How do I use a relay in a charging circuit?

I am trying to make a charging circuit where the power switches to a secondary load once 14.8 volts is reached.  This is to charge 5 amp hour SLA battery.  Someone told me to use relays but I am not sure how to do it.  Do I need a relay that switches at exactly 14.9 volts?

Extending my answer a little, yes you do need a relay with sensitive swithcing, its usually done with a simple thing called a comparator circuit. You need to have some hysteresis in the switch, so it doesn't make the relay chatter.