LiFePo4 max charge voltage

[blackrose]

Now that we're getting a bit more sunshine I'm trying to optimise my solar charging regimen.

 

I know that the maximum charge limit for 12v LiFePo4 batteries is 14.4 -14.6v but this shouldn't be used as a target as it stresses the cells and 14.2 -14.4v is a more reasonable maximum limit. However, for daily solar charging would 14.2v reduce battery longevity? If so what should the daily charge limit be? 14.0v or even less? 

 

I have a relay connection between my BMV battery monitor and MPPT and currently it's set to switch off charging at 14.2v and resume charging when the voltage drops below 13.3v.

 

While a daily 14.2v charge might seem a bit high it doesn't stay at that voltage for any time because as soon as it reaches that voltage the relay switches off the MPPT.

 

Is it ok or should I reduce it?

 

 

Edited March 28 by blackrose

[GUMPY]

Mine is set at 14.2v it only stays there for 30mins then drops to 13.5v where it spends most of the time? It's below the max cell voltage so I wouldn't worry. Remember the 80% rule was for a different type of lithium chemistry.

 

Am I going to change mine for the summer?

No.

 

As an addendum if I am leaving the system for a couple of weeks I will shut the MPPT off remotely once the battery is charged and turn it back on before I need to use it.

Edited March 28 by GUMPY

[blackrose]

Ok thanks 👍🏻 

[IanD]

My daily solar charging is to 54V (CVL set by the BMS) which would be 13.5V for your system -- typically gets there by lunchtime, stays there for the rest of the day while the sun is out, then drops away when it goes in.

 

Once a month it does a cell balancing/100% SoC charge to 57.3V (14.3V for you), once the cells are balanced CVL drops back to 54V.

[nicknorman]

4 hours ago, blackrose said:

Now that we're getting a bit more sunshine I'm trying to optimise my solar charging regimen.

 

I know that the maximum charge limit for 12v LiFePo4 batteries is 14.4 -14.6v but this shouldn't be used as a target as it stresses the cells and 14.2 -14.4v is a more reasonable maximum limit. However, for daily solar charging would 14.2v reduce battery longevity? If so what should the daily charge limit be? 14.0v or even less? 

 

I have a relay connection between my BMV battery monitor and MPPT and currently it's set to switch off charging at 14.2v and resume charging when the voltage drops below 13.3v.

 

While a daily 14.2v charge might seem a bit high it doesn't stay at that voltage for any time because as soon as it reaches that voltage the relay switches off the MPPT.

 

Is it ok or should I reduce it?

 

Trying to control Li solar charging by means of voltage basically doesn’t work. For example on a strong sun day with big panels and a small battery, you might reach 14.2v whilst the SoC is still quite low. The high current into a smallish battery means that only holding the 14.2v for an hour or two will get you to 100%.

 

By comparison with a big battery and small panels/weak sun, by the time you get to 14.2v the SoC will be 99.9%.

 

it doesn’t matter hugely because LiFePO4 is fairly tolerant of being charged close to 100% every day provided you don’t hold high voltage and low current for long periods, but if you are trying to treat your batteries kindly, doing it on voltage alone simply doesn’t work.

[blackrose]

Ok, so how should I be doing it?

 

I can either set the relay by voltage or SOC and last time it was discussed everyone was telling me to use voltage. 

 

9 hours ago, IanD said:

My daily solar charging is to 54V (CVL set by the BMS) which would be 13.5V for your system -- typically gets there by lunchtime, stays there for the rest of the day while the sun is out, then drops away when it goes in.

 

Once a month it does a cell balancing/100% SoC charge to 57.3V (14.3V for you), once the cells are balanced CVL drops back to 54V.

 

So you think going up to 14.2v daily is too much? The batteries aren't actually held at 14.2v. In sunshine the battery voltage rises pretty quickly and as soon as they hit that figure charging stops.

Edited March 29 by blackrose

[IanD]

38 minutes ago, blackrose said:

Ok, so how should I be doing it?

 

I either set the relay by voltage or SOC and last time it was discussed everyone was telling me to use voltage. 

 

 

So you think going up to 14.2v daily is too much? The batteries aren't actually held at 14.2v. In sunshine the battery voltage rises pretty quickly and as soon as they hit that figure charging stops.

