Getting SoC from voltage

[RDaugherty]

Hey there.

I had a question for you guys.

Let's say we have a 24 Cell battery. How would I get a state of charge from the voltage only? I've heard about getting the SoC from specific gravity but surely there's someway to get a decent SoC from voltage purely.

So lets say that at 100% charge it has an overall voltage of 52.8v (2.20 volts per cell)

So if a battery has an overall voltage of 48.2v (~2.01 volts per cell) what is its SoC?

The math seems to be: 48.2 / 52.8 which comes to 91.2%. But to me this doesn't make sense because I would think the lower voltage of 2.00 vpc would equate to something more like 40-60% SoC.

Thanks for the help!

[Sunking]

OK but you will not like the answer.

The only time the voltage means anything is if the battery is Open Circuit (not connected to anything electrically) and has rested for many hours. Even then it is only a Ball Park and not accurate. Voltage on an operating system is pretty much useless and doe snot tell you much of anything unless the battery is about to be fully discharged.

I can take any new fully charged battery, apply a load, and the voltage will indicate the battery is completely dead. The Inverter will trip off line from under voltage when the battery is 100% fully charged. I can take the same battery completely discharged, connect a charger to it and the voltage will read above 100% charged up despite it is completely discharged.

It all has to do with the battery Internal Resistance, Surface Charge, and the fact it is a slow chemical reaction. So unless the battery is Open Circuit and rested, the voltage does not tell you much on its own. The only way is to use a temperature corrected battery hydrometer.

So if the battery is open circuit and you read 48.2 volts, your battery is less than 50% SOC and in desperate need of being recharged yesterday. A fully charged 48 volt battery will be roughly 50.1 volts, not 52.8 volts as that is the charger voltage.

[RDaugherty]

Alright, I got you. Yeah, I knew that it would only be an estimate and not a science. But did you get the 50% SOC via that graph meaning that's still just from the specific gravity reading and not just purely voltage?

If the case is just that you should never and can't really ever get a SoC from voltage purely than so be it I'll go simply off of the SG.

Now if that is the case will the SG ALWAYS = the voltage of the cell?

IE: If my SG rating is 1.285 at its lowest specifications and I see its at say 1.250 (Using the chart above and just replacing numbers) my SOC is always going to be ~80% charged? Because we have a variance of 35, and on the graph the 1.285 would = 1.277 and the 1.250 would = 1.238 roughly.

And what if the same battery has a SG of 0 (The hydrometer doesn't float at all and reads less than 1.200, so we'll call it water and the battery just came off charge) we know that the SoC is SUPPOSED to be 100%, but the SG doesn't show that. I know that if this is the case this battery is in bad shape and has either heavy sulfation or has has the acid boil out due to overcharging/over watering but I'm just trying to get a firm ground to base everything else off of.

[Sunking]

But did you get the 50% SOC via that graph meaning that's still just from the specific gravity reading and not just purely voltage?

If you meant from your voltage of 48.2 volts I pulled from the Graph. If that were on a rested battery should raise a Red Flag and make you get it on a charger.

Now if that is the case will the SG ALWAYS = the voltage of the cell?

If the SG is good, the voltage will be good, but just because the voltage is good does not mean th eSG is good. Just an indicator.

IE: If my SG rating is 1.285 at its lowest specifications and I see its at say 1.250 (Using the chart above and just replacing numbers) my SOC is always going to be ~80% charged? Because we have a variance of 35, and on the graph the 1.285 would = 1.277 and the 1.250 would = 1.238 roughly.

I think you are getting things mixed up. If you have a cell at 1.285 in a string and another at 1.250 you are in trouble and in need of an EQ charge.

And what if the same battery has a SG of 0 (The hydrometer doesn't float at all and reads less than 1.200, so we'll call it water and the battery just came off charge) we know that the SoC is SUPPOSED to be 100%, but the SG doesn't show that

It means your battery is a BOAT ANCHOR. It means at some point you poured out the electrolyte and refilled it with water. Actually I do see that from time to time. It is caused by excessive over charging boiling the electrolyte and causing it to spew out of the battery, then being replaced by water.

You use Voltage and SG to determine when it is time to equalize the battery. Read your battery Owners manual.

You can use voltage to determine th eSOC if you are willing to spend a day or two charting things out. READ THIS