Battery SOC vs Internet myth

Grumpy_G

In another recent thread the "50% rule" came up, which recommends to not discharge a lead acid battery below 50%. There are quite a few tables floating around the Internet that show the relation between the battery SOC and battery voltage. Based on these tables 12.0-12.1 V indicates 50% charge and often people state they don't discharge below 12.1V. Well, I've got bad news: These tables are for a specific load and different loads yield different voltages. See table below shamelessly taken from here: http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf



C is the capacity of the battery (bank). The graphs show that depending on discharge current, 50% SOC can be over 12.4V or below 11.2V. Conversely the lower the discharge rate, the lower the SOC is when the voltage reaches 12.1V. For example a 200Ah battery bank that only delivers 2A (C/100) is at 20% SOC when it reaches 12.1V. 

Bottomline: To make a somewhat educated guess at the SOC the battery discharge rate based on capacity and current needs to be taken into account. Alternatively use a gizmo that measures actual Ah delivered by the battery. 

Discuss ! 

Sharon_is_SAM

I learned this from trying to run down our battery by having the fan on high.  Due to the high load, the actual B capacity is less.  You will see a discrepancy with % of discharge and Ahr that is not logical.  High discharge current changes the available capacity value.  If the discharge is slow, you see a higher capacity than normal.

Voltage is not a good indicator under active discharge.  Shunted monitors actually work better on discharge.  

Mickerly

Agreed, I found the same thing while looking at choices for batteries. Unfortunately, the 'gizmo' that reads amps has a constant draw. It's a small draw, but all the little ghost draws do add up.

MuttonChops

Isn't the voltage measurement method of SOC . . . .
To be completed with the Battery at rest and No Load.
i.e.  Cut-off switch in Off position and any solar disconnected.
         Then you measure the terminal to terminal voltage after 30-minutes of 'full off'
          Those voltage vs SOC tables then are fairly correct.

Issue is few of us are willing to fully disconnect the battery.

So AHr Meters are a far better method to estimate SOC.

Grumpy_G

MuttonChops said:

Isn't the voltage measurement method of SOC . . . .
To be completed with the Battery at rest and No Load.
i.e.  Cut-off switch in Off position and any solar disconnected.
         Then you measure the terminal to terminal voltage after 30-minutes of 'full off'
          Those voltage vs SOC tables then are fairly correct.

Issue is few of us are willing to fully disconnect the battery.

So AHr Meters are a far better method to estimate SOC.

Good point, and I thought so too. However if you look at the graph, 10.5V as per the widely used SOC tables can't be the no-load voltage as it is in between the discharge curves for various loads. Digging around I found this table which shows the no-load voltage at 0% charge being in the 11.8V range depending on temperature. Which makes sense as that voltage is above all of the discharge curves. 

http://wiki.orbswarm.com/index.php?title=File:Pb_battery_state_of_charge.png

Dutch061

The problem is known as the "Peukerts Law".

Peukert's law, presented by the German scientist Wilhelm Peukert in 1897, expresses the capacity of a battery in terms of the rate at which it is discharged. As the rate of discharge increases, the battery's available capacity decreases. The greater the discharge rate, the lower the delivered capacity.

This can be observed when the load is removed and you see the voltage increase. 

Brad