LFP's Charging Voltage and State of Charge

[john61ct]

Originally Posted by Cpt Pat

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.

Well, another alternative is simply stopping charge while you know SoC is still well below 100%.

All this depends on whether the user is regularly getting enough solar to get back to full in the first place.

Many if not most cruisers have an alternative ICE source available, which may need to be used once or twice a week to compensate for not enough solar input.

> I'm not in disagreement with your assertion that storing for a short period at a full charge is acceptable.

Same here. But the definition of Full being "14.6V held to a .005C taper" is very different from your and my definition.

And the question of "how long" is short vs long is all grey scale no hard line as well, just a judgment call.

[Bob James]

Well there is a bit of meat there to digest.
So are you using a BMS on your systems?

[john61ct]

For me "a BMS" is just one implementation of several different types of protective functionality.

That BMS functionality does not require buying "a BMS".

And those protections may be implemented on three separate levels,

whole-bank, pack-level or per-cell.

By avoiding the voltage shoulders, the need for cell-level functionality is greatly reduced.

Letting the BMS do active balancing is unnecessary and potentially harmful, it is very common for the complexity of cell-level BMS devices to be the cause of problems.

Thousands of owners do just fine with only the higher pack- or bank-level protections.

Check cell-level voltage balance say monthly to start, then quarterly, finally every six months if there are no imbalance issues, but only if that seems safe to you.

IMO just look for LVD / OVD and temp protection. Multiple layers of protection are advised if it is a very expensive bank, so you don't rely on any one device to keep working.

[Bob James]

Hmm . But for $20 USD for a 100Amp waterproof 8S you would be pretty silly or stuborn if you didn't put one on. You can still draw/charge from the battery negative terminal at any time and have it to just swap onto if you go away or if a controller or other feed in device plays up you know the pack is protected.

[john61ct]

Could you link to one you recommend for marine use at that kind of price?

Personally, sounds like something I'd hook up for checking cell V occasionally, not leaving attached.

And only for that functionality, never seen one with voltage setpoints I'm happy with, or adjustable ones. By the time LVD / OVD kicks in more damage than I'd accept has already been done.

Any temperature protections?

And do you believe that 100A tolerance spec? For how many minutes, or seconds?

I'd be much more interested in learning what specific devices Cpt Pat actually has installed, if they're willing to share. . .

[Cpt Pat]

Originally Posted by john61ct

Could you link to one you recommend for marine use at that kind of price?

Personally, sounds like something I'd hook up for checking cell V occasionally, not leaving attached.

And only for that functionality, never seen one with voltage setpoints I'm happy with, or adjustable ones. By the time LVD / OVD kicks in more damage than I'd accept has already been done.

Any temperature protections?

And do you believe that 100A tolerance spec? For how many minutes, or seconds?

I'd be much more interested in learning what specific devices Cpt Pat actually has installed, if they're willing to share. . .

I started with something complex: voltage and current sensors, analog-to-digital converters, a Linux computer, graphing software, etc. You know... all the things that working at NASA should make me drool over.

But complexity is the enemy of reliability so, after a lot of observation and research, that design evolved into something far simpler and off the shelf.

1) Victron Energy BVM-702 https://www.victronenergy.com/blog/2...-702-insights/
2) APO3 low voltage disconnect: APO3: Automatic Power Off 3 - APRS World, LLC
3) A 30 amp power relay: https://www.digikey.com/products/en?...N2C-DC12&v=236
4) A 50 amp breaker: https://www.bluesea.com/products/713...face_Mount_50A

The BVM-702 shunt is connected to the LFP negative terminal, so it totalizes only the LFP battery. The BVM-702 has two voltage sense lines: "starter" and "main" sense. "Starter" sense is connected to the power bus (lead-acid batteries). "Main" sense is connected to the LFP battery positive terminal.

Charge termination control: The BVM-702 relay is configured to open at 80% SoC (config menu #17). It has no "low" SoC setting, i.e., it's set to zero (config menu #16). This causes the BVM relay to receive an "open" command at 80% SoC, but no close command. There is only one BVM relay, but the relay responds to multiple independent commands.

