Here's some more fuel for the charger control discussion. I think I posted this a year or so ago. This is basically the battery characteristic my charger controller uses to measure SOC. The circuit is described in a link in the for sale listing on elec-trak.org|
From the Battery University website:
State-of-charge reading based on terminal voltage
The state-of-charge of a lead-acid battery can, to a certain extent, be estimated by measuring the open terminal voltage. Prior to measuring, the battery must have rested for 4-8 hours after charge or discharge and resided at a steady room temperature. A cold battery would show slightly higher voltages and a hot battery would be lower. Plate additions of calcium and antimony will also vary the open terminal voltage with calcium being a little higher than antimony. Furthermore, AGM has a higher voltage plateau than the flooded lead acid and the readings on Figure 3 may not apply for AGM systems. Due to surface charge, a brief charge will raise the terminal voltage and provide inflated state-of-charge reading. For example, a 30 minute charge could wrongly indicate 100% SoC if no rest is applied.
With sufficient rest and stable temperature, voltage measurements provide an amazingly accurate SoC estimation for lead acid batteries. It is important that the battery is free of polarization. If connected in a system, such as in a car, there are steady auxiliary loads, not to mention frequent starting and driving.
Note: The BCI readings apply to flooded batteries with antimony doping. Calcium will raise the voltage by 5 - 8%. Calcium is commonly used for maintenance-free lead acid batteries.
After charge or discharge, allow the battery to rest for a minimum of eight hours before assessing the state-of-charge by measuring the terminal voltage.
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