It can be a bit frustrating understanding battery voltage and amperage. At a basic level, a battery is just a simple storage device. Just like any other storage device, it is filled and then drained. During the day, the sun fills your batteries with electricity, then you drain that electricity at night through the use of your loads in the house. In order to understand batteries better, we like to compare batteries to water pressure-tanks.
The Key
Water = Electricity
Water Pressure (Pounds per Square Inch or PSI) = Battery Voltage (Volts or V)
Water Flow Rate (Gallons Per Minute or GPM) = Battery Amperage (Amps or A)
Water Pump = Solar Panels
The Analogy
- As water pumps into a pressure tank, the pressure (PSI) of the tank increases over time. In the same way, electricity pumps into the batteries by the solar panels, the voltage (V) of the battery increases. A typical 12V flooded battery at rest (no charge or loads on the battery) is full at approximately 12.7V.
- As water flows from the tank, the PSI of the water in the tank decreases. Similarly, as electricity flows from the battery, the voltage of the battery decreases. A 12V lead-acid battery at rest fully discharges at approximately 10.5V.
- GPM is the measurement of the water flow rate in or out of the pressure tank. For batteries, Amps are the measurement for electrical flow.
- During high flow rates of water into a pressure tank, the PSI of the tank may read artificially high. For batteries, high amperage flow into the battery will also cause the voltage to read artificially high. In the opposite way, pressure and voltage may artificially drop when high rates of water or electricity are removed.
How it works
The following chart can be used to determine the state-of-charge of a battery at rest (no charge or discharge).
Most off-grid solar electric system batteries are in the process of charging or discharging. This makes it difficult to determine your battery state-of-charge purely based on voltage. This is why an additional battery monitor that computes state-of-charge based on an Amp-Hour calculation can be most accurate.
If you plan to only use voltage to determine your state-of-charge, the most accurate time to do so is at least an hour after sunset and when there is a very minimal load in the house.
During charging, most solar charge controllers or inverter/chargers will raise the battery voltage up to a range of 14.4V -14.8V. Once charging has stopped, the battery voltage will begin to drop to the maximum (at rest) voltage of 12.7V. This drop may take some time, but generally it will fall to the “at rest” voltage within an hour.
