Hello All
Looing to get the LEd bulb glow lomger than usual 8 hours with the 1.5 v battery.
planning to put another one to it so it can go for at least 14 hours

 

Welcome to AAC!

Show us the circuit and how the existing battery is connected. If this is some cheap LED gadget, it probably doesn't limit LED current. In that case, adding another battery with sufficient capacity might cause damage.

What is the chemistry of the batteries in question?

 

planning to put another one to it

Two batteries, parallel or series? 1.5V or 3V?
What LED are you using? I need to know what is its turn on voltage?

 

Drive a while or blue LED with a single 1.5 volt cell. May give you trouble getting all the way down to red LED without adding a diode or two in series with the LED.

This is sometimes called a Joule Thief that anniel747 mentioned above.
http://cappels.org/dproj/ledpage/leddrv.htm