[acropolite (Phil)]

Rodstar, without actually monitoring the current going out of the battery it's nearly impossible to tell the state of charge. Factors that effect the reading are temperature, SG of electrolyte and the open circuit terminal voltage. A rough and ready rule is that a 12V lead acid battery with a voltage reading of 12 Volts has 50% charge. When a battery is discharged the lead inside the battery is converted to lead suplhate (and reconverted back to lead during chargeing) and if left discharged for any appreciable time (as little as a couple of weeks) the lead suplhate hardens and the battery won't take a charge. Deep cycle batteries have materials added to the lead to make them more tolerant of deep discharge. Gel or SLA batteries aren't really all that suitable for deep discharge. The best way to prolong battery life is to recharge as soon as possible after discharge and try to keep the discharge as shallow as possible.