peukert Exponent vs SOC

[Mike Wilson]

I almost understand peukert exponent.
I do do understand SOC.
I don't understand why when I changed the peukert exponent from factory of 1.25 to recommend 1.01 in my Victron Smart Shunt it lowered the SOC from 75% to 30%.
I can move it up or down with the same results using different values.

400 ah Amperetime Battery
Victron Smart Shunt
Victron Smart Solar Controllers

I just don't understand the correlation.
Thanks Guys.

 

[sunshine_eggo]

LFP doesn't suffer from Peukert to a significant degree.

To my knowledge, a Peukert exponent shouldn't change the SoC instantaneously.

I just confirmed the same thing happens on my BMV-702. Very odd.

Set to 1.01 (or 1.05 as Victron recommends), ensure your sync settings are: 0.2V below absorption voltage and 6% tail current.

Manually set SoC to your best guess or wait until the next sync.

EDIT: Apparently, you're not the only one confused...

Decreasing Peuker's coefficient decreases SOC? - Victron Community

community.victronenergy.com

Been an issue for 3+ years I guess.

EDIT #2: I think I understand it...

The unit tracks the "Ah consumed value" SoC is computed based on this value using Peukert, so if you change peukert, it changes the SoC%.

Once you have Peukert set, you shouldn't need to change it again, and SoC will be accurate once it's synced again and synched at least a few times a month.

 

[Hedges]

I haven't worked with Peukert other than maybe putting battery parameters into Sunny Island.

I think that under high current, changing Peukert parameter changes what SoC a given voltage represents.
I don't think it should affect SoC indicated by resting voltage.
I would think the inverter could use it to adjust low-voltage cutout, allowing high loads down to desired SoC.

I don't think Peukert has any effect on Gozinta/Gozouta amp-hours. It would affect watts in/out splitting between heat and useful charging/power.

 

[Mike Wilson]

LFP doesn't suffer from Peukert to a significant degree.

To my knowledge, a Peukert exponent shouldn't change the SoC instantaneously.

I just confirmed the same thing happens on my BMV-702. Very odd.

Set to 1.01 (or 1.05 as Victron recommends), ensure your sync settings are: 0.2V below absorption voltage and 6% tail current.

Manually set SoC to your best guess or wait until the next sync.

EDIT: Apparently, you're not the only one confused...

Decreasing Peuker's coefficient decreases SOC? - Victron Community

community.victronenergy.com

Been an issue for 3+ years I guess.

EDIT #2: I think I understand it...

The unit tracks the "Ah consumed value" SoC is computed based on this value using Peukert, so if you change peukert, it changes the SoC%.

Once you have Peukert set, you shouldn't need to change it again, and SoC will be accurate once it's synced again and synched at least a few times a month.

Thanks, glad I'm not the only one with the (issue) that's not really an issue, just a curiosity.
I'll set it up and wait for the sun to fill it up again.

 

[Mike Wilson]

Set to 1.01 (or 1.05 as Victron recommends), ensure your sync settings are: 0.2V below absorption voltage and 6% tail current.

I'm afraid I can't find how to set the sync settings.
I see how to set "Charged Voltage" but I don't think that would be 0.2 below absorption.
I did move the tail current to 6%.

 

[RCinFLA]

If the SoC instantly changed when you changed the Peukert setting it is a software bug. Probably resetting a register they should not be. I am sure they do not keep actual samples that make up Columb count as it would be too much memory so there is no possibility of recalculating new count based on new fudge factor.

Peukert is just a multiplier fudge factor to amp-secs of measured samples based on larger current draw having less battery efficiency, eating up more effective capacity of battery.

i.e., 100 amps for 10secs results in more battery capacity consumption compared to 10 amps for 100 seconds.

LFP cells are very efficient with low charge and discharge losses.
0.1 C(A) cell current about 99.0% effic.
0.2 C(A), about 98.5%
0.5 C(A), about 97.5%
1.0 C(A), about 95.9%

This is just discharge (one-way). LFP are very close to same efficiency on charging although absorb top off charging degrades efficiency for charging slightly. You can pretty much just square the one-way number for round-trip.

A good number for LFP for Peukert is 1.01 to 1.03 depending on your battery cable losses and average amperage draw across discharge capacity. Your battery cabling losses have an impact on the Peukert number with LFP batteries because LFP batteries are so good. Lead-acid batteries are much less efficient as discharge current increases, needing a greater Peukert number. Typically, manufacturer installs a lead-acid battery default value in firmware.

Attached is Victron document on the subject.

Taking the above LFP efficiency numbers for 0.1 C(A) and 0.5 C(A) in the calculator:


Like I said, the battery cable losses drives the 1.01 calculated number a bit higher for the exceptional LFP efficiencies.

 

[sunshine_eggo]

If it's using Ah and the time to achieve that Ah, i.e., average current, changing Peukert would change SoC.

 

[RCinFLA]

The main objective is to get the best fudge factor number so the battery monitor can run longer on 'dead reckoning'

If it's using Ah and the time to achieve that Ah, i.e., average current, changing Peukert would change SoC.

It should keep existing SoC and apply the revised Peukert to amp-secs collected going forward.

Next time battery gets full charge, the Columb counter is reset, clearing any cumulative errors caused by old, less accurate Peukert number.

 

[sunshine_eggo]

I'm afraid I can't find how to set the sync settings.
I see how to set "Charged Voltage" but I don't think that would be 0.2 below absorption.

Victron explicitly recommends this for solar charging.

I did move the tail current to 6%.

 

[sunshine_eggo]

Victron explicitly recommends this for solar charging.

