Battery DC adapter mod

Not sure if this is the right place to post, kind of looked like it. Don't know where else to ask.
I have a small 7" LCD monitor that takes a DC transformer adapter, with a barrel connector. The LCD screen is rated 9v-12v DC <25w. With my universal transformer (1800mA max), set to 12v it works just fine.
I soldered a new plug of the same size to some battery terminals for a 12v 4aH battery. For all my calculations it should work, but it does nothing, cant get it to power on. I do have the polarity correct. I am trying to make the monitor portable off battery power.
What am I missing here? Thanks.

Justin wrote:
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Get a car lamp bulb (a 27W one will do nicely), solder a couple of wires to it and put the wires on your battery terminals. It should light, brightly. If not, charge the battery.
If it lights brightly there, put the wires (carefully) on to the new plug. It should light brightly - if not, your wires going to the battery are too long and/or too thin.
If you have a test meter, put it on volts across the battery, with the new plug not plugged in - then plug it in and attempt to switch the monitor on. The voltage should initially be >12v, should drop, but not much. If it isn't 12v, even after a good charge - it's duff. If it drops very quickly -it's duff.
-- Sue

Justin wrote:
  Quoted Text Here  
Be sure your plug is really the same size. There are several different ID's available with the same OD, and it might not be making contact with the center post.

Good question on the barrel connector. I'll check on that. Although I am pretty sure it is the same size. As far as the battery goes I know it is good. I have several that I have tried, all good, different sizes. A 4aH and a 8aH, neither worked. The wires are not that long, only about 12", and 18 awg. Im guessing the error is in my calculations for amps/watts etc. But it looks pretty straight forward for what I am dealing with. Calculating these things is not my strong point.