharge, if not getting topped up regularly (I would at least monthly). "Lower the better" say many more recently, as long as you take **zero chances** letting self-discharge drop below 10-15% SoC, IMO not below 3Vpc.
Isolated from everything, **including your BMS** and the cooler the better.
Letting the batts go "dead flat" = **permanent unrecoverable** damage, possibly instantly rendering that expensive bank into worthless scrap.
_______
Same with charging in below 32°F / 0°C freezing temps.
Note that all voltage / performance specs assume a temperature near 25°C/77°F. both charging and discharging behaviours change more significantly the farther you get from there. Persistently hot ambient temps is the #1 lifetime shortener, both in cycles and calendar life in storage.
And although cold temperatures are great for calendar longevity, they drastically impair cycling performance.
1 of 2 parts
