Telephone caller ID on auxillary supply

Hi,
I have a telephone caller ID, which runs on 3AA batteries. The AA batteries dont last long, so i was planning to hook it up with a 6v 3.2Ah sealed lead acid battery.. It has 3 terminals inside. 2 AA 's for the Device and the 3rd AA in series with the first two to light up the blue backlight.. The block diagram looks like this


The circuitry draws 2.5mA for the first 15 seconds when working, then only 0.5mA which I think is power save mode.. When I tried a 1k resistor, the LCD works good for 15 secs, then display completely lights up making it unable to read what is displayed. if I increase the resistor value, nothing shows up. So I paralleled a 2v7 zenner across the circuitry section, then it works like as if on battery and completely normal. But now the led doesnt go off. It has a good resistance in series and worked well till I connected that zenner. What could be the problem?? Is there any other approach other than this?? that zenner if used gives a good display and bad light, if not used gives bad display and correct led working... Somebody please help..

Hello,

Would this idea work?


I have assumed the + is at the left, you did not state where the + of the batteries are.

Greetings,
Bertus

Sorry that I didn't indicate polarity. The Rightmost end is negative, obviousely middle becomes 3v+ and left end 4.5v+... By the time after I posted this, i did a little experiment and connected the 6v through a 220r resistor to the negative and 4.5v+ ends.. then placed a useless rechargable single Ni-cd AA battery to make the drop to below 3v and placed a 2v7 zenner across the ckt section and now it works fine... I think If your ckt is modified for the polarity mistake I made, it would also work fine.. Will try it out.. thanks for the idea !!