Good Video Explaining The Puekert Effect

[tictag]

I thought this video explains the Puekert Effect very well, including the significant differences between lead-acid and lithium-ion.

 

[snoobler]

Have a nice vacation, stranger?

 

[tictag]

Starting college again after 31 years has been ... taxing; not so much free time on my hands. I'll be back on form soon enough!

 

[gnubie]

I'm not sure I could handle going back to full time study myself. ?

 

[Sabre36]

If Victron would only add a Peukert compensation for "low current" (slower discharge than the 20 hour rate) to their BMV battery monitors, not just high current Peukert compensation, we'd be getting better accuracy for SoC..

 

[snoobler]

Starting college again after 31 years has been ... taxing; not so much free time on my hands. I'll be back on form soon enough!

We're close to the same age, and I can't imagine something that would compel me to go back to school...

Good on you.

If Victron would only add a Peukert compensation for "low current" (slower discharge than the 20 hour rate) to their BMV battery monitors, not just high current Peukert compensation, we'd be getting better accuracy for SoC..

This isn't a Victron thing. Peukert doesn't work that way. It's an exponent. It's only designed to find reduced capacity due to higher current.

You could enter your own by using a longer C period.

use https://planetcalc.com/2268/ to calculate the exponent for your situation between C?? and C20.

Enter the C?? capacity instead of C20 and the calculated exponent.

Or you can just accept the more conservative SoC based on C20 and be content in the knowledge that you have more than it says.

 

[Sabre36]

We're close to the same age, and I can't imagine something that would compel me to go back to school...

Good on you.



This isn't a Victron thing. Peukert doesn't work that way. It's an exponent. It's only designed to find reduced capacity due to higher current.

You could enter your own by using a longer C period.

use https://planetcalc.com/2268/ to calculate the exponent for your situation between C?? and C20.

Enter the C?? capacity instead of C20 and the calculated exponent.

Or you can just accept the more conservative SoC based on C20 and be content in the knowledge that you have more than it says.

I am well versed in what Peukert is. It is a "thing" in many Ah counters, including Victron, that take Peukert into consideration to aid in determining the SoC of the battery. Sadly, the algorithms in most of them, only account for high rate discharge Peukert effect, not for low current discharges.

 

[fat_old_sun]

I thought this video explains the Puekert Effect very well, including the significant differences between lead-acid and lithium-ion.

Responding to all Peukert's law relating postings & comments on this website and not necessarily to this post.

There's a lot of misunderstanding as to what Peukert's law is why is it is (ir)relevant in many instances. Hope this write up clears it up.

What is Peukert's Law: simply stated, the law shows a correlation between the rate at which an electro-chemical battery is discharged and its 'useful' capacity. Higher the discharge rate, the lower the effective capacity.

Physics based reason for Peukert's law: limited mobility of ions in the electrolyte and its effect on the chemical reaction rate. During high discharge rates, the electrolyte in the region around the plates gets severely depleted. This causes a depression in the cells electrical potential. When the discharge rate is reduced (or goes to zero), diffusion of ions from the undepleted zones causes the cell potential to increase and the 'lost' capacity is regained. A battery's is considered fully discharged when its voltage drops below some arbitrary threshold - typically 10.5V for 12V lead acid batteries. There is no permanent loss of capacity since the reactants (Pb, PbO, H2SO4, e-) are all still in the cells.

Peukert's correction is most useful in applications where a battery is consistently discharged at a higher than the name-plate specified rate. USe Peukert's correction to derate the capacity for the expected discharge rates. It is least useful in instances where the discharge rate is usually small (running LED lights, CO & propane detectors for example), with brief periods of high discharge (5-10 mins of running a microwave oven). In this case, Peukert's correction will underestimate the remaining capacity. It would be as useful as the file download progress indicator on MS Windows - goes from 30 seconds remaining to 4 hrs remaining & back to 30S as the download speed fluctuates.

 

[tictag]

@fat_old_sun, nice write up.

I think I have fallen fowl of this misunderstanding before. I mean, I've always known that if you draw more [than the C20 rate] then you get less, but I have been guilty of extended this further to include heavy discharges where, for example, you draw 100A for 0.5 hours (50Ah) you will have actually 'withdrawn' say 75AH from the battery. But in fact this is a temporary effect because, as you say, "the reactants are all still in the cells" so I guess after some recovery time [a 100AH] battery will be back at 50AH remaining.

I do wonder, though, if I discharge the hell out of my battery but then stopped at say 10% SoC (peukert corrected coulomb count), would my battery's SoC then increase over time? I guess the answer is yes, though this doesn't seem intuitive.

Love the download progress indicator analogy. Very true!

 

[fat_old_sun]

@fat_old_sun, nice write up.

I do wonder, though, if I discharge the hell out of my battery but then stopped at say 10% SoC (peukert corrected coulomb count), would my battery's SoC then increase over time? I guess the answer is yes, though this doesn't seem intuitive.

Yes, the SoC would increase over time, as would the battery's voltage. And the (resting) voltage is a good indicator of SoC when temperature corrections are applied.

 

[snoobler]

Yes, the SoC would increase over time, as would the battery's voltage. And the (resting) voltage is a good indicator of SoC when temperature corrections are applied.

Why would the SoC raise? If the SoC is based on voltage, it's not accurate, and you're the only one on hear that thinks voltage alone is a good indicator of SoC. The only way I'll agree with you is if it has been at rest for several hours.

 

[fat_old_sun]

Why would the SoC raise? If the SoC is based on voltage, it's not accurate, and you're the only one on hear that thinks voltage alone is a good indicator of SoC. The only way I'll agree with you is if it has been at rest for several hours.

It appears that you overlooked the word resting in my response. Temperature corrected resting voltage is a good indicator of a battery's SoC. However, it not a practical method for determining the SoC of a battery in use.

 

[snoobler]

Not overlooked at all. I even quoted it. "Resting" simply means no current is flowing. The "resting" voltage of a 12V can change substantially over time, and that change is NOT dependent on temperature. "Resting" voltage is only meaningful after several HOURS of rest.

 

[fat_old_sun]

Not overlooked at all. I even quoted it. "Resting" simply means no current is flowing. The "resting" voltage of a 12V can change substantially over time, and that change is NOT dependent on temperature. "Resting" voltage is only meaningful after several HOURS of rest.

if the voltage is still changing, it simply means that the battery has not fully rested. go for a long walk & re-measure. it's not hard!

 

[fat_old_sun]

An electro-chemical cell's electrical potential, electrolyte concentration and its charge capacity are very closely related. I'm happy to explain this relation, but it would involve some advanced chemistry & thermodynamics concepts. When a cell is being discharged, the system is not in equilibrium. There's a spatial variation in concentration of the reactants & by-products. The depression in the cell's voltage is a reflection of this. When the discharge stops, the system slowly moves towards an equilibrium and the cell's potential recovers. As long as the reactant concentration is not zero, there's some current that can be extracted. However, this current may not flow with the 'electrical pressure' (voltage) to be of any practical use.

 

[snoobler]

I've referred to that phenomenon as "chemistry lag" for years in other applications.