Bat. Charge Circuit

I'm wondering if someone has a circuit to charge a gel cell back-up battery, 3ah 12vdc. Need to charge the battery without overcharging and diode isolate it from the main battery. Also the voltage going into the bat. needs to be below 13.5vdc. Yes commercial units are available, but what fun is that? $30 is crazy! Any ideas or thoughts! Thanks

I don't like to discourage anyone from trying there hand on building something they require. But, speaking from personal experience. I have read and built 12 power supplies to which you could use it for various tasks, that being a charger for a battery. I can tell you from first hand experience that, the time and material costs are more than $30.00 for even the most basic power supply. The only thing that would lower the costs is if you had alot of dead electronics laying around with transformers which would meet the requirements you need. Building the circuit is not really that hard, it is actually building a circuit that stable and has all the built in protection which one requires when, regulated recharging IS required. If you require links to some sites where they have from simple to full out chargers let me know, and I will provide them to you. If you want something out of the box that has been track proven to recharge, and desulficate a battery, then that too can be provided. At the end of the day, it is about consistancy, reliability, and safety to you, and the equipment that is paramount. Regards EVIL Teken . . .