Ademco Vista 20p Voltages?

I have installed a Vista 20P panel in my condo in China. This panel is from the US so it runs on 16.5/60hz transformer. I used a 220/110vac transformer which I plug in the Vista panel transformer. Everything is working fine although the dc output voltages from the panel are at 18vdc and not the normal 13.5 vdc. I suspect the panel's power supply dosen't like 50hz. The kypd's and devices (pir) don't care and work fine. The panel functions fine. BUT, the battery connection from the panel also outputs this 18vdc which of course does effect my 12.5 dc battery. I need to reduce this to the 13.5 dc needed to keep my battery happy. Without a battey, I will not have any siren operation. So I need to know what I can do to reduce this voltage to 13.5 dc. I tried to build a a simple voltage drop circuit which does output the correct 13.5 dc, but it overheats this circuit quickly. So I need some sort of dc regulator circuit (dc to dc @ 750ma)I can place between the panel's charging output and my battery that will function correctly and not damage my panel. Any ideas????