a fully charged 12 volt battery should read 12.73 volts, according to the trojan battery users guide. a reading of 12.5 volts indicates 80% of full charge.
while your charger is running you will get a higher voltage reading - that's how a battery is charged - sometimes as high as 14.6 volts, depending on the charger regulator
, although 13.8 is probably more common.
for a good guide to deep cycle batteries go to www.trojanbattery.com
and download their users guide ( i think its in the 'technical' section). it's an excellent guide and i refer to it often when checking my battery state of charge....