Estimating SoC% from battery voltage for custom built monitor

[Xplode]

Hi Guys,
I've been custom building a solar system reporting/monitor solution for my off-grid cabins for around a year now. The electronincs poll the 4 solar charge controllers at the various cabins (all stand alone systems except for one that has two arrays: east, west). The parents that spend a lot of time there are not the most electronic oriented, and so I don't think they'll be climbing under the main cabin weekly to check the electrolyte gravity. Hence, I would like to include a line on the web display that estimates the state of charge each morning so we know approximately where they are at from the overnight usage.


This is the current calculation I'm using to grab a rough-guess SoC in the morning. 0% is based on the Crown Battery supplied charge voltage of 11.6V * 2 (24v bank), and the 100% is higher than their recommended number, but is based on what i've seen when no one is at the cabins. I figured the system is safer to report a lower SoC so they'll be more careful.


Battery SOC% Calculation
23.2V = 0%SOC
25.6V = 100%SOC
25.6V-23.2V = 2.4V Spread from 100% to 0%
25.6 minus Morning Battery Voltage (ie 25.1V) = 0.5V
2.4V spread - .5V = 1.9V
1.9V / 2.4V * 100 = 79.2% SoC



I would like to add a correction for battery temperature, since in the summer we have outside temps up to 35degC, and in the winter we have temps down to -30degC.


Any suggestions on how to be as accurate as possible with including a temperature correction? Is there a rough degC adjustment I can apply to the formula? Should I adjust the 100% SoC number with this temp-correction, or the read battery voltage?

Thoughts and suggestions please!

Thank you!

[Mike90250]

Here's a link to a variety of useful battery graphs

https://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf

Maybe some of them can guide you

[Xplode]

Thank you, that looks like an excellent resource. I'll give it a better read tomorrow and apply what I learn.

[LPG]

How are you handling voltage sag due to load?

[Xplode]

How are you handling voltage sag due to load?

I am not.

While it would be imperative if I was trying to get a highly accurate number, there is almost never any load early in the morning and that is the only time I want to take a snapshot. Even if the inverter is left on overnight accidentally (which it's usually not) AND the fridge happens to be running, we're only talking about 7-8 Amp draw on the batteries. I don't think that's going to put a lot of stress on the batteries and draw their voltage down. (They're 430Ah @ 20hr rated)