I have a 12v battery (lithium ion pack) and a 12v wall wart. I want the battery to give me a 12v output when the wallwart is not plugged in and the wallwart to give me a 12v output while charging my battery. I dont want to use normal toggles/push buttons to do this, i want it to be automatic. The best i could come up with is relays. Here is my wiring diagram. Will this work? Can you suggest another way of accomplishing this? Thanks in advance

Mod edit: reduced image filesize

 

Typically this would be done using whats called "diode or'ing" and if you use schottky diodes you only get "lower" voltage drop compared to regular diodes
http://www.codemsys.com/smps/ORingDiodes.gif

 

Mcgyvr, You are correct. However: as stated by you the Vf is affecting the output.
Pfet could be used as a diode for this application gate to gnd (use a Vg>=20V I=10A int Rsd > 15mOhm ).
10A @ 15mOhm=> 150mV and 100*15*10^-3 =1.5Watt
Diode .6V approx 10*.6=6Watt ( agree som diodes do better)

 

I'd love to see a 10A @12V wallwart
But yes a fet might be a better solution depending on the specifics of the OP's needs..

Maybe the OP should just use a change over power jack (not sure if thats the right term) but a power jack that can disconnect another source upon insertion..

 

Thanks for all the help. Ive seen those power jack at radio shack before, but didnt even think about it. Ill try one out.

 

the wallwart to give me a 12v output while charging my battery

A 12V wallwart will not charge a 12V lithium battery pack safely. Lithium cells need a special charging profile to avoid overheating/fire risk. I suggest you visit www.batteryuniversity.com.

 

Its a 12v pack that has a lithium charger built in.

 

One relay will do, you need a diode in series with the coil to the NC contact, to prevent the relay from being powered by the battery, the wallwart powers the coil only.

 

I just didnt want to lose any voltage, but I guess 0.3V isnt much lol