Monitoring the State of Charge (SOC) of the battery is a most important task with a solar system, but is difficult to assess accurately.
Here are the main methods.
Voltage: The voltage must be measured when the battery is ‘at rest’. This means that there is no charge going into the battery and no load going out. Ideally, the battery should be ‘at rest’ for 20-30 minutes before you measure the voltage. The voltage range is narrow, so you need to use a good digital multimeter.
Approximate values for a 12 Volt battery are:
Specific Gravity: You can use a hydrometer to measure the density of the electrolyte. This is prone to interpretation and inaccuracies.
When a battery becomes discharged, the electrolyte becomes lighter. When you charge the battery up, this lighter electrolyte will tend to float on top and give you a pessimistic reading. This is known as electrolyte stratification and is only overcome when the electrolyte is mixed up again by the bubbling action of a good charge.
Adding distilled water to your battery will affect the reading in the same way. There are also issues with how to read a hydrometer, and the quality of hydrometers. A dirty hydrometer can contaminate a battery.
Plasmatronics PL regulator: This monitors amp-hours in and out of your battery, and reads SOC as a percentage estimate of how full the battery is, based on a running amp hour balance counter.
The SOC display shows this balance as a percentage of battery size, so BCAP (battery capacity) must be entered by the installer for SOC to be meaningful.