That shouldn't be a problem, if it's the way your system works. Mine uses an external BMS which controls all charging sources using CVL/CCL, so is different to yours.

 

(and also they're 700Ah cells, so different solar charging rates like Nick described make very little difference, they're *all* "low" as a fraction of C...)

Edited March 29 by IanD

[nicknorman]

44 minutes ago, blackrose said:

Ok, so how should I be doing it?

 

I can either set the relay by voltage or SOC and last time it was discussed everyone was telling me to use voltage. 

 

 

So you think going up to 14.2v daily is too much? The batteries aren't actually held at 14.2v. In sunshine the battery voltage rises pretty quickly and as soon as they hit that figure charging stops.

By SoC. Which is what I do for my caravan. “Everyone” was not telling you to use voltage, I originally advised you to use SoC and helped you set it up. At some point you decided to change to voltage, based on a majority cry - a majority most of whom probably didn’t have Li batteries!

 

Ian is right that since the charging is all low fractional C, using voltage sort of works in a rather imprecise way. But because it works in an imprecise way there is no “right answer” to the correct voltage threshold, especially as the charge rate is quite variable (cloudy vs blue sky day). All of which is why I use SoC. But yes using voltage also sort of works in an inexact way.

[GUMPY]

KISS

 

The way it's set up works.

 

Don't let yourself be drawn into overcomplicating things.

Edited March 29 by GUMPY

[Quattrodave]

1 hour ago, nicknorman said:

By SoC. Which is what I do for my caravan. “Everyone” was not telling you to use voltage, I originally advised you to use SoC and helped you set it up. At some point you decided to change to voltage, based on a majority cry - a majority most of whom probably didn’t have Li batteries!

 

Ian is right that since the charging is all low fractional C, using voltage sort of works in a rather imprecise way. But because it works in an imprecise way there is no “right answer” to the correct voltage threshold, especially as the charge rate is quite variable (cloudy vs blue sky day). All of which is why I use SoC. But yes using voltage also sort of works in an inexact way.

 

How are you calculating SoC?

Not trying to pick a fight, genuinely interested 😁

[IanD]

1 hour ago, nicknorman said:

By SoC. Which is what I do for my caravan. “Everyone” was not telling you to use voltage, I originally advised you to use SoC and helped you set it up. At some point you decided to change to voltage, based on a majority cry - a majority most of whom probably didn’t have Li batteries!

 

Ian is right that since the charging is all low fractional C, using voltage sort of works in a rather imprecise way. But because it works in an imprecise way there is no “right answer” to the correct voltage threshold, especially as the charge rate is quite variable (cloudy vs blue sky day). All of which is why I use SoC. But yes using voltage also sort of works in an inexact way.

Given that SoC is usually calculated from voltage and current (and integrated current), I'd argue that you're still using voltage, just indirectly... 😉

 

The problem with using SoC is that it almost always drifts over time, until you do another cell balance/100% SoC reset charge at a high voltage, and this drift is much worse with some setups than others. While relying on daily solar charging only (to the 54V float voltage) my SoC starts at 99% and then drops by about 1% per week, and others have reported similar figures.

 

Which is why the LFP battery and BMS manufacturers invariably make charging decisions on measured voltage -- with adjustments for charging current -- not the calculated SoC, because this is subject to error.

 

I know that you disagree with this approach, but it has to be pointed out that you're very much in the minority here... 😉

Edited March 29 by IanD

[Tony Brooks]

29 minutes ago, Quattrodave said:

 

How are you calculating SoC?

Not trying to pick a fight, genuinely interested 😁

 

In a question I posed about how accurate the Ah counters on the % charged scale with Li, I was assured that for several reasons they are way more accurate than they are for LAs. It seems to me that as long as you set the parameter correctly the way the voltage seems to be regular raised on Lis to top balance the cells the meter synchronisation will be far more frequent than with LAs and you are not trying to guess or calculate the loss of capacity caused by sulphation. However, I bow to the likes of Ian and Nick.

[Quattrodave]

@Tony Brooks thats just what I was wondering... Knowing that Ah counters and SoC with LA batteries can be wildly inaccurate got me wondering how it differed (and how well it worked) with LI 🤔

 

I have LA at the moment and they work well for me but at some stage I would like to upgrade...

