How can you tell if a battery is fully charged?

The only true way to tell if a VRLA battery is fully charged is by using a good voltmeter to determine the open circuit voltage (OCV). Accessible flooded type batteries can also use a hydrometer.

 Charge %  Digital Voltmeter Open Circuit Voltage (OCV)
            Flooded             Gel             AGM

100%        12.80-12.60     12.95-12.85     12.90-12.80
75%         12.40           12.65           12.60
50%         12.20           12.35           12.30
25%         12.00           12.00           12.00
0%          11.80           11.80           11.80

Divide the above values in half for 6 volt batteries or by six to determine cell voltage. The TRUE OCV of a battery can ONLY be measured after the battery has been removed from the charge or discharge load for 24 hours.