Comfortable LFP State of Charge

[wholybee]

Originally Posted by CaptainRivet

To 10% of the SOC between 2,5V and 3,65V, means 2,5V is 0% SOC?
Depending on how BMS actually shows the SOC, some have start charge=0% SOC and end of charge =100% SOC others 2,3V =0% SOC and 4,2V as 100%SOC while other use under and overvoltage lock as 0/100% SOC…

I am having 4-5 days capacity without any recharged in my bank. That there is no recharge at all is seldom which means I can survive a week of bad weather from my starter/house bank without using backup gen or engine.
Why because like this the capacity is large enough so charge and discharge is below 0.3C even with cont. 7kw of inverter power. Cells keep more balanced, life span improves significantly, if a cell dies I have more then enough backup. Why because it’s speced as bluewater Cruiser for remote areas.
If you are a costal sailor, your daily AH use+50% is more then enough. You cycle it harder and go down to 20% but actually use what you have. Life’s a bit shorter but I am sure in 7-10years there are other Typ of batteries on the market that does better, cheaper and safer what you get know.

Good observations. I consider 0-100 as 2.5 to 3.65. A whole lot of BMSs set protection wider. I think this is more to prevent protection from activating when a charge regimen is set to cycle the full 100% range, not because the BMS considers 2.3v 0% and 4.2v 100%. Also, by the battery specs, damage happens at 4.2v. So a "last resort" protection would by some engineers be set to that voltage.

I set my protection at 2.5v and 3.7v. So i can sometimes charge to 3.65v to balance. Although i actually balance at 3.55v

[sailorboy1]

the reason i attach polls to these threads is because i know by around reply 6 the question drifted off the simple asked question :-)

[goat]

Originally Posted by wholybee

Some errors in that thinking. For LFP, voltage based measurements work well in the knees. So, the highest 5% and the lowest 5%. So it works well for a BMS to use that as LVC and HVC. But anything between those voltages is a crapshoot. If someone gave you a LFP battery what would you use to discern approximate state of charge, a volt meter or a battery monitor SOC display?

Second, it is Voltage that damages a cell, not SOC. Set a high current charger to 15v, and the BMS will protect before the battery is at 100% SOC, as it should. Using SOC would be inappropriate to protect cells. Agreed. That's kind of my point; SOC for our convenience, voltage for protecting battery.

Third, BMSs use Ah counting for SOC, not voltage. Voltage is used for protection, because its required for that. Ah counting is used for soc because thats best for that. I'm curious about what kind of BMS you are using, I've just got a simple BMS that shuts down high and low voltage, temperature etc. it would work in the absence of a shunt or coulomb counter, it wouldn't know State Of Charge from Spray On Cheese

Anyway, I guess what I'm saying is, if I had to use strictly one or the other I'd rather have voltage than SOC to operate the battery. Since I have both the chart I linked to in the other post can give you a ball park to check if your SOC needs to be reset. I know it's not a popular opinion and, as always, I discourage anyone from following my advice.

goat

[newhaul]

Originally Posted by sailorboy1

the reason i attach polls to these threads is because i know by around reply 6 the question drifted off the simple asked question :-)

Agreed but question on poll is that low at end of days charging or lowest overnight before charging g begins?

I do lowest as 40% as that is about the lowest I can recover from in a day without charging from generator or iron wind.

[newhaul]

Originally Posted by sailorboy1

Just answer the question or not

You nailed it at the 6 posts till drift .

[CaptainRivet]

Originally Posted by goat

This will be an unpopular opinion; I trust the voltage more than the state of charge. *forum rises up with pitchforks and torches*

Hear me out. What state of charge does your BMS use to shut down for high or low values? Also at what SOC does your solar controller go to float? Can you reset you're battery monitor to show 14.6 volts even though it's not? If you are using a shunt on your negative pole, does several days of not returning to full require resetting your SOC?

To me, SOC is an easy way to extrapolate how long you can keep using your battery until it runs out, where voltage is an absolute that gives you the "pressure in your balloon"

Of course even voltage is subject to interpretation, "it only works if you have no load and the battery has had time to rest!" I can hear you screaming.

