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.

Digital Voltmeter Open Circuit Voltage (OCV)
Charge % Flooded Gel AGM
100% 12.95-12.85 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 can ONLY be measured after the battery has been removed from the charge or discharge load for 24 hours.