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Peter Hanslope avatar image
Peter Hanslope asked

Understanding Smart Shunt readout

I have x2 100 Ah Leisure Plus sealed lead acid batteries wired in parallel and have just installed a 500 A smart shunt to monitor battery usage. From the product description it suggests the best way to do this is through the State of Charge SOC readout. Following a installation I used the van for two night stop and had a reading 94% SOC but a battery voltage falling to 12.4V at which point I was minimising usage to protect the batteries.

From the above the SOC indicates I should be good to go for a significant amount of time but the this is not reflected in the actual battery voltage which clearly points to needing to put them on charge.

Apologies for my lack of understanding but hopefully someone can point me in the right direction

SmartShunt
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Matthias Lange - DE avatar image Matthias Lange - DE ♦ commented ·
Please add some screenshots from VictronConnect (Main screen and settings).

What other components do you have in the system?
Did you do a full charge cycle after the installation to synchronize the SOC?

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pwfarnell avatar image pwfarnell commented ·
How do you charge your batteries, do you have solar
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Peter Hanslope avatar image
Peter Hanslope answered ·

Yes did a full charge cycle on hook up overnight.

No other components victron-image.png


victron-image.png (174.7 KiB)
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cybermaus avatar image cybermaus commented ·
I suspect Mathias would like the settings also, specifically the battery settings. They are hiding behind the settings cogwheel top-right.
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Peter Hanslope avatar image
Peter Hanslope answered ·

The requested settings page

img-6841.png


img-6841.png (188.2 KiB)
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Kevin Windrem avatar image
Kevin Windrem answered ·

If you shut down the battery (specifically the shunt) the SOC will be reset to 100% the next time you power up. Battery SOC on reset should be turned off unless you can guarantee that the battery is fully charged any time you power up the shunt. This is hard to guarantee.


This does not explain what you are seeing necessarily.

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Peter Hanslope avatar image Peter Hanslope commented ·
Ok I can change that setting
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kevgermany avatar image kevgermany ♦♦ commented ·

I did this some time ago. It caused low battery alarms. I don't know if this was changed in later firmware.

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delf67 avatar image
delf67 answered ·

18.6Ah is approx 9% of your capacity, so the SOC should be reading 90/91%. That voltage looks pretty low for that amount of power drawn. How old are the batteries?

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kevgermany avatar image kevgermany ♦♦ commented ·
Battery capacity varies according to load. Usual manufacturer statement is at the 20 hour to discharge rate (C20).
There's a very low current draw here, so the calcs in the shunt will compensate and reflect a higher SOC than you calculate.
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Peter Hanslope avatar image
Peter Hanslope answered ·

Unfortunately I don’t know the age of the batteries as I’ve only had the van they are installed in for 12mths. One has the indicator disc that is still coloured green ie ok but I guessing this is a pretty crude indicator.


I’m leaning towards it’s the batteries but was trying to understand what the readings were before jumping in and changing them

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delf67 avatar image
delf67 answered ·

You're sure all of your loads are wired through the shunt? You haven't got anything else that is draining the batteries that the shunt is not measuring? 12.4V for a sealed lead acid battery should correspond to a SOC of about 60%.

I have a 48V system using SLA batteries that are a few years old and definitely below original capacity, and they read 50.6V @ 93%, so 12.63V/ battery.

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Peter Hanslope avatar image Peter Hanslope commented ·
Hi can you just provide info on how you relate the measured voltage to a SOC. eg you say that 12.4 should correspond to around 60% - that would be a big help in understanding the relationship between them thsnks
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cybermaus avatar image cybermaus Peter Hanslope commented ·

On a healthy battery, when idle (so not being charged or discharged), the voltage is a useable indicator. There are various tables on the web, and as long as you use one for the same chemistry they all fairly match.


But if you make the mistake of interpreting the voltage on a non-idle battery, you will indeed get weird results. Especially if the battery is not healthy, has a high internal resistance, the voltage may be different at even minor charge/discharge rates.


In other words, if you want to estimate SOC based on voltage, disconnect it and wait for it to stabilize, then measure with a multimeter.


Edit: picked up a ref. Just the first one google fed me, but it is probably correct.

https://octopusasia.com/battery-soc-to-voltage-chart/

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Peter Hanslope avatar image
Peter Hanslope answered ·

Single cable coming off the battery through the shunt so can’t see missing anything but will check.

Your point made about SOC should be around 60% is exactly why I’m scratching my head. Battery voltage clearly doesn’t align with the SOC. Would I be right in thinking the SOC is a calculated figure while the voltage is measured. Just trying to understand how the unit works - are the batteries at fault or something else

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pwfarnell avatar image
pwfarnell answered ·

As you have said that you charge with solar once out on your journey you should change the settings in the SmartShunt because the default ones can confuse low solar power with a fully charged battery and synchronise the SOC to 100% before the batteries are full so you think you have more capacity than you really do. With the default settings, a limited amount of solar can get the battery above 13.2V with a low current which resets the SOC to 100%

Set the charged voltage to 0.2V or so below the absorption voltage, so if the battery charges at 14.4V set the charged voltage to 14.2V. Set the tail current to 2%. This means that the solar has to get the battery up to full charge voltage before synchronisation to 100%.

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Peter Hanslope avatar image Peter Hanslope commented ·
Thanks I will setup as you suggest
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