Anyway, for anyone still reading, here's a link to a printable chart that I use in conjunction with SOC. While I don't rely solely on voltage, it's nice to have corroboration of your SOC meter. The difference between fully charged and dead are much closer in LifePo4 and the discharge is so flat (13.0 is +-30% while 13.3 is +-90%) that many don't trust voltage readings for approximate SOC.

https://footprinthero.com/lifepo4-ba...voltage-charts

Apologies to sailorboy for derailing your thread and not voting, as I couldn't find 52 volts on the poll.

goat

It just depends who is calculating SOC how accurate it is and if you can use it.
My BMS Electrodacus calculates SOC very precise. Advantage of SOC it excludes the voltage drop under load or rise under charge and depending if you pull/charge 20 or 200A that is very significant differnce. Means these voltage chart are good to use on a cell(!) at rest, on a bank one cell can be low while others are high, so the SOC available depends on that low cell while your chart would tell you you are at x SOC….means it’s basically unusable except of the high and low end.
I can choose if SOC or voltage with my BMS and the for limits means HVC and LVC and Under-/overlock I use voltage as this is for cell protection on cell level as wholybee said correctly 0% SOC doesn‘t destroy the cell but 2,2V will…
For battery optimizing management (which my Electrodacus BMS does additionally) I use SOC to eg shut off of big loads like my 2x3,5kw inverters (incl. hot water heater) or 12V watermaker I use 30% SOC. Same automatically shutting on the hot water heater and icemaker as dump load if SOC is at 85% and shutting of at 75%. Both can be operated manually too.
Planning to get a small AC for the owner cabin, that will be then SOC 85% dumb load too.
Like this you don‘t get above 85% SOC…

[CaptainRivet]

Originally Posted by newhaul

Agreed but question on poll is that low at end of days charging or lowest overnight before charging g begins?

I do lowest as 40% as that is about the lowest I can recover from in a day without charging from generator or iron wind.

Correct the correct questions should be:
A) what is the technica optimal lowest state of charge of my bank? =>30%
B) what is my lowest end of charge daily SOC before night so I end up at a) => 30% + capacity typically needed during night.
C) how many % reserve capacity of my daily AH usage is comfortable and what minimum. Minimum should be the 15% SOC between 15% and 30% SOC.

That gives you additional info if your bank is big enough or you need more AH or use the iron wind/Benny more often or need to get more solar installed.

You have an untypical small bank for a cruiser (also because of size limitations of your vessel) if you need 40%SOC to make it through the night.

By the way why is 30% SOC optimal? Simple the voltage drop when discharging 0.3C-0.5C will get below 2,8V, most likely under or close to 2,5V especially if you use a big inverter.
So you cells don‘t get stressed unnecessarily.

[newhaul]

Originally Posted by CaptainRivet

Correct the correct questions should be:
A) what is the technica optimal lowest state of charge of my bank? =>30%
B) what is my lowest end of charge daily SOC before night so I end up at a) => 30% + capacity typically needed during night.
C) how many % reserve capacity of my daily AH usage is comfortable and what minimum. Minimum should be the 15% SOC between 15% and 30% SOC.

That gives you additional info if your bank is big enough or you need more AH or use the iron wind/Benny more often or need to get more solar installed.

You have an untypical small bank for a cruiser (also because of size limitations of your vessel) if you need 40%SOC to make it through the night.

By the way why is 30% SOC optimal? Simple the voltage drop when discharging 0.3C-0.5C will get below 2,8V, most likely under or close to 2,5V especially if you use a big inverter.
So you cells don‘t get stressed unnecessarily.

So you are saying that I am wrong for using my bank how I want to use it?

I say 40% dod due to that leaving me a days use without charging . Actually with my usage pattern if I have no charging at all I'm can comfortably run for 5 days before I fire up the generator to charge .

He was asking for personal opinions not your technical this is how everyone should do what I do bit.

[CaptainRivet]

Originally Posted by newhaul

So you are saying that I am wrong for using my bank how I want to use it?

I say 40% dod due to that leaving me a days use without charging . Actually with my usage pattern if I have no charging at all I'm can comfortably run for 5 days before I fire up the generator to charge .

He was asking for personal opinions not your technical this is how everyone should do what I do bit.

No you can use how you want it.
Misunderstood your 40% DOD, thought you need 10% SOC to make it during the night.
DOD 40% and capacity for a day is well balanced setup regarding capacity of your bank.
Your having a small vessel if I remember correctly and therefor a small bank.

