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lenarda avatar image
lenarda asked

Why isn't SoC adjusted with load, according to Peukerts Law?

This should be simple enough.

Peukerts law states that the larger the instantaneous load, the smaller the available total capacity.

Ergo, at a given depth of discharge, when the instantaneous load increases, the displayed SoC should be reduced (assuming Peukerts exponent is >1), as the heavier load means a lower available remaing capacity at the new discharge rate before reaching min cell votlage

So - why is this not the observed behaviour? Ever with Peukerts exponent st at max value, my unit shows no change in displayed SoC, even with loads as great as 1C.

This cannot be correct behaviour. It results in scenarios where I have discharged large amounts of charge (~80%) at C20 rates, then increased the load to 1C, only to go under minimum cell voltage with 20%+ still still on the gauge - because it hasn't compensated for the load-induced reduction in total usable (and thus remainaing) capacity - but rather incorrectly treats the SoC as constant - as if the exponent is =1.

Why is this the case?


PS - strangely the "Time to go" does seem to reduce based on instant load....but not in line with expected reduction based on the applied exponent. Even more strangely, changing the exponent to 1, results in the remaining capacity reducing - which makes no sense at all, and pe=1 is an ideal battery with maximum available capacity under all loads.

BMV Battery MonitorSOC
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lifeingalicia avatar image lifeingalicia commented ·

My understanding is SOC is a snap shot in time. As loads are fluctuating there is not much milage in calculating a future. Time to go - may use present or historical based curves or adjusted future predictions - still it is is a prediction as the future is unknown.

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lenarda avatar image lenarda lifeingalicia commented ·

That 'snapshot' in time is still relative to the avaialble total capacity, which is in turn dependant on the load at the point of that 'snapshot'. Its still entirely applicable are relevant as an instantaneous metric.


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2 Answers
jetlag avatar image
jetlag answered ·

Are you talking about lead-acid batteries? Because lithium batteries do almost not show the Peukert effect.

But independant from the battery type, what Victron gear you refer to? Do you use a smart shunt or a BMV? These components can count the amperage (coulomb count) what makes them quite accurate. I rember, that in one of the battery monitors you can even adjust the peukert factor.

So please give us some more details about your battery and your setup at all.

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lenarda avatar image lenarda commented ·

It's obviously above the smartshunt and bmv lines. Peukerts Law does apply to lithium chemistries - they just have a different exponent. This has nothing to do with my specific setup - it's a general problem with the hardware that is present regardless of the individual setup

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Alex Pescaru avatar image
Alex Pescaru answered ·

Starting from the assumption that you are right and it's a bug inside the SOC computing algorithm...

If this is the case, then it will be addressed, for sure, at some point. No respectable manufacturer will leave things like that.

But in your case and configuration do you have something that really depends on this Peukert exponent? And if this Peukert dependence is not properly followed, it will have serious consequences?

Or is just a simple annoyance for you, considering your experience with this?

Because, you know, we live in a world full of things that work on batteries - laptops, phones, cars, etc - where all their functioning is governed by this Peukert law, but almost no one really get involved in its details.

When the battery gives the signs that it's becoming too old or is malfunctioning, we just change it and that's it. At most we are trying to use one with a better reliability, efficiency and endurance.

But all thing considered, I feel you and without wanting to be patronizing and considering that I am not an English speaking person, I know that there is a saying: you'll catch more flies with honey than vinegar.

Am I wrong?

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Fideri avatar image Fideri commented ·

Just to add that the SOC figure can come from the batteries, the shunt or the inverter depending on what you have and your settings. It seems to me that that is relevant. I'm not implying that your main point is necessarily wrong.

F.

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