Overvoltage disconnect: The BVM relay opens at 13.9 volts on the "starter" sense (bus) voltage (config menu #23). This control voltage is seldom reached, the 80% charge disconnect above usually (but not always) is hit first. It does get hit at high (above 25C) ambient temperatures - in my climate, that's only a few days a year.

Parallel connection control: The LFP is connected in parallel with the bus at a bus ("starter sense") voltage of 13.2 volts. (config menu #22)

Low voltage disconnect: The APO3 is configured to remove power at its output at 12.7 volts. It is connected to the bus voltage (NOT the LFP). The APO3 has a manual override for defeating the low voltage disconnect in an emergency.

Interconnection: The BVM relay switch applies power to the high current relay from the APO3. I used a double-pole relay and connected the N.O. terminals in parallel, cutting the contact resistance in half and getting a "sort of" a 60 amp current rating. The power for the relay is derived from the APO3 through the BVM relay, so the combination of the BVM relay and the APO3 are configured in series: a "wired And" configuration.

I don't have an engine to start: my maximum current draw is 20 amps and the maximum charge current is 30 amps. I really recommend against starting an engine with a LFP, so lead-acid batteries still have a place for starting engines (the idea of drawing an LPF down to a low voltage during engine starting just makes me cringe).

Implementation note: The high power relay has a reversed polarity snubber diode across its coil to absorb inductive flyback. Otherwise, the BVM relay contacts would arc every time they open. In series with the LFP positive terminal is the breaker for storage isolation and fault disconnect.


All together, pretty simple and cheap. The most expensive component is the BVM-702, but I wanted the BVM-702 anyway.

[Delfin]

Originally Posted by Cpt Pat

>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.

Actually, I was referring to John, not you. John has taken the position many times on many threads that manufacturers think that screwing their customers by recommending battery killing charge rates is a way to sell more product. Cars last longer today than they did 30 years ago because cars that die young, also die as brands, so I consider John's view on this to be frankly nuts.

He has also taken the position, and this is the one I was referring to that has zero empirical evidence behind it, that having an LFP bank at full charge for more than "a few minutes" is harmful. This is also nuts. As the thesis paper you reference above notes, capacity losses following full charging are both minor and reversible except at very high storage temperatures and then only when stored for months and months, so John's statement is not supported and one should not hold one's breath waiting for substantiation.

Additionally, he takes the position that one must avoid "the shoulders" of charge rates like one would avoid ingesting polonium. What he fails to mention, and as your post on Nordkyn and your own comments support, is that it isn't the charge voltage at the "shoulders" but how long you keep the battery at that voltage that matters. Whether you charge at 13.6 volts or 14.4 is immaterial for longevity as long as you stop charging when the battery stops accepting current. You can kill a battery at both voltages because any charge current applied after the battery is "full" just goes to heat and heat kills. You're aware of this per your comments, but John seemingly is not. And like you, I use a coulomb counter to determine when the battery is full. For me, it is around 5% of capacity.

We are substantially in agreement on handling of these banks, BTW, and I'm sorry you confused my criticisms of John's unsupported opinions with your real world experience, none of which John has, as near as anyone can tell. John could, of course, correct the record on his qualifications and experience, but having been asked many times and never offering any, one has to draw their own conclusions. The nutshell summary of managing an LFP bank for me, and one I think you will agree with is:

1. Charge at 14.0 - 14.4 volts
2. Disconnect charging when CAR < 5%
3. Provide a parallel lead acid bank (starter bank for me) to accept charge current when the LFP bank is full and charge sources have to stay on.
4. If you leave the boat for a few weeks at a time, store the batteries at 50% or so.
5. Avoid very high and very low temperatures when charging or storing.

Finally, I would note that if one is really concerned about getting the maximum number of thousands of cycles out of an LFP bank, sizing it so that you don't generally draw it down below 40% SoC will extend the life of the bank vastly more than most anything else you can do.

[john61ct]

Originally Posted by Cpt Pat

All together, pretty simple and cheap. The most expensive component is the BVM-702, but I wanted the BVM-702 anyway.

Cool setup, and writeup.

100% agree on reliability through simplicity, constantly having to battle my "too much" OCD-spectrum tendencies.

Lot to absorb there, will be back with Qs for sure, hopefully others will participate, constructively.

