Comfortable LFP State of Charge

[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)

[Pete7]

We have seen 33% at dawn just before solar rescued the situation. If it had been 33% in the early evening, then that would be more of a concern and we might have to do something about it.

[Off Trail]

Our overnight drop this time of year is about 30%, so the lowest I'm comfortable with for late afternoon tends to be around 50%. Which I guess means 20%?

Also everything's much more relaxed. If it's low in the morning I'm comfortable just seeing how the day's solar turns out. No need to jump in and add charge right away.

[fxykty]

We’re OK going as low as 10% SOC, which if we don’t turn things off overnight means around 30% SOC the night before. But those levels will be after a few grey days and no engine use. Generally we like keeping above 20% SOC. Note that our LVC based on cell voltages is around 5% SOC.

I recently spent 3 days on a boat with lead acid batteries and we ran the engine whenever they got to 70%. What a pain, I’d forgotten those days.

[Cheyne]

I will run my little Honda until it is out of fuel if I get under 30% late in the afternoon and I know it is going to be rainy or overcast the next day or two. 30% late in the after normally gives me just under 20% the next morning. In three months in the Bahamas this past season that happend 3 times. My generator gets started more to just test it than use it.

Cheyne

[wholybee]

The key here is regularly, and it has nothing to do with what is safe for LFP. I sized my battery to be able to go and extra day if its cloudy and my solar can't keep up. So regularly, not less than about 40-50%. I don't get concerned until it is less than 10%, and then because I don't want the lights to go out, not because it will harm the LFP. And since I am not regularly down there, I can take the LFP all the way to 0% without any issues.

I wouldn't take the LFP to zero regularly, but getting to 10% regularly is fine for the LFP, if not careless power management.

[CaptainRivet]

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)

How do you know your SOC of your LFP? Forget voltage based tables…
Even my Electrodacus BMS who is performing several calculations (like coulomb counting, amps in/out of the bank, voltage, charge current into each cell…) simultaneously is only reliable after 100 cycles or more, and it’s Self-calibrating with each cycle getting it more precise. And it does that because I can steer via SOC or and/or voltage it’s actions.

The other point is simply how much reserve capacity did you calculate into your bank? And how many different charge sources do I have to recharge? And are you in safe bay on anchor with super calm and sunny the whole week or is a weather front and rain for a week forecasted.
Technically optimum is 30% SOC, 20 is no problem. All theory, why because even LIfepo4 and their charge and discharge curve for each manufacturer are different, also depending on how many C you dis-/charge.
To get to know it for your drop ins get a battery tester and run a charge and discharge with your 0.x C you are commonly using. Then you exactly see at which voltage level your drop in is entering the steep part (low and high end) of the curve and which SOC it roughly is. To discharge that voltage level is optimum and this is quite different from manufacturer to manufacturer.
Done that for my 272AH Lishen cells…

[sailorboy1]

Just answer the question or not

[CaptainRivet]

Watch this videos from Andy‘s offgrid garage, you will learn a lot. Understanding how this chemistry works, what are myths and important factors are much more helpful then trying to define a discharge point to be comfortable with by a poll. If you understand you know what you are comfortable with.

https://m.youtube.com/watch?v=uNFIkNtsd1c

https://m.youtube.com/watch?v=hLf1FdRjlaw&t=476s

https://m.youtube.com/watch?v=qZg4Jxftw-c

https://m.youtube.com/watch?v=H2c_suhosOI

[CaptainRivet]

Originally Posted by wholybee

The key here is regularly, and it has nothing to do with what is safe for LFP. I sized my battery to be able to go and extra day if its cloudy and my solar can't keep up. So regularly, not less than about 40-50%. I don't get concerned until it is less than 10%, and then because I don't want the lights to go out, not because it will harm the LFP. And since I am not regularly down there, I can take the LFP all the way to 0% without any issues.

I wouldn't take the LFP to zero regularly, but getting to 10% regularly is fine for the LFP, if not careless power management.

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.

[sailorboy1]

Guess I will leave my alarm set at 30%. Should never get near that as I would perfer to manage the generator use to only have to listen to it for an hour each day instead of 3 hours every 3 days.

[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

[PaulCrawhorn]

I'll go further and say 99% of SoC readings are wildly inaccurate quesstimates or even pure fiction.

Very few instruments are consistently within 4-6% accurate, and that only for a couple cycles after a reliably calibrated 100% reset, ideally manual.

The problem with voltage is only "isolated at rest" has any relation to SoC, but a given rig with predictable discharge C-rates you learn to trust your intuition.

Right after running the windlass is not the time to try to guess, when in doubt LVC higher rather than lower.

[wholybee]

Originally Posted by PaulCrawhorn

I'll go further and say 99% of SoC readings are wildly inaccurate quesstimates or even pure fiction.

Very few instruments are consistently within 4-6% accurate, and that only for a couple cycles after a reliably calibrated 100% reset, ideally manual.

The problem with voltage is only "isolated at rest" has any relation to SoC, but a given rig with predictable discharge C-rates you learn to trust your intuition.

Right after running the windlass is not the time to try to guess, when in doubt LVC higher rather than lower.

That's lead acid thinking, and a result of lower efficiency and higher Perkurt effects of lead acid. With LFP efficiency >99% and Perkurt very near 1, accurate SOC is easy to achieve, even with many days between resets.

My accuracy is about 5% off after 2 weeks since reset.

Using voltage is very inaccurate with LFP, even at rest.

[wholybee]

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

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.

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.

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.