48-volt battery bank SOC chart

[MountainmanBill]

I've looked all over the web only to find conflicting charts as to the SOC of 48-volt batteries. Anywhere from 50 volts being 100% to 52 being 100% SOC. I found some info on the manufactures (Leoch) website depicting a line graph that shows the battery's "Relationship of OCV and State of Charge (77 degrees F.)". The line shows "Battery voltage (V/cell)" being approximately 2.17 volts per cell. Assuming there are three cells per battery, a battery's 100% OCV SOC would then be 6.51 volts. 6.51X8=52.08 volts.

I found a chart that lists 100% SOC being 52.0 volts, 51.1 90%, 50.0 80%, 49.2 70%, 48.6 60%, 48.2 50%, 47.8 40%, 47.3 30%, 46.6 20%, and 46.0 10% SOC. The manufacturers line graph shows 50% SOC being 49.44, quite different than the chart numbers listed above.

Is the manufactures line graph accurate for real world conditions or are they laboratory conditions only? They are also for OCV. How am I to know the actual, or in the ballpark figure, of the battery bank when in use? What do Y'all do to get a reasonable assumption of your battery's SOC without shutting down your system and resting the batteries to get an accurate reading? And where can I find a real time/real world SOC chart for a 48-volt battery bank when it's load or charging rates are low?

 

[Troubleshooter]

It's not that simple, which is why you can't just find one chart.

What Is Peukert's Law and What Is Its Effect on Batteries?

Learn what the Peukert effect is, how it’s calculated, and which type of battery is least affected by this energy-draining phenomenon.

battlebornbatteries.com

 

[robby]

Post #5
48V LFP Battery Voltage vs SOC
And yes some charts are slightly different depending on the battery but this is a good starting place.

 

[crossy]

@MountainmanBill what battery chemistry are you looking at? Lead Acid, Li-ion, LiFePO4? All are different.

2.17V per cell would suggest lead-acid, just want to be sure.

 

[sunshine_eggo]

Is the manufactures line graph accurate for real world conditions or are they laboratory conditions only? They are also for OCV. How am I to know the actual, or in the ballpark figure, of the battery bank when in use? What do Y'all do to get a reasonable assumption of your battery's SOC without shutting down your system and resting the batteries to get an accurate reading? And where can I find a real time/real world SOC chart for a 48-volt battery bank when it's load or charging rates are low?

You can't. Voltage is a complete shit way to tell SoC.

Invest in a quality battery monitor. It removes 100% of the guesswork. If you get a victron smartshunt or BMV, they factor in the Peukert effect and can even factor in temperature effects on battery efficiency if the manufacturer makes that available. Incredibly accurate SoC for lead acid.

 

[robby]

You can't. Voltage is a complete shit way to tell SoC.

Invest in a quality battery monitor. It removes 100% of the guesswork. If you get a victron smartshunt or BMV, they factor in the Peukert effect and can even factor in temperature effects on battery efficiency if the manufacturer makes that available. Incredibly accurate SoC for lead acid.

It sure is crap but the one good thing is that under load it will give the Inverter the proper voltage to shutdown any battery usage and prevent damage.
That is why I used voltage mode and ditched the Sol-Arks own calculated SOC. Thank God I am now back to using a closed loop connection again, it is the best of both worlds.

 

[OffGridInTheCity]

One sure approach is to do a discharge test over the operating range you plan to use and plot the metrics / see how many kwhs you have.

 

[Steve_S]

Information for LFP / Lithium Iron Phosphate / LiFePo4 chemistry.
Peukerts Law does NOT Apply to LFP Chemistry.
LFP has TWO Voltage Ranges - Allowable & Working, just like all other batteries.
Allowable Voltage Range which is the voltage range that will not cause harm/damage. This is from 2.500-3.650 Volts per Cell (VPC).
! NB ! below 2.500 or above 3.650 results in damage to the cells.
Working Voltage Range which delivers your functional AmpHours. This is from 3.000-3.400Vpc. NOMINAL VOLTAGE (
50% SOC) = 3.200VPC *This is also the Voltage Range that Manufacturers test the cells at for capacity validation.
Voltage Readings are not the same as when under load or being charged, Static Voltage (no charge/discharge) is commonly referenced for SOC.
Temperatures also affect Voltage Readings. Nominal Temp is 25C/77F.

 

[MountainmanBill]

The batteries in question are Leoch 6-volt AGM 400Ah, wired in series for 48 volts. All I'm looking for is a guide to let me know where I stand on SOC. Most of the time I'm only running an upright freezer in an outbuilding that, according to the Kil-o-watt meter I had connected to the freezer for a five-day period, used 26 watts an hour on average, and a small heater that uses around 200 watts per hour when the temps drop well below freezing. So, can I run the heater or do I drain my water system that is also housed inside the same outbuilding? If I'm at 60% SOC I'd drain the pipes. If I'm at 90% I'll run the heater if it will be sunny the next day. Running the freezer that uses 26 watts per hour, I wouldn't think, is going to affect the meter's ability to somewhat accurately read the true voltage and give me a ballpark figure of where I stand on SOC. I'll look into the suggestions offered above. Thank you for them...

 

[PunchySunshine]

Attached sheet

 

[MountainmanBill]

The temperature overnight was 30 degrees F., so there was no need to run the heater in the outbuilding. The inside temperature of the outbuilding sits at 34.9 degrees at 7am. So the only appliances running overnight was the 26 watts-per-hour freezer, and whatever the inverter uses just to be turned on. 26 watts according to the manual. The SCC meter has been sitting at 49.5 volts for an hour now. Again, I would think that that reading would be somewhat accurate of the battery banks actual voltage given the low load placed upon it for the last15-hours since the sun went down. Using one of the insulated walls of the outbuilding is an enclosed shower. Running a heater in the shower room is going to be essential before I can turn on the water to that room today, but I'll shower in a cold room if I knew the batteries were closing in on 50% SOC. But how do I know what the SOC really is, or close to what it really is? Living in a forest with a very limited supply of direct sunlight on my panels per day I need to closely monitor the battery bank's SOC. Yesterday I made only around 2,600 watts. Help me out here fellas. Get me close to a usable SOC.
Again, 6-volt AGM batteries, wired in series for 48 volts.

 

[RV10flyer]

My 10 BMS’s get me close on SOC. I go by the lowest readings. If I ever go below 20% and need to run the Gen, then I’m buying more batteries and panels. I don’t need the worries.

 

[sunshine_eggo]

Again, 6-volt AGM batteries, wired in series for 48 volts.

Again,

You can't. Voltage is a complete shit way to tell SoC.

You can't will something impossible into existence just because you want it. If Leoch doesn't publish SoC to voltage curves for different current levels, you're up shit creek.

The only way you can accurately correlate SoC to voltage on your system is with... a battery monitor.

Furthermore, your freezer doesn't use 26W on average. It uses something like either 5W (powering the electronics) or 100W when it's running the compressor. It cycles the compressor on and off as needed. In such cold weather, it's likely not cycling on very often...

So you don't even know if you're pulling any current when you take a voltage reading.

The only thing you can do is find the charts for AGM and know that you are at or above any value you see in the chart, e.g.,

49.5/4 = 12.375V (6.19 for each 6V)

Given that you have 8 6V in a string, it's important that you confirm all 6V are within about 0.05V of each other, especially when at absorption voltage.