Lithium? The big con?

[autorouter]

My days of calculating formulae with exponents are long gone but fortunately Peukert calculators can be found online.
Peukert Number Battery Life Calculator

Sadly this calculator is like a lot of them, giving incorrect results if you just put in the information from the battery manufacturer. A 200Ah battery discharged at the 20-hour rate will discharge at 10A. Its 20-hour rate capacity is 20 x 10 = 200Ah. If you put those figures into the calculator, with a Peukert index of 1.25, you should get 200Ah. But it gives an answer of 112Ah. Which is nonsense.

The description says that you need to enter the 'theoretical amp-hour capacity when discharged at a rate of 1A', but does not give you any method of calculating that value. The manufacturer won't give it because the 20-hour rate at 10A is the only information they give you.

 

[Raul]

New cards arrived and Pi up and running I risked putting V 3.3 onit as Raul said that works and all good. My touch screen is broken but still works just got to program it in now.

Lenny HB , I'm running the 3.33 on Pi 3B+, with 7"screen from PI shop. Last time I found the file for the screen all on Github. Before that, I had a card from Paul, Two on Tour, but later on as I upgraded, I had to start all over again with the screen. Thanks to Paul I can now do SHH with putty and by miracle it works.

 

[Lenny HB]

Haven’t you just described the Charge efficiency factor rather than the Peukert’s exponent?

Ian

Probably but it is what you use to set the Peukert Exponent on the shunt.

 

[AdrianChen]

Probably but it is what you use to set the Peukert Exponent on the shunt.

I have the Peukert set at 1.05, I thought from reading the blurb this is supposed to manage the difference from using a very high vs low discharge, and I guess helps keep the shunts accuracy when we use the inverter at 2600w vs when we are in storage and using 8w.

When charging I thought the charge efficiency (99% on mine) was used in a similar way to compensate for losses in the BMS and chemistry of the battery - my lithiums warm up quite a lot when charging, even at 3-4amps that the solar is producing at the moment, and my old LA jobbies did too, with possibly a bit of gassing using energy as well.

So I simply thought that the Victron Shunt at least used Peukert for discharge, and Charge efficiency for charging. Other systems may be different?

In practical terms I found the standard settings are close enough given the variance in the vans solar yeild / usage etc, doubtless my settings aren't perfect, and to some extent I may loose on one and gain on the other! I'm not trying to understand if I am at 62.3333% or 62.5%, just if I have enough to make a coffee and get through the hair preparation going on in the back of the van!

 

[Lenny HB]

I have the Peukert set at 1.05,

Mine is set to 1.05 I was thinking of dropping it to 1.02 but as the shunt appears to be accurate to 1 - 2% I'll probably leave it alone.

Marked difference with Lithium is I set the shunt up first time results have been rock solid, never had the adjust the settings.

Previously with LA Gels it would take a week or two of tweaking the settings to get reasonable results, the best I ever got was 5% and that was difficult to achieve. It would drift out and need tweaking or a manual sync and you had to remember to reset the battery capacity each year for natural losses.

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[AdrianChen]

Mine is set to 1.05 I was thinking of dropping it to 1.02 but as the shunt appears to be accurate to 1 - 2% I'll probably leave it alone.

Marked difference with Lithium is I set the shunt up first time results have been rock solid, never had the adjust the settings.

Previously with LA Gels it would take a week or two of tweaking the settings to get reasonable results, the best I ever got was 5% and that was difficult to achieve. It would drift out and need tweaking or a manual sync and you had to remember to reset the battery capacity each year for natural losses.

I agree, lithium is much less work!

The best monitor I found for lead acid was the Smart Gauge, I never really understood how they worked, but they certainly did. https://www.smartgauge.co.uk/smartgauge.html - Don't work with Lithium though. They must have tested well because they were taken up by Balmar in the US who made expensive alternators and regulators - Sadly now part of Dometic.... https://balmar.net/products/smartgauge-battery-monitor/

 

[Lenny HB]

Lenny HB , I'm running the 3.33 on Pi 3B+, with 7"screen from PI shop. Last time I found the file for the screen all on Github. Before that, I had a card from Paul, Two on Tour, but later on as I upgraded, I had to start all over again with the screen. Thanks to Paul I can now do SHH with putty and by miracle it works. View attachment 996690

Yahoo, I'm up and running again, I was tearing my hair out yesterday. This morning with the help of Tobi's video I installed Kwind SuperHelper file and then found I could install the Display setup & Shutdown Monitor from Package manager, that made it a doddle.
Just need to make a backup to a USB stick & copy the SD card.