I meant to include:

The default recommendation to set 0.2V below float is only applicable to reliable constant current/constant voltage charging, i.e., chargers that don't vary their current.

Using 0.2V below float with solar charging will regularly result in false synchronizations to 100% as the variable nature of solar charging can easily meet the float-based "charged" criteria before the battery is at or near 100%.

 

[RCinFLA]

Relying on taper off current for PV charge controllers doesn't often work well. The charge controller must recognize any drop off in charge current is not due to reduction in PV illumination level due to a cloud going by.

Most common PV charge controller method is to use timed absorb cycle. It may stop countdown timer if PV drops out but also has a raw maximum safety time limit for absorb time to prevent continuous situation of almost meeting absorb voltage due to overcast skys.

 

[Mike Wilson]

Relying on taper off current for PV charge controllers doesn't often work well. The charge controller must recognize any drop off in charge current is not due to reduction in PV illumination level due to a cloud going by.

Most common PV charge controller method is to use timed absorb cycle. It may stop countdown timer if PV drops out but also has a raw maximum safety time limit for absorb time to prevent continuous situation of almost meeting absorb voltage due to overcast skys.

Funny you mentioned the times absorption, once in awhile this setup will extend that 20 minutes just a little.
They did struggle to maintain the absorption voltage (I'm on a Sailboat and it's partly cloudy today) and eventually went back into bulk while I worked on my dinghy motor. When I got to look at it again it had went into absorption again 20 minutes this time and was in float.
Boating is an interesting life.

 

[sunshine_eggo]

Funny you mentioned the times absorption, once in awhile this setup will extend that 20 minutes just a little.
They did struggle to maintain the absorption voltage (I'm on a Sailboat and it's partly cloudy today) and eventually went back into bulk while I worked on my dinghy motor. When I got to look at it again it had went into absorption again 20 minutes this time and was in float.
Boating is an interesting life.

MPPT behavior is independent of the shunt settings. That's dictated by the MPPT settings and their interaction (do you have them in a VE.Smart network?).

If you have the MPPT set for LFP, they default to a 2 hour absorption @ 14.2V, float at 13.5V and 0A tail current.

If your MPPT dropped to 0A or to a different value you have specified, that will end absorption before the 2 hour mark. There are also cases where re-bulk may trigger a longer absorption time as well.

If you set to custom and are using their adaptive algorithm, all bets are off. Not recommended for LFP as the algorithm is targeted at optimizing the health of lead-acid.

Post settings including Expert?

 

[Mike Wilson]

MPPT behavior is independent of the shunt settings. That's dictated by the MPPT settings and their interaction (do you have them in a VE.Smart network?).

If you have the MPPT set for LFP, they default to a 2 hour absorption @ 14.2V, float at 13.5V and 0A tail current.

If your MPPT dropped to 0A or to a different value you have specified, that will end absorption before the 2 hour mark. There are also cases where re-bulk may trigger a longer absorption time as well.

If you set to custom and are using their adaptive algorithm, all bets are off. Not recommended for LFP as the algorithm is targeted at optimizing the health of lead-acid.

Post settings including Expert?

Expert is off.

 

[sunshine_eggo]

Expert is not off.

Expert settings are not displayed.

Please slide bar to right and provide those settings.

 

[Mike Wilson]

OK, I'm no expert so I didn't fiddle with them lol.

 

[sunshine_eggo]

You are set to fixed duration and 20 minutes. Given that your absorption is longer than 20 minutes, this means that it was unable to maintain absorption voltage after switching out of bulk - likely due to inconsistent input and/or loads.

Once it has transitioned from bulk to absorption, it stays in absorption even if it can't maintain the voltage. If it drops to the re-bulk voltage, then it re-bulks. I don't know for certain, but I believe that the absorption time is cumulative even if it re-bulks, i.e., 20 minutes at the absorption voltage, and it's charged.

Personally, due to the perceived inconsistent PV availability, Victron's default absorption time of two hours is likely preferred. It is important that batteries stay at elevated voltage long enough to allow for any needed cell balancing to occur.

 

[Mike Wilson]

You are set to fixed duration and 20 minutes. Given that your absorption is longer than 20 minutes, this means that it was unable to maintain absorption voltage after switching out of bulk - likely due to inconsistent input and/or loads.

Once it has transitioned from bulk to absorption, it stays in absorption even if it can't maintain the voltage. If it drops to the re-bulk voltage, then it re-bulks. I don't know for certain, but I believe that the absorption time is cumulative even if it re-bulks, i.e., 20 minutes at the absorption voltage, and it's charged.

Personally, due to the perceived inconsistent PV availability, Victron's default absorption time of two hours is likely preferred. It is important that batteries stay at elevated voltage long enough to allow for any needed cell balancing to occur.

I like to see it go into and complete absorption at least once a week, I'm thinking that'll keep the battery healthy enough. We're now in FL. Heading to the Bahamas soon and the short days are over so it'll probably be more than the once a week now.

 

[Mike Wilson]

For future reference.
Amperetime got back to me and said the peukert exponent that should be used is 1.05.

 

[hogback]

I meant to include:

The default recommendation to set 0.2V below float is only applicable to reliable constant current/constant voltage charging, i.e., chargers that don't vary their current.

Using 0.2V below float with solar charging will regularly result in false synchronizations to 100% as the variable nature of solar charging can easily meet the float-based "charged" criteria before the battery is at or near 100%.

Mind clearing something up for me? Wouldn't the voltage need to be over a certain point, AND the current to drop for the 100% to be synced? Wouldn't this avoid the false sync to 100% when the current drops due to low solar only?