Hi there, I'm a novice at electronics but I'm an electrician by trade. I want to design (hopefully cheaply) a circuit that would take 12v in from a 10A powersupply, float charge SLA battery/s (7AH or 14AH), if loss of power switch to battery power, have an 12v output for normal conditions and battery back up and a 12v output for just on battery. Basically this is for a haunted house where our lighting system is 12v LEDS and if we were to lose power it would keep the normal lighting on but also let white LEDs turn on to mark out exits. So far I was thinking of just using a relay with the battery on normally closed and have the normal 12v power keep the coil energized so it fails-safe to battery mode......good idea? It's the float charging i'm having an issue with, I saw some boards for float charging but they require 13+v so do I have to use a step up trans for it?

 

I've not designed a UPS system before but do have Haunted House experience. Mine was in a building with generator backup, so having a UPS was not issue.

However, I am aware that when designing this, you/we will need to know the total current draw of your emergency lighting, exit lighting and anything else running off that battery. This is needed for a couple of reasons. One, you need to know how long your batteries will last. Two, you need to know in order to properly size the driving components.

Secondly, we purchased commercial exit signs from Grainger that had a battery backup already installed.

 

We are looking at a maximum draw of 8A. We are trying to keep cost down, as we have many white leds to use if we got this circuit to work. We have non illuminated exit signs currently.