 

[Raul]

I came across a graph some time ago, and was showing the peukert effect vs rate of discharge. Up to 0.2C was negligible, or no peukert at all. This was done across few LFP manufacturers. At 0.3C showed a consistent 1.02 peukert value or above, and climbing to 1.05 at 1C. That's the bit I do remember, if your discharge is up to 0.2C a 1.02 is fine, but, if you have a smaller bank and you draw 0.3-0.5C, then the value needs increasing.
Same goes for the charging process, if your rate is low 0.1-0.2C 99-98% efficiency will work fine.
At the moment, after a very long months without a full charge, last week I plugged in to Balance. Energy gone in was a bit more vs energy reserve claimed by the shunt. Hence I upped the peukert from 1.02 to 1.03; the batteries are since 2019, and my large draws have increased. I left charge efficiency at 98% for now.

 

[autorouter]

I have the Peukert set at 1.05, I thought from reading the blurb this is supposed to manage the difference from using a very high vs low discharge, and I guess helps keep the shunts accuracy when we use the inverter at 2600w vs when we are in storage and using 8w.

When charging I thought the charge efficiency (99% on mine) was used in a similar way to compensate for losses in the BMS and chemistry of the battery - my lithiums warm up quite a lot when charging, even at 3-4amps that the solar is producing at the moment, and my old LA jobbies did too, with possibly a bit of gassing using energy as well.

As you say, the two ideas of charge efficiency and charge rate variation are distinct. The Peukert index accounts for the change in battery capacity as the charge/discharge rate varies. The charge efficiency is ratio of the charge out to charge in, at the same rate of charge/discharge.

There are some items like the battery internal resistance that have an effect on both of those. There are other factors that only affect one of them, like slow electrolyte dispersion and diffusion. So a higher Peukert index usually means a worse charge efficiency.

If you want the full story, in minute detail, it's here:

SmartGauge Electronics - Peukert's Equation - what it means to you

The Peukert calculator on the same website works perfectly, but involves downloading a spreadsheet onto your computer and running it, which is a step too far for most people - they just want a simple web page calculator that can be used in a browser.

 

[AdrianChen]

As you say, the two ideas of charge efficiency and charge rate variation are distinct. The Peukert index accounts for the change in battery capacity as the charge/discharge rate varies. The charge efficiency is ratio of the charge out to charge in, at the same rate of charge/discharge.

There are some items like the battery internal resistance that have an effect on both of those. There are other factors that only affect one of them, like slow electrolyte dispersion and diffusion. So a higher Peukert index usually means a worse charge efficiency.

If you want the full story, in minute detail, it's here:

SmartGauge Electronics - Peukert's Equation - what it means to you

The Peukert calculator on the same website works perfectly, but involves downloading a spreadsheet onto your computer and running it, which is a step too far for most people - they just want a simple web page calculator that can be used in a browser.

Yes - in the past I read quite a lot of stuff on their site - It's kind of boffin level in parts, but the stuff about battery wiring - alternator wiring - charger sizing etc is quite practical and helpful stuff, sadly mostly related to LA and older electronics.

I guess

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[AdrianChen]

I came across a graph some time ago, and was showing the peukert effect vs rate of discharge. Up to 0.2C was negligible, or no peukert at all. This was done across few LFP manufacturers. At 0.3C showed a consistent 1.02 peukert value or above, and climbing to 1.05 at 1C. That's the bit I do remember, if your discharge is up to 0.2C a 1.02 is fine, but, if you have a smaller bank and you draw 0.3-0.5C, then the value needs increasing.
Same goes for the charging process, if your rate is low 0.1-0.2C 99-98% efficiency will work fine.
At the moment, after a very long months without a full charge, last week I plugged in to Balance. Energy gone in was a bit more vs energy reserve claimed by the shunt. Hence I upped the peukert from 1.02 to 1.03; the batteries are since 2019, and my large draws have increased. I left charge efficiency at 98% for now.

Good stuff - thanks for posting it.

