LFP's Charging Voltage and State of Charge

[john61ct]

Out of the Victron solar voltage thread
http://www.cruisersforum.com/forums/....php?p=2788036

Originally Posted by Cpt Pat

I can't stress this enough: you can overcharge a 12 volt (4 series cells) LFP battery with a terminal voltage as low as 13.6 volts (with some chemistries, even lower). This is especially insidious when charging the battery with a low current - like a solar array. This is why "trickle charging" an LFP is so strongly discouraged. Charging begins at about 13.4 volts -- and with a low current charging source it needs to end after 13.6 volts has been achieved for some random amount of time. That's only a 0.2 volt difference! When charging at higher constant currents, such a 0.2C rate or higher, you can detect the 100% SoC point by a tapering off of the charging current. That detection cannot be performed with low and varying charging current from a solar controller.

Employing only terminal voltage detection to determine when to stop charging is "lead-acid thinking." This is the hardest concept to get over with LFP batteries: terminal voltage only reflects state of charge at a very specific charge current and temperature. That charge current is neither constant nor achievable with a solar power source (unless you have thousands of watts of panels).

The manufacturer's voltage curves assume you are charging the battery with far more current than you are likely to ever see from a solar charging source. If you have a 100 AH battery, you'd need a constant-current source of 50 amps for those curves to be valid.

The charge/discharge versus voltage curves for LFP batteries presume a high charge current. Usually the lowest charge current shown in those curves by the manufacturer is 0.2C (if you can find the curves at all). When charging with a solar controller with 10% or less of that current, you need to use a combination of "coulomb counting" (counting amp/hours in and out) and terminal voltage to detect SoC. Here's a paper on the topic: https://arxiv.org/ftp/arxiv/papers/1803/1803.10654.pdf.

Fortunately, the charge controller can do terminal voltage detection (within the accuracy constraints you have already described) and the BVM-712 can do coulomb counting by configuring it to open its internal relay contacts at your chosen SoC set point (I use 80% SoC) for charge discontinuation. I also set a charge cutoff voltage at 13.8 volts, but that only applies to charging with my (0.2C) shore power charger. With solar panels, 80% SoC occurs at 13.6 volts (with some small variance due to temperature). That a per-cell voltage of only 3.40 volts.

I calibrate the BMV to 100% SoC every few months by continuing the charge from my shore power charger at a constant current rate of 0.2C, at or near an ambient temperature of 25 degrees C, until the charge current tapers to 0.05C.

Since you can safely use terminal voltage for discharge discontinuation, I use the BVM low voltage detection for stopping discharge (I use 12.7 volts). In my installation, the BVM relay contacts drive a high-current external relay (be sure to install a reverse polarity snubber diode across the relay coil to absorb inductive flyback).

I would not trust the BMV switch contacts to carry more than 2 amps, so beware of some high current latching relays that draw 7 amps when actuated. Such as this one: https://www.bluesea.com/products/cat...Relays/ML-ACRs.

The reason I chose 80% SoC as the charge cutoff threshold (which appears wasteful) is to accommodate the reduced charge capacity of the LFP at lower temperatures. I couldn't get temperature-dependence data from the manufacturer for inclusion of temperature compensation to the charge cutoff threshold. It's a tradeoff. If the LFP has a built-in BMS that takes the battery offline, I would only trust that as a last ditch "ejection seat" control.

There are so many points in this post that are of interest to me, but none related to Victron MPPT controllers, the topic of that original thread I thought it better to start a new one.

So far I've been very impressed with Cpt Pat's approaches to managing their LFP, but I don't believe we've seen a compromise "build thread" with the details on control systems, hardware selected, relays etc?

If I missed it, please provide a link. Obviously only post what you feel comfortable with, do not want to pry or probe beyond that, but would consider it very helpful.

[john61ct]

Originally Posted by Cpt Pat

I can't stress this enough: you can overcharge a 12 volt (4 series cells) LFP battery with a terminal voltage as low as 13.6 volts (with some chemistries, even lower). This is especially insidious when charging the battery with a low current - like a solar array. This is why "trickle charging" an LFP is so strongly discouraged. Charging begins at about 13.4 volts -- and with a low current charging source it needs to end after 13.6 volts has been achieved for some random amount of time.

I have long been a strident advocate for lower-than-mfg-spec charging in order to maximize longevity.

The following is from my "standard LFP care blurb"

_______
My end-charge voltage setting for charging LFP is 3.45Vpc, which for 4S "12V" = 13.8V max. Note that is usually at an amps rate of around .3C, or 30A per 100AH. At higher rates, to shorten ICE run-times it is safe to go to 3.5Vpc / 14.0V. Also at **very** low charge rates, I back off to 3.40Vpc / 13.6V.