[IanD]

14 minutes ago, Quattrodave said:

@Tony Brooks thats just what I was wondering... Knowing that Ah counters and SoC with LA batteries can be wildly inaccurate got me wondering how it differed (and how well it worked) with LI 🤔

 

I have LA at the moment and they work well for me but at some stage I would like to upgrade...

They're more accurate after a balancing charge than LA because current integration is more accurate, there's no Peukert coefficient or temperature variation to speak of so charge out is almost identical to charge in, and SoC is pretty accurate, at least initially -- also there's almost no drop in capacity over time because there's no sulphation.

 

However the SoC errors can build up over time because no BMS current integrator is perfectly accurate, so there's a (hopefully!) slow drift of the SoC away from the actual value -- but 1% per week SoC drop like mine (1Ah per day) can be ignored, that's only a drop from 99% to 95% (at 54V) over a month before the next balancing cycle.

 

Unfortunately some BMS are much worse than this, and the SoC errors can then be huge (tens of percent) so they're effectively meaningless.

 

It's like anything else -- a well designed system works well, a badly designed one doesn't... 😉 

Edited March 29 by IanD

[nicknorman]

3 hours ago, Quattrodave said:

 

How are you calculating SoC?

Not trying to pick a fight, genuinely interested 😁

 

I am not calculating it, the BMV712 is! But it is integrating current. The integral of current is charge. We know that when you integrate something without absolute precision, errors can accumulate and that is why one needs to occasionally reset to a known datum - in this case a nominal 100% SoC, ie voltage up around 14.4v and charge current less than 5% capacity. Because the charge efficiency factor is close to 100% and the self-discharge is very low for Li batteries, the errors from a correctly calibrated BMV712 accumulate quite slowly and it is only necessary to do a "datum charge" to 100% every month or so. And lets face it if the indicated SoC is in error by a few %, it doesn't really matter.

2 hours ago, IanD said:

Given that SoC is usually calculated from voltage and current (and integrated current), I'd argue that you're still using voltage, just indirectly... 😉

 

The problem with using SoC is that it almost always drifts over time, until you do another cell balance/100% SoC reset charge at a high voltage, and this drift is much worse with some setups than others. While relying on daily solar charging only (to the 54V float voltage) my SoC starts at 99% and then drops by about 1% per week, and others have reported similar figures.

 

Which is why the LFP battery and BMS manufacturers invariably make charging decisions on measured voltage -- with adjustments for charging current -- not the calculated SoC, because this is subject to error.

 

I know that you disagree with this approach, but it has to be pointed out that you're very much in the minority here... 😉

No SoC is not calculated from voltage and current. Here is a clue - "state of CHARGE". Charge is the integral of current and so in order to measure charge you only need to measure current and time, not voltage. It is true that voltage is occasionally used when re-datuming the meter, but voltage does not form part of the calculation of SoC. It is true that some BMSs hybridise integration of current with some sort of rested voltage table, but that is mainly because the current measurement they use lacks resolution and accuracy, and using voltage is a fudge to cater for that imprecision. And to be fair, limits the maximum error to something not too big

 

I don't know how you are measuring SoC. Obviously you have a lot of parasitic drain from your various gadgets and if it is consistently dropping 1% a week then clearly the zero offset needs adjusting. But even at 1% a week, that is 4% a month which I suggest is completely unimportant.

 

I am  not at all surprised that you think me in the minority - in my professional and hobby life this has often been the case. But usually it transpires that I was right. If you think that the "right answer" to technical issues is based on democracy then that is up to you. I don't.

47 minutes ago, Quattrodave said:

@Tony Brooks thats just what I was wondering... Knowing that Ah counters and SoC with LA batteries can be wildly inaccurate got me wondering how it differed (and how well it worked) with LI 🤔

 

I have LA at the moment and they work well for me but at some stage I would like to upgrade...

They work much better with Li compared to LA because of the ~100% charge efficiency, low self-discharge and unchanging capacity.

36 minutes ago, IanD said:

They're more accurate after a balancing charge than LA because current integration is more accurate, there's no Peukert coefficient 

 

 

Peukert doesn't come into SoC calculations either for LA or Li. It only comes into the "time to run to flat" display which is pretty useless for boating. But there is some Peukert co-efficient for Li, although it is much closer to unity.