I'd like to think my settings are about right for when we are using the van, which is when it's most helpful, during storage the current flows are very small by comparison. The measure of success is there have been no times when I have run out of power, other than once when I forgot to turn the inverter off, and the fridge gobbled the batts down to 26% by the morning. Even then there was enough for coffee!

 

[autorouter]

I came across a graph some time ago, and was showing the peukert effect vs rate of discharge. Up to 0.2C was negligible, or no peukert at all. This was done across few LFP manufacturers. At 0.3C showed a consistent 1.02 peukert value or above, and climbing to 1.05 at 1C.

Not very convinced about that. If you need to adjust the Peukert index to allow for varying discharge rates, that just shows you got the Peukert index wrong to start with. Have you got a link to that graph? Given that a lot of websites are very slapdash when dealing with Peukert effect, it may well be one of those.

On the same website as I linked to before, there is another spreadsheet that calculates the Peukert index from two pairs of capacity/time measurements. For example, if the 20-hour rate gives a capacity of 100Ah (ie 5A), and the 5-hour rate gives a capacity of 75Ah (ie 15A), then the Peukert index is 1.26.

 

[Raul]

Not very convinced about that. If you need to adjust the Peukert index to allow for varying discharge rates, that just shows you got the Peukert index wrong to start with. Have you got a link to that graph? Given that a lot of websites are very slapdash when dealing with Peukert effect, it may well be one of those.

On the same website as I linked to before, there is another spreadsheet that calculates the Peukert index from two pairs of capacity/time measurements. For example, if the 20-hour rate gives a capacity of 100Ah (ie 5A), and the 5-hour rate gives a capacity of 75Ah (ie 15A), then the Peukert index is 1.26.

If you look deep into original equation, not the reformulated version, it asks for 1A draw, exactly at what lithium gives true capacity. Try separate the lead behaviour from Lithium. Smart gage have theoretically proved right that lead needs calculated differently.
However those values are still TABULATED figures, and in real life, measured figures do change.
The graph I mentioned was based on measured discharge rates, time, and energy needed to be replaced. You can't beat that with any tabulated formula. There is a difference, even on same chemistry, but different manufacturers. Hence, even a different built cell, it will have an impact on efficiency.
Wether the graph was flawed or true, I wouldn't know for sure, but, what made me sway towards it, was measured values. You are right in being skeptical about it, but until I come across something better, I'll stick with it for now.
Also, my personal experience with lead deep cycle does not ring true with the peukert claimed at 1.25, but more like 1.35. I do think the peukert exponent it is not a fixed value, but dynamic in line with battery resistance and degradation. It does have a theoretical base line, but real measured value it changes.
And I'm still wiling to learn, I'm open to consider any study you may find and wish share. If I come across that graph again, be sure I will post it.

 

[mikash]

Not very convinced about that. If you need to adjust the Peukert index to allow for varying discharge rates, that just shows you got the Peukert index wrong to start with. Have you got a link to that graph? Given that a lot of websites are very slapdash when dealing with Peukert effect, it may well be one of those.

On the same website as I linked to before, there is another spreadsheet that calculates the Peukert index from two pairs of capacity/time measurements. For example, if the 20-hour rate gives a capacity of 100Ah (ie 5A), and the 5-hour rate gives a capacity of 75Ah (ie 15A), then the Peukert index is 1.26.

I think it is well westablished that the Peukert number is just an approximation and the reality is often quite different. Even a same battery will have different Peukert number when measured at different stages of its lifespan and it is also well known that with high enough currents (not relevat to our use case) Peukert is nigh useless at predicting battery behaviour.

https://www.researchgate.net/profile/Stelios-Ioannou/publication/301496300_Battery_Capacity_and_Discharge_Current_Relationship_for_Lead_Acid_and_Lithium_Batteries/links/59d7171ba6fdcc52acab07d4/Battery-Capacity-and-Discharge-Current-Relationship-for-Lead-Acid-and-Lithium-Batteries.pdf

That is one example of a paper where a variable Peukert was found at different discarge rates (and a more complex, more accurate formula proposed) for lead acid batteries but not lithium batteries. There is a lot more, about different batteries, and often contradicting each other. So I don't really doubt that some paper exists where variable Peukert has been found for lithiums as well (in fact the sources used in the above paper hint at at that too). It is a complex matter and the Peukert's law is just an empirical model with no exact theory and physics behind it, so some inaccuracy is to be expected.