These days I guess the 712-BMV would be worth substituting.

On behalf of all us lifetime-learners, thanks so much!

[Bob James]

They have dropped the 100 Amp off this listing but a little searching will find it .
I don't go above 100 amps on anything . If I did I would move up in voltage to 48.

Check the specs . High and low voltage, High current ,Short circuit small balancing charge at set voltage. If something gets out of wack it shuts the whole Neg line down so I have 4 strings ,1 BMS each

https://www.aliexpress.com/snapshot/...Id=32873151474

[john61ct]

@Delfin, please don't put straw man positions in my mouth to battle like that.

Twisting, misrepresenting my words, some from long past posts I've since changed my mind about. Not to mention outright lies.

Some of which I just refuted within this very thread!

Makes you seem… well I won't go there.

_______
Again, dozens of times now, I do not hold any voltage for **any** amount of time, pick a setpoint depending on current amps level, and Just Stop charging when it hits.

As a result of Cpt Pat's points, will go down to 13.6V at higher currents than I have done before.

Going any higher than 13.8V in normal cycling serves no useful purpose, IMO can only be harmful.

[john61ct]

Originally Posted by Bob James

They have dropped the 100 Amp off this listing but a little searching will find it

Maybe because the spec is a lie.

All the cheap Chinese gear I've tested, overestimates ampacity, often by like double or triple.

Be willing to bet that would burn out at 50A continuous.

BTW can't see the specs without logging in via that link.

[Bob James]

Originally Posted by Delfin

Actually, I was referring to John, not you. John has taken the position many times on many threads that manufacturers think that screwing their customers by recommending battery killing charge rates is a way to sell more product. Cars last longer today than they did 30 years ago because cars that die young, also die as brands, so I consider John's view on this to be frankly nuts.

He has also taken the position, and this is the one I was referring to that has zero empirical evidence behind it, that having an LFP bank at full charge for more than "a few minutes" is harmful. This is also nuts. As the thesis paper you reference above notes, capacity losses following full charging are both minor and reversible except at very high storage temperatures and then only when stored for months and months, so John's statement is not supported and one should not hold one's breath waiting for substantiation.

Additionally, he takes the position that one must avoid "the shoulders" of charge rates like one would avoid ingesting polonium. What he fails to mention, and as your post on Nordkyn and your own comments support, is that it isn't the charge voltage at the "shoulders" but how long you keep the battery at that voltage that matters. Whether you charge at 13.6 volts or 14.4 is immaterial for longevity as long as you stop charging when the battery stops accepting current. You can kill a battery at both voltages because any charge current applied after the battery is "full" just goes to heat and heat kills. You're aware of this per your comments, but John seemingly is not. And like you, I use a coulomb counter to determine when the battery is full. For me, it is around 5% of capacity.

We are substantially in agreement on handling of these banks, BTW, and I'm sorry you confused my criticisms of John's unsupported opinions with your real world experience, none of which John has, as near as anyone can tell. John could, of course, correct the record on his qualifications and experience, but having been asked many times and never offering any, one has to draw their own conclusions. The nutshell summary of managing an LFP bank for me, and one I think you will agree with is:

1. Charge at 14.0 - 14.4 volts
2. Disconnect charging when CAR < 5%
3. Provide a parallel lead acid bank (starter bank for me) to accept charge current when the LFP bank is full and charge sources have to stay on.
4. If you leave the boat for a few weeks at a time, store the batteries at 50% or so.
5. Avoid very high and very low temperatures when charging or storing.

Finally, I would note that if one is really concerned about getting the maximum number of thousands of cycles out of an LFP bank, sizing it so that you don't generally draw it down below 40% SoC will extend the life of the bank vastly more than most anything else you can do.

I'd have to take you to task on that.
When Landrovers came out in the war and just before they were all positive earth ,as were most cars . Shortly thereafter most changed to negative earth . Reason? Frankly the body stays charged up and that assists rust . Believe it or not is up to you but I know its true because I still have one with a damn good chassis and steel work under the alloy sheet skin. Now thats 70 odd years old and runs round the farm like it was a youngster. Outdoes the 4 WD tractors because it has little weight. You can today go buy a washing machine and find the back panel is made of masonite . They even paint it sometimes.

