Real world power from stated voltage level

softdown

I'd think there is a bell curve, of sorts, that shows how much power is really available at various voltage levels. For example, the difference between 13 volts and 12.8 volts is almost nothing. Voltage above 12.8 is a "surface charge" and evaporates quickly. 

To me it seems like it is pretty easy to draw a battery down to ~12.5 volts. Then the "meat" of the battery power prevents farther voltage drop from occurring so readily. At the other end of the scale, any voltage below ~ 11.6 represents a battery that is pretty well dead. 

My suspicion is that ~80% of a typical 12 volt battery charge is found in between  ~12.6 volts and 11.6 volts. 

My other suspicion is that 12 volts is closer to 50% charge/discharge than 12.2 volts. But I'm a rabble rouser often seeking to challenge "established dogma".

softdown

This chart begs to differ. Perhaps they are right.  This seems to represent established dogma.

mike95490

Once you get past the residual surface charge, lead acid has a very linear discharge curve and voltage is a good indicator of SoC. Specific Gravity even better when configuring your system setpoints.

Estragon

I think the shape of the curve would also depend on the rate of discharge.  At high discharge rates, I'd expect apparent capacity to drop off faster at lower voltage / higher current.  At slow discharge rates, I'd expect the apparent capacity to be pretty linear, as in the chart above.

BB.

Here is an article about battery voltage vs current (charging and discharging, different C/x values):

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

-Bill

softdown

BB. said:

Here is an article about battery voltage vs current (charging and discharging, different C/x values):

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

-Bill

Awesome - I'll assume that typical home usage might approximate the C10 discharge values? I have no real idea on that one. So I picked an average. 

Marc Kurth

Voltages are design specific. I deal with AGM's that show 2.166c/cell @ 100% and 1.75v @ 0% SOC under load.That same battery at 0% SOC will read 1.93v as a standing voltage.

Example: If I drag a fully charged 12v battery down to 0%, it will read 10.5v at 0% SOC under load. almost regardless of the size of the load. Extremes like C/0.5 or C/1000 can be corrected for. Remove the load and wait, and that same battery will rebound to 11.6v over time. It is still a dead battery at 0% SOC. WAAAY too many people don't get this part! I can assure you that my customers know it well.

The operating voltage at various loads, charge level and temperature is published data from at least one manufacturer that I work with.  

Properly designed off grid systems allow some level of autonomy, generally 2-3 days depending upon specific needs. The C/1 to C/10 rating would be more tailored to a UPS or industrial type application.

Edit to add a rough chart to help explain the concept

Marc

softdown

13 volt AGMs? Huh. Did they juice up the SG? Or magic pixel dust? 

Marc Kurth

softdown said:

13 volt AGMs? Huh. Did they juice up the SG? Or magic pixel dust? 

You would have to call Concorde to ask about magic dust. Been the same for at least 14 years that I know of and verified by my own experience. Concorde is a respected aircraft battery manufacturer and very engineering oriented. I am not a battery engineer, but I know what their batteries can and cannot do, and how they perform in real life applications.

BB.

C/X energy design really depends on your electrical needs... Just as an example, say you want to use power in the evenings, 5 hours per night, for 2 nights (poor weather during the day) and 50% maximum discharge... That means 10 hours  running * 1/0.50 max discharge = 20 hour discharge rate.

When you run a refrigerator... Say 1/3rd duty cycle (or 20 minutes per hour) for 2 days/50% max discharge:

• 1/3 rd duty cycle * 24 hours = 8 hours per day running
• 8 hours * 2 days storage * 1/0.50 max discharge = 32 Hour discharge cycle (to run a fridge, 2 days, 50% discharge, 1/3rd duty cycle)

If you run something that draws a lot more power (say a well pump) that runs for ~1 hour per day at 1.5 HP--That is a much harder draw on the battery bank--You are running the pump for 2 hours over 2 days/50% discharge:

• 2 hours running * 2 days * 1/0.50 max discharge = C/8 discharge rate...

Which is about the maximum suggested continuous draw for a FLA battery bank... But the problem is the starting surge... Which can be 5x the running power which would be a ~C5/8 = C/1.6 draw--Which is really too much draw for a FLA battery bank... Either a larger bank (C/2.5 to C/5 draw) or a different chemistry/construction--AGM, Li Ion, etc...

If you have 24 hour per day loads, 2 days storage * 1/50% discharge * 24 hours = 72 hour discharge rate...

-Bill