For daily use cycling, best and simplest is to "just stop" charging when your end-point voltage is reached. A long Absorb stage is holdover "lead thinking".

For precise benchmarking of 100% SoC, an endAmps spec of .03C (3A per 100AH) or even .05C is fine, but otherwise Absorb time only gets you another percent or two actual SoC capacity, mostly surface charge or dissipated as heat, and at charge voltages over 3.5Vpc will reduce longevity.

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, and will reduce lifecycles.

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, at say 13.1-13.2V. But that is a compromise, and *may* somewhat 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 more than a few minutes. Therefore only "fill up" if consumer loads are present, ready to start discharging, ideally right away.

______
Now, it seems to me that Cpt Pat and I agree on a fair number of points, but I would appreciate feedback on the above, especially where we may differ,

or where we can perhaps add some more specific and helpful details on the charge rate vs ending voltage issue.

[Cpt Pat]

John,

I would love to have a dialog with you on points of disagreement - but I can't find any. I completely concur.

Nordkyn Design has done some excellent research and experimentation, and this page offers some elaboration and illustration on what we both are saying: Marine lithium batteries in operation | Nordkyn Design. Here's an excerpt that is well explained in the linked page:

LFP cells simply don’t really charge at voltages up to 3.3V [per cell] and then fully charge already at 3.4V and upwards. The transition is so abrupt that claiming to control the charging process by adjusting the voltage is purely and simply bound to fail.

Charging at reduced voltages, down to 3.4V/cell, only increases the absorption time and therefore the overall charging time, but achieves strictly nothing in terms of preventing the battery from getting fully charged and then overcharged. It only takes longer for this to happen.

At low charge currents, one MUST monitor amp/hours in and out (coulomb counting) to prevent overcharging.


The "factory" BMS for my LFP doesn't take the battery offline until it reaches a terminal voltage of14.4 volts. Depending only on that BMS would certainly result in a destroyed battery.

My own design places my LFP in parallel with my pre-existing lead-acid batteries, so there is no interruption to bus operation at the charged/discharged thresholds of the LFP. I place the LFP on the bus when the bus (lead-acid batteries) voltage declines to 13.2 volts, and float the lead-acids on shore power at 13.4 volts (temp compensated) with the LFP off the bus. This resolves the issue of supporting loads when the LFP is off the bus, and allows me to use the voltage settings and charging configuration designed for the lead-acids. The configuration resolves an otherwise complex design to accommodate both battery types (LA & LFP) with my solar and hydrogenerator charging sources.

NONE of the charging sources I've found for LFP batteries have met my specifications, and my design is the only one I could devise that accommodates charging both lead-acid and LFP batteries from the same charging sources.

My LFP it taken off the bus when its SoC is approximately 20%. At the end of a sail, at the dock, I switch the LFP off the bus at the dock at whatever SoC it's at (as low as 20%), and I leave it off the bus for storage. I then fully recharge my lead-acids on shore power. I don't recharge the LFP until just before departing on my next sail. That way, the LFP is always stored at a reduced SoC and spends as little time as possible at 80% SoC.


Cell balancing is somewhat a separate topic. This may be controversial: I don't have any cell balancers. My cell voltages never reach the threshold when the balancers come into play. The cell voltages have never reached the 3.55 volt cell balancer threshold (many are set on other systems at 3.65 volts), and they only increased the self-discharge rate and increased complexity and risk of failures. I removed them. I monitor the cell balance, and because the cells are never operated in the "knee" areas of the voltage curves, the cell balances have remained very closely matched. If I were to overcharge the cells, that's where I would need cell balancing (so I would ruin all the cells at the same time and rate).


The need for cell balancing is, in my opinion, symptomatic of a bad charging design. Critical charging and cell balancing makes sense for electric vehicles that squeeze every last amp/hour out of the battery. LFP use on a sailboat at low fractional charge/discharge storage battery regimes in an 80-20% SoC range is a completely different application.

[john61ct]

Originally Posted by Cpt Pat

I would love to have a dialog with you on points of disagreement - but I can't find any. I completely concur.

Great minds think alike 8-)

To be continued, thanks so much for replying.

[john61ct]

Cross pollination http://www.cruisersforum.com/forums/...id-206572.html
http://www.cruisersforum.com/forums/...ml#post2441975

[Bob James]

Originally Posted by john61ct

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 more than a few minutes. Therefore only "fill up" if consumer loads are present, ready to start discharging, ideally right away.

I am intrigued by where you guys get these statements from?
Do you have some research papers to back this up?

[Bob James]

Originally Posted by Cpt Pat

My LFP it taken off the bus when its SoC is approximately 20%. At the end of a sail, at the dock, I switch the LFP off the bus at the dock at whatever SoC it's at (as low as 20%), and I leave it off the bus for storage.