[blackrose]

11 hours ago, nicknorman said:

By SoC. Which is what I do for my caravan. “Everyone” was not telling you to use voltage, I originally advised you to use SoC and helped you set it up. At some point you decided to change to voltage, based on a majority cry - a majority most of whom probably didn’t have Li batteries!

 

Ian is right that since the charging is all low fractional C, using voltage sort of works in a rather imprecise way. But because it works in an imprecise way there is no “right answer” to the correct voltage threshold, especially as the charge rate is quite variable (cloudy vs blue sky day). All of which is why I use SoC. But yes using voltage also sort of works in an inexact way.

 

Yes ok the majority here were telling me to use voltage.

 

The problem is that SOC on my battery monitor tends to indicate 100% far too early while battery voltage suggests it's much lower. 

[David Mack]

7 hours ago, IanD said:

However the SoC errors can build up over time because no BMS current integrator is perfectly accurate, so there's a (hopefully!) slow drift of the SoC away from the actual value -- but 1% per week SoC drop like mine (1Ah per day) can be ignored, that's only a drop from 99% to 95% (at 54V) over a month before the next balancing cycle.

Is that 1% per week drop due to some small current being drawn by permanently on equipment? Or does it still happen with the batteries isolated?

I bought two Fogstar batteries in their Black Friday sale and I have yet to fit them in the boat. They arrived with 50% and 51% charge according to the Fogstar app. Checking again today I get the same figures, so apparently no perceptible drop in SoC in about 4 months. 

I was thinking I should probably put them on charge (which would mean buying a suitable charger), but it looks like I don't need to.

[GUMPY]

47 minutes ago, blackrose said:

 

Yes ok the majority here were telling me to use voltage.

 

The problem is that SOC on my battery monitor tends to indicate 100% far too early while battery voltage suggests it's much lower. 

In which case your monitor is set up wrongly.

Mine is spot on.

Try these for a Victron monitor 

Obviously the battery capacity is different 

 

 

Edited March 29 by GUMPY

[nicknorman]

59 minutes ago, blackrose said:

 

Yes ok the majority here were telling me to use voltage.

 

The problem is that SOC on my battery monitor tends to indicate 100% far too early while battery voltage suggests it's much lower. 

If you post the settings of the BMV, I can comment and suggest.

[BEngo]

33 minutes ago, David Mack said:

I was thinking I should probably put them on charge (which would mean buying a suitable charger), but it looks like I don't need to.

The Fogstar xiaoxang or JBD BMS  does not detect self discharge, or low discharge currents.  Self discharge can be 1-3%.  IIRC Fogstar recommend you give the batteries a charge every three months or so.

 

If you would like to borrow my spare charger you are welcome.  Let me know and I will bring it to the basin this weekend. It is a 10 A charger but that is enough for a refresh charge.

[IanD]

13 hours ago, David Mack said:

Is that 1% per week drop due to some small current being drawn by permanently on equipment? Or does it still happen with the batteries isolated?

I bought two Fogstar batteries in their Black Friday sale and I have yet to fit them in the boat. They arrived with 50% and 51% charge according to the Fogstar app. Checking again today I get the same figures, so apparently no perceptible drop in SoC in about 4 months. 

I was thinking I should probably put them on charge (which would mean buying a suitable charger), but it looks like I don't need to.

No, that's 1% indicated SoC drop per week when the batteries are charged back up to 54V every day by solar. After a balancing charge to 57.3V/100% SoC the reading the next day when float is reached is 99%, a week later it's 98% and so on. Every night the SoC drops below this, then goes back up when the sun comes out. So I know the battery state is the same every lunchtime, but the reported SoC drops by 1% each week -- which doesn't matter, given that this isn't used to control charging... 😉 

 

Since the reported SoC at a given battery voltage (54V) is dropping, the BMS must think that there has been a net current drawn from the batteries (about 1Ah per day) when it hasn't been, which is the opposite way round to what a parasitic load would do. So I suspect it's just an offset error in the BMS current reading, 1Ah per day is about 40mA error, with a 1000A current shunt this is 0.004%FS error (1 part in 25000) which is not unreasonable, that's a 2uV error (50mV shunt drop).

