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Battery SOC vs Voltage

I have a small 24v off grid setup (remote monitoring capable) for the last few months and when I checked the VRM this morning I noticed that the battery voltage had dropped down to 23.77 V. However the SOC still shows ~73% capacity on the batteries. I know voltage isn't always accurate especially under a load (currently a 24/7 ~1 A load on the batteries) but was just looking for clarification if I should be concerned regarding the voltage dipping below 24 V even if the SOC still shows 70%. It has been quite cold (-10 C) and snowy here for a few days, so not much charging time for the batteries, and the batteries are stored outdoors. I turned off the inverter remotely and dropped the load down to ~0.5 A (still have some essential DC loads) and the voltage has risen slowly to 23.86 V after about 30 min.


Should I be concerned? Is the smart shunt not reporting SOC acurately due to temp and or drift? (Last 100% sync was 9 days ago before cold temps)


System specs:

Batteries: Rolls FLA 6 FS GC HC (6V x 4 in series for 24 V) C20 Capacity of 235 AH

Smart Shunt monitoring batteries and SOC

~1000 W of panels

SmartShuntSOCbattery system voltage
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smallsolar avatar image smallsolar commented ·
Doing a bit more research, I am pretty sure that the SOC is reporting incorrectly due to the temperature. I need to add a temperature sensor, but I am currently using the aux input of the Smart shunt to measure the V of a secondary battery. Any suggestions on how to add a temperature sensor for the SOC to be more accurate while still maintaining the ability to monitor secondary battery voltage?


I have Venus running on an RPI and the charge controllers, smartshunt, and inverter are all connected to that.


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2 Answers
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JohnC answered ·

Hi @SmallSolar

The Shunt can't do both those things together. But you could try adjusting battery capacity yourself when it gets cold. I think Victron suggest 1% of batt capacity lost per degC below 25. This is ballpark stuff, as cold temp fouls up some of the other settings to a degree anyway. The result might worry, as SOC under discharge will be lower. But still, might be more truthful.

I use clones of your batts. This is just me, but 75% SOC is emergency level, genset time. Going without a sync for 9 days would have me beginning to doubt SOC completely. I don't like going under my 48V, but I've learnt to manage it (err, suck it up).

All chemical reactions slow in the cold. These batts are susceptible, so that low V under load won't go away. By all means accept it as normal, but not with a blindfold on. Check your data and be aware of the why..

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Alexander Bartash answered ·
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Related Resources

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SmartShunt Product Page

SmartShunt Installation Manual

SmartShunt Quick Installation Guide

SmartShunt Datasheet