karnes314 avatar image
karnes314 asked

Question on readings from smart shunt

Hi - I was wondering if someone might be able to help me understand the SOC reading and how its calculated. I have a 100Ah LiFePO4 battery. I believe I've set all the correct settings to reflect the battery type per the manual (See attached), but I'm not sure I understand the SOC number. If, I've used 73.7Ah of the 100Ah capacity, so that should be ~26% SOC. Looking at the documentation for my battery, a voltage reading of 13.0 is ~30% SOC. Is the shunt "smart" enough to tell from the voltage or other data that the battery might actually have a little more than 100Ah capacity (this is the first discharge cycle since initially charged, so there is no history to draw from). Any info or pointers on how the numbers are being calculated or what to expect over time would be greatly appreciated.

Thank you!


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2 Answers
wkirby avatar image
wkirby answered ·

Battery Voltage is not used in the BMV's SOC calculations at all. Using Voltage to estimate SOC is a very coarse method and is dependant upon so many different factors that it's almost useless.
The BMV calculates SOC by counting the energy going out from and into the battery.

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karnes314 avatar image karnes314 commented ·
Yup definitely agree that voltage is the last thing I'd use if there is other/better data available, but that still leaves me with what is it using? If I've used 73.7/100Ah that doesn't sound like SOC should be 34%
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JohnC avatar image JohnC ♦ karnes314 commented ·

Hi @karnes314

Please post a pic(s) of your Smartshunt settings.

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karnes314 avatar image karnes314 JohnC ♦ commented ·

I did, they are above?

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karnes314 avatar image karnes314 JohnC ♦ commented ·
Not sure what's going on - seems some of my images are not getting attached properly... Here are my settings

Battery Capacity: 100Ah

Charged voltage 13.2V

Discharge floor 20%

Tail Current 4.00%

Charged detection time 3m

Peukert Factor: 1.05

Charge efficiency factor 99%

Current threshold 0.10A

Time-to-go averaging period 3m

Battery start synchronized On

State-of-Charge 34.2%

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JohnC avatar image
JohnC answered ·


Victron won't be using Ah directly to calculate SOC. Ah needs a Voltage at the time of discharge to be meaningful as energy.

The Charge Efficiency is fairly straightforward, for you every 99 Wh counted going to the batts needs 100Wh applied.

When discharging, the Peukert Exponent is applied. This is (sort of) a measure of efficiency, and if you discharge quickly you'll pay an SOC penalty. If more slowly than the batt C20 rate, you'll see more SOC remaining than you might expect. Might be hard to get your head around this, but you could google re Peukert, and there's a bit in the Shunt manual. After some discharge you can even change Peukert to see the difference it makes - this is reversible, so you can revert and it will recalculate.

In practice you can adjust either/both to control day-to-day 'drift' in the SOC at a fully charged state, but you'll need to gain some mastery of doing this. And you may even find the difference in batt Temperature between seasons requires a tweak.

This drift is of course controlled/repaired ultimately by the sync to 100% function, and you may have to set up the Tail % and Charged V settings to have that happen when you want it to.

Have fun..

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