Convert Battery to AC @ same voltage
Simple &amp; cheap with stuff from Wallmart is to use a 12V battery, a car inverter, and the wall-wart.

Simple &amp; cheap with stuff from DigiKey is to build a MOSFET H-bridge that's driven by a 555 timer running at 60Hz.  But that's a lot more complex than the other simple &amp; cheap.  You need a pair of gate drives that have built-in shoot-through protection, you need to generate a +12V gate &amp; control electronics supply, and you need four suitable FETs.  Then you need to wire it all up.

There may be a simple &amp; cheap eBay solution that'll generate 30V square-wave AC from your 30V battery, but if there is I don't know it.