[CatNewBee]

Originally Posted by sailorboy1

Trying to unlearn all my old lead acid battery charging rules and get myself aligned with how I use my batteries and the LFP world.



One of the internal issues I am battling with is state of charge % levels. In the past being less than 70% would get me taking/considering action. This morning I was at 66% and it tossed me into a emotional verse logic battle because I know that SOC is OK, but feel it is bad. (am running the generator to charge because I don't want to have do it longer period tomorrow, but wouldn't get fully charged today)



So for you longer term LFP users. how low are you comfortable allowing you SOC getting. (make a poll answer please for when the thread drifts)

I can go as low as 0% as my BMS/BMV reports the measured usable capacity of 1200Ah for my 1000Ah system between full charge 3.65V and empty at 2.9V (Winston cells are rated for 1C discharge currents and are up to 30% overprovisioned / less rated to cover the mfg guaranteed cycles at 80%DOD), but usually I would recharge or take measures at 45%, shallower discharge ensures a longer life. Deep discharge can cause bulging and unrecoverable damage.

[team karst]

Surprised that "remaining capacity" hasn't raised its ugly head. I have professional handheld radios that measure that one. The charger discharges, charges, then discharges while counting coulombs. It also knows the rated capacity of the battery pack. That works, but takes time. At the end, you get a % of new. 0 to 100.

You can have 100% SOC, but crappy capacity...

[newhaul]

Originally Posted by CaptainRivet

No you can use how you want it.
Misunderstood your 40% DOD, thought you need 10% SOC to make it during the night.
DOD 40% and capacity for a day is well balanced setup regarding capacity of your bank.
Your having a small vessel if I remember correctly and therefor a small bank.

I am a 29 ft columbia defender so yes small vessel ( by current standards ) but it's bigger than my islander bahama 24 I did the sail to Hawaii in many years ago. And a 250ah lifepo4 bank so I'm considered big bank for my boat size . My average usage is 30ah to 35ah daily .

[newhaul]

Originally Posted by team karst

Surprised that "remaining capacity" hasn't raised its ugly head. I have professional handheld radios that measure that one. The charger discharges, charges, then discharges while counting coulombs. It also knows the rated capacity of the battery pack. That works, but takes time. At the end, you get a % of new. 0 to 100.

You can have 100% SOC, but crappy capacity...

Well I do an annual capacity test and I have 264ah every time in my 250ah bank .
Now I use up to 60% of capacity which leaves 40% . Or approx 100ah available .

The question posed was what do you use comfortably not how much do you have left!
The numbers are different if you have a 1000ah bank and use down to 30% 700ah used 300ah remaining.

[CaptainRivet]

Originally Posted by CatNewBee

I can go as low as 0% as my BMS/BMV reports the measured usable capacity of 1200Ah for my 1000Ah system between full charge 3.65V and empty at 2.9V (Winston cells are rated for 1C discharge currents and are up to 30% overprovisioned / less rated to cover the mfg guaranteed cycles at 80%DOD), but usually I would recharge or take measures at 45%, shallower discharge ensures a longer life. Deep discharge can cause bulging and unrecoverable damage.

Winston rated 2C continuous and 4C Peak
Well 0% is not 2,9V, that’s 2,5V by common standards. 2,9V is somewhere between 5% and 15% SOC depending if Rest or under heavy load voltage. Using 2,85V with 1s delta when cell is 1s under that threshold.
45%SOC is very conservative but given your capacity is quite achievable, mine is 40% when taking additional action but 30% is a safe limit with minimal reduction in cycle life and no bulging or unrecoverable damage happen.
Do you charge daily to 3,65V if possible? Your Absorption voltage and time in the mppt please.

[CaptainRivet]

Originally Posted by newhaul

I am a 29 ft columbia defender so yes small vessel ( by current standards ) but it's bigger than my islander bahama 24 I did the sail to Hawaii in many years ago. And a 250ah lifepo4 bank so I'm considered big bank for my boat size . My average usage is 30ah to 35ah daily .

30-35AH daily, that’s very low….that’s the usage of 130l top loader fridge during night. Add to that 60AH for my 155l freezer alone during night using 0,5A at 230V or 9A at 13V, run it via a small 300W inverter continuously. Much lower consumption then my big inverters, which are continuously on during day but got shut off at night.