The charge state of a lead acid battery (% charged) can be estimated from the voltage measured at the terminals of the battery. The chart below shows the relationship.
First, it is important to understand the terminology. “C” is the capacity of the battery in amp-hours. For example, this battery has a rating of 215AH when discharged over 20 hours. Note: The Duracell GC2 battery is a 6V golf cart battery. So, the chart is applicable to two of these batteries connected in series.
For easy reference, here are calculations of the currents shown on the chart:
||Approximate Wattage Draw/Charge
Here are several examples of interpreting voltage:
It is night and no solar energy is available to charge the battery. Several small pieces of electronic equipment are connected to the battery. They consume about 25W or so. The battery voltage is 12.5V. This means that the battery is about 60% charged.
If the battery voltage were 12.0V, then the battery would only be 10% charged. Likewise, if the voltage were about 12.7V, the battery would be pretty much fully charged.
It is night and no solar energy is available to charge the battery. An inverter, which draws about 2A (~25W) of current is powering a small air conditioner and 6 high intensity recessed LED lights. The air conditioner consumes 500W of power and the lights consume 75W. So, the total power drawn from the battery is about 600W, which is between the C/3 and the C/5 curve – but closer to the C5 curve.
If the battery voltage measures 12.0V, then the battery is almost certainly fully charged. If it measures 11.5V, the battery is about 60% charged. And if it measures 10.5V, it is somewhat less than 10% charged.
Note that discharging a battery at the C/3 or even the C/5 rate decreases the life of the battery. C/10 or C/20 is much safer. In addition, battery life is shorted by discharging a battery below 50% of its capacity. 600W represents 50A of current. So, using a Duracell G2 battery pair like this for more than two hours decreases life because of the discharge rate as well as discharging too deeply. For usage such as this, it is advisable to use at least 4, if not 6 of these batteries, in series/parallel combination. 6 batteries would reduce the discharge rate to C/16.7 and would increase the safe usage time to about 6 hours.
In the chart below, the area in green represents the safest discharge rates and amounts for best battery life. Note that battery terminal voltages under 12.0V are always suboptimal.
It is a sunny day, nothing is using power from the battery, and a solar panel system is producing about 10A of current. The voltage at the battery terminals measures 14.0V. This means that the battery is approximately 90% charged. On the other hand, if the voltage at the battery terminals measures 13.0V, the battery is approximately 50% charged.