Edited March 30 by IanD

[wakey_wake]

My batteries are like https://www.amazon.co.uk/KEPWORTH-Lithium-Batteries-Rechargeable-Phosphate/dp/B0BNT23DT5

except at £379 that one seems priced from five years ago 🤑

 

On 28/03/2026 at 23:36, nicknorman said:

Trying to control Li solar charging by means of voltage basically doesn’t work. For example on a strong sun day with big panels and a small battery, you might reach 14.2v whilst the SoC is still quite low. The high current into a smallish battery means that only holding the 14.2v for an hour or two will get you to 100%.

 

By comparison with a big battery and small panels/weak sun, by the time you get to 14.2v the SoC will be 99.9%.

 

it doesn’t matter hugely because LiFePO4 is fairly tolerant of being charged close to 100% every day provided you don’t hold high voltage and low current for long periods, but if you are trying to treat your batteries kindly, doing it on voltage alone simply doesn’t work.

I had simply planned to keep my LFPs on the plateau and try to avoid crossing either 'knee' in the SoC:voltage curve, and to do this by stopping charge around 14V. By the shape of the curve, going above gains very little energy storage.

This is somewhat reinforced by the manufacturer's instructions (voltage range 10V~14.6V for this one here, I think the others are the same) and the practice of their included (cheap and cheerful) charger, which is to apply up to 14.6V at up to 7A, into a three pin 'aviation'-style plug in the side.

 

However as @nicknorman is warning not to, then I do want to hear what can happen. Better to find my plans are bad before I follow through all the way, eh?

 

I have observed that after a "full" charge the voltage will sag a little. What I haven't tried is doing this repeatedly and counting how much energy I've added after the initial "full" - by whatever voltage level. That sounds tedious enough to automate, whereas the discharge curve I plotted was adhoc by drawing about 25A off it and taking readings every few minutes.

 

I've never observed these batteries to get even slightly warm during charge or discharge, and that must partly be because I don't go much past -0.5C / +0.25C , but it does raise the valid question: if I can "fill" them by voltage and then put more in the next day, where did that energy go? Does it just mean the battery has more capacity if I'm patient with the charging, or am I stressing the chemistry beyond what is advisable?

 

It does appear that care of LFPs is not as well understood as the manufacturers might have us believe.

 

On 29/03/2026 at 11:14, IanD said:

Which is why the LFP battery and BMS manufacturers invariably make charging decisions on measured voltage -- with adjustments for charging current -- not the calculated SoC, because this is subject to error.

 

I had been relying on voltage for safety and estimates of my coddling of the battery.

 

I have the hardware for charge counting but it's not together. I believed it's only for my convenience and amusement.

[David Mack]

22 hours ago, BEngo said:

The Fogstar xiaoxang or JBD BMS  does not detect self discharge, or low discharge currents.  Self discharge can be 1-3%.  IIRC Fogstar recommend you give the batteries a charge every three months or so.

 

If you would like to borrow my spare charger you are welcome.  Let me know and I will bring it to the basin this weekend. It is a 10 A charger but that is enough for a refresh charge.

PM sent.

And for my next question: I need to give the Fogstars a top up charge.  I assume the SoC won't reset correctly unless I fully charge them, but as I won't be using them after charging, I want to leave them charged to somewhere between 30% and 50% (according to the Fogstar FAQ) so how do I know how long to charge them for?

[IanD]

1 hour ago, David Mack said:

PM sent.

And for my next question: I need to give the Fogstars a top up charge.  I assume the SoC won't reset correctly unless I fully charge them, but as I won't be using them after charging, I want to leave them charged to somewhere between 30% and 50% (according to the Fogstar FAQ) so how do I know how long to charge them for?

You don't. You would have to charge them up to 100% and do cell balancing, then discharge them by about C/2.

 

Alternatively, since keeping them for long periods at 13.5V float probably only reduces the lifetime from 200% of the lifetime of the boat to 180% of the lifetime of the boat, discharge them a bit after charging to 100% SoC and then stop worrying about it... 😉 

Edited March 30 by IanD

[wakey_wake]

27 minutes ago, IanD said:

200% of the lifetime of the boat to 180% of the lifetime of the boat

Surely "20% of the lifetime of the boat to 18% of the lifetime of the boat" unless the blacking's not done?

 

(Mine's >30 years old already, it's trying to catch up with me!)