[I don't recharge the LFP until just before departing on my next sail. That way, the LFP is always stored at a reduced SoC and spends as little time as possible at 80% SoC

Again what sources are you using to choose to follow this procedure ?

40% SOC is accepted for transport and storage isn't it?

[john61ct]

Yes accepted, but IMO higher than I'd want for long term.

Lower the better, as long as ensuring no danger of self-discharging to your Zero definition.

Mine is 2.99Vpc or under 12V for 4S.

_______
Industry and academic papers are oriented around EV & other high current usage, not maximizing longevity, long term House bank use is not even on their radar.

This thread is long but informative
http://www.cruisersforum.com/forums/...nks-65069.html

, make sure to give both Maine Sail and Ocean Planet your close attention.

Also MS' summary notes here
https://marinehowto.com/lifepo4-batteries-on-boats/

**Everything** at that site is worth reading, very valuable. https://marinehowto.com/support, feel free to make a donation to help with those expenses. He also has great articles in Practical Sailor.

[Delfin]

Originally Posted by Bob James

I am intrigued by where you guys get these statements from?
Do you have some research papers to back this up?

Don't hold your breath, because there isn't any, at least with respect to the few minutes at full charge harming LFP. True, you don't want to store them for long periods of time fully charged, and 40 - 50% is certainly the consensus opinion on a good SoC to use. They self-discharge at 3% per month, which s/b considered as well.

Much of this poster's approach to his advice is based on the belief that LFP battery manufacturers think that if you give the consumer bad advice on how to charge their product and it dies early, this will cause you to buy their product again. In line with this spurious and odd perspective, the poster believes that limiting charge current to 13.8/27.6 volts is vital to longevity. This is also incorrect, as the comments from Nordkyn above from Cpt Pat would attest. These words mean something: "Charging at reduced voltages, down to 3.4V/cell, only increases the absorption time and therefore the overall charging time, but achieves strictly nothing in terms of preventing the battery from getting fully charged and then overcharged. It only takes longer for this to happen."

Mainesail has made the point more than once that charge voltage within reason is unimportant in impacting longevity, while time at charge voltage most certainly is. What Nordkyn is pointing out is that at the poster's 3.45 volt recommendation, you can kill the battery as easily as charging it at 3.6 volts. It just takes a bit longer for the murder to occur.

[Cpt Pat]

Originally Posted by Bob James

I am intrigued by where you guys get these statements from?
Do you have some research papers to back this up?

Please see my link in post #3 above for a practical discussion.

Also see https://arxiv.org/ftp/arxiv/papers/1803/1803.10654.pdf as regards the need to do coulomb counting. That paper has a number of cross-references.

[Cpt Pat]

Originally Posted by Bob James

Again what sources are you using to choose to follow this procedure ?

40% SOC is accepted for transport and storage isn't it?

See my post #10 for references.

There's nothing wrong with storage at 40% SoC. The higher the SoC, the more rapid is the calendar aging. 40% is a reasonable number, especially if you have cell balancing cards that are constantly discharging the cells (albeit at a very low rate).

[Cpt Pat]

Originally Posted by Delfin

Don't hold your breath, because there isn't any, at least with respect to the few minutes at full charge harming LFP. True, you don't want to store them for long periods of time fully charged, and 40 - 50% is certainly the consensus opinion on a good SoC to use. They self-discharge at 3% per month, which s/b considered as well.

Much of this poster's approach to his advice is based on the belief that LFP battery manufacturers think that if you give the consumer bad advice on how to charge their product and it dies early, this will cause you to buy their product again. In line with this spurious and odd perspective, the poster believes that limiting charge current to 13.8/27.6 volts is vital to longevity. This is also incorrect, as the comments from Nordkyn above from Cpt Pat would attest. These words mean something: "Charging at reduced voltages, down to 3.4V/cell, only increases the absorption time and therefore the overall charging time, but achieves strictly nothing in terms of preventing the battery from getting fully charged and then overcharged. It only takes longer for this to happen."

Mainesail has made the point more than once that charge voltage within reason is unimportant in impacting longevity, while time at charge voltage most certainly is. What Nordkyn is pointing out is that at the poster's 3.45 volt recommendation, you can kill the battery as easily as charging it at 3.6 volts. It just takes a bit longer for the murder to occur.

>Much of this poster's approach to his advice is based on the belief that LFP battery manufacturers think that if you give the consumer bad advice on how to charge their product and it dies early, this will cause you to buy their product again.

It seems a bit presumptuous to state my beliefs.