I'd also agree that avoiding the knees and you will be pleased is valid. But my concern is the heat that is produced in the center of the cell. That's what swells LFP because the center heat is the last to dissipate and it boils the small amount of electrolyte that is in the separators and you cant put it back in.

I have not seen any technical information from anywhere that would support your 40 % SOC claim.

I have two years only experience specifically in LFP and 24 V only at this point but my first LFP pack is as good as it was at the start, no corrosion ever and excellent voltage profiles. I have 5 battery pack installations on the go now.

The bms's are everywhere if you bother to look. I'v had them at 50 amps and not even warm . You are wrong. They make good stuff these days if you are careful and get one to test first

https://www.aliexpress.com/wholesale...ort+with+balan

[Delfin]

Originally Posted by john61ct

@Delfin, please don't put straw man positions in my mouth to battle like that.

Twisting, misrepresenting my words, some from long past posts I've since changed my mind about. Not to mention outright lies.

John, you have taken every position I mentioned, so no lies, no straw men. You reserve the right to change your opinion as you learn more about this subject, and that is fine. What would be great would be if you could avoid making declarative statements, that are then refuted with similarly expert sounding declarative statements. You don't own a boat, right? You don't maintain a house LFP bank, correct? So the fact that your information is purely what you learn on the Internet is no shame, it is only that you pose whatever your current state of knowledge is as if it were gospel. It would be a pity if people who know less than you do on this subject took your words to heart, only to find out that notwithstanding prior certainty, you've changed your mind.

Originally Posted by john61ct

Again, dozens of times now, I do not hold any voltage for **any** amount of time, pick a setpoint depending on current amps level, and Just Stop charging when it hits.

As a result of Cpt Pat's points, will go down to 13.6V at higher currents than I have done before.

Going any higher than 13.8V in normal cycling serves no useful purpose, IMO can only be harmful.

And that is one of the positions you still take that I pointed out is wrong, and that I suppose you will disavow at some point in the future, then get huffy when someone reminds you of your prior position. Charging at a higher voltage than 13.8 does not cause harm as you suggest, or anymore harm than would be experienced if you hold charging voltage at whatever level longer than it takes to fill the battery. The only thing that your lower charge voltage accomplishes is precisely what Nordkyn noted - you lengthen the time your genset or engine runs to charge them. That's it.

By the way, how do you propose to "go down to 13.6v at higher currents?" Not sure what that means.....

[john61ct]

I have not found speed of charging to be any issue with LFP, in many cases under an hour is plenty for many days' usage.

Originally Posted by Delfin

By the way, how do you propose to "go down to 13.6v at higher currents?" Not sure what that means.....

What isn't clear? How to change a voltage setting?

That's all I mean, vary the target setpoint for a given charge session depending on the expected charge current. Assuming 4S, say 400AH

A 150A source can be set much higher, to 13.8V

In the past, would not have gone all the way down to 13.6V until the source was say 5A or less.

Now I'm thinking say 20A?

Will take some experimentation to establish actual numbers, I don't think I need to go so far as Cpt Pat's use of AH counting to control my end charge,

but always defaulting to a lower SoC, lower voltage is sensible.

[tanglewood]

When I look at the various LFP charge curves plotting voltage against SOC at various charge rates, it looks to me like a strictly voltage based end of charge protocol would be fine, and that coulomb counting is not required. Coulomb counting does give an extra refinement, but doesn't seem required.


At different charge rates, the battery will reach the charge cut-off voltage a somewhat different states of charge. At a fast charge rate, it might reach 3.45 vpc @ 80% SOC, where at a lower charge rate it will reach that same voltage at 90% SOC. This is the basis for the concern about over-charging at very low charge rates, right?



But it seems that if you just judiciously pick your terminal voltage, you will be fine. A fast charge may leave you with a lower SOC, but so-what. Does it really matter whether you are at 80% or 90% SOC?


I'm not saying this is the only way to charge, or the best way to charge, but it does seem like a perfectly acceptable approach if you are OK with a somewhat variable final SOC. If your use requires always getting back to the same SOC, then some accommodation needs to be made for charge rate, either by adjusting the terminal voltage, or coulomb counting, or something else.