> limiting charge current to 13.8/27.6 volts is vital to longevity


Do you mean voltage, not current? My assertion was that any charging voltage over 13.6 volts (3.4 volts per cell) will overcharge a LFP battery if applied long enough. But my countermeasure is to count amp/hours in and out (coulomb counting), NOT limiting the charging voltage. Go ahead, use a higher voltage so the battery charges more quickly (within its charging current rating) -- but STOP charging at the latest at 100% SoC. If the charge current is so low or variable (solar charging) that the ramp-down of charging current when approaching 100% SoC can't be detected then you MUST detect the charge cutoff point by counting amp/hours. Nearly none of the charging schemes use a shunt to allow counting amp/hours. Instead, they inaccurately cheat by trying to guesstimate SoC with voltage. That approach will eventually fail due to overcharging.


I suspect most of the misinterpretation here comes from people who just start their engine and use a high current alternator to charge their LFPs. In that case, the charge current ramp-down can be detected. I don't have an alternator, and even if I did, I'd still risk overcharging my LFP if I also had low current and variable current charging sources (like solar).

> Don't hold your breath, because there isn't any, at least with respect to the few minutes at full charge harming LFP. True, you don't want to store them for long periods of time fully charged, and 40 - 50% is certainly the consensus opinion on a good SoC to use. They self-discharge at 3% per month, which s/b considered as well.

See post #1 at http://www.cruisersforum.com/forums/...ml#post2753841 for a reference to a research study at https://uwspace.uwaterloo.ca/bitstre...pdf?sequence=3. (You can stop holding your breath now...)

I'm not in disagreement with your assertion that storing for a short period at a full charge is acceptable. Arguing otherwise would be logically irrational: how else would you ever utilize a full charge state? It's long term storage (days, weeks, months) at a high SoC that causes capacity fade.

It's important to understand how LFP characteristics correlate. Any single didactic statements are bound to be inaccurate and lead to circular arguments. If there is an argument or counterpoint here with what I posted -- I don't see it.

[john61ct]

I do not prescribe any Absorption time at all in normal cycling.

There is no doubt that avoiding the voltage / SoC shoulders greatly extends longevity, but what I mean by that is lifetime far beyond vendor ratings.

If you're happy with that X thousand cycles, then feel free to follow vendor voltage specs.

Planned obsolescence is not some crackpot conspiracy theory, just common sense in a free market where consumers have limited knowledge.

Why are there no cars designed to be used for 50 years? Because the market is driven by factors other than value and utility, and that is no accident.

I have offered an alternative possible explanation for why vendors specs are not in line with maximizing longevity.

In any case whatever the reason, their motivation is irrelevant to the fact that the discrepancy exists.

[Cpt Pat]

This discussion on charging LFP batteries seems a little wordy. I'm going to try to simplify my point:


Question: what is the terminal voltage of an overcharged four-series-cell LFP battery during charging?
Answer: 13.6 volts or higher at 25 degrees C.
Question: Then if you are charging that battery at 13.6 volts or higher, how do you know when it's at 100 percent state of charge (SoC)?
Answer: 1) If you have a substantial constant current charging source, you will see the charge current ramp down as the battery approaches 100% SoC. That's when one of the cells stops accepting full charge current because one or more cells are approaching their charge limit. If you always have exactly the same constant current charging source, you can also get a close estimate of SoC from the terminal voltage rise during charging - but any changes in the charging rate or temperature will destroy your estimate. Many people choose a voltage slightly higher than 13.6 volts (like 13.8 volts) to estimate the SoC for charge termination -- but this will not work with low current/variable current charging sources that behave like a trickle charger.
2) If you have a small variable current source or you want to terminate charging before one or more cells reach 100% SoC, then you will have to count amp/hours replaced from the measured amp/hours discharged. That's called coulomb counting. You will have to accurately set the charging efficiency of your battery on your counter because it is slightly less than 100%. You will occasional have to recalibrate your amp/hour counter to accommodate capacity fade. To play it safe, use a maximum of 80% SoC charge to accommodate temperature changes and estimation inaccuracies (this is really a risk-acceptance personal choice).
Question: what happens if you apply 13.6 volts or higher at any charging current (greater than the tiny battery self-discharge rate) for a long period of time?
Answer: you ruin the battery by overcharging it. As the cells will fall out of balance, you may be fortunate to ruin only one cell.
Question: then why do people use cell balancers?
Answer: so all of the cells get ruined by overcharging equally. It let's them preserve a battery a little longer when it is being charged by a bad charging source that uses only terminal voltage sensing.


Note: all of voltages listed above have some variation based on aging, temperature, and chemistries.

[john61ct]

Originally Posted by Cpt Pat

40% is a reasonable number, especially if you have cell balancing cards that are constantly discharging the cells (albeit at a very low rate).

Absolutely. IMO should not buy a system that does not allow isolating the cells from all control electronics.

And that they should be so isolated before any long term storage.

If not, then 60% may be too low without periodic checkup visits.

Which last I recommend anyway.