Re: how will you convert a periodic digital signal into an analogue one

Greetings to the subcontinent, and thank you for your question.  I do not wish 
to appear overbearing, and I respect your question in English because it is 
certainly better than my Hindi.  But I must redirect your intentions and 
clarify what you mean because I see difficulties you must avoid.

A battery is a DC voltage source -- it is not a "... periodic digital signal."  
It is an energy source whose voltage is constant over time.  In electrical 
engineering, we reserve "signal" to mean a "voltage or current carrying 
information," usually with little power.

You wish to convert DC high power to AC sine wave high power.  For low power, 
this is easy.  You build an "oscillator."  For 100 amperes of sine wave AC 
power -- it's not easy anymore.  100 amperes is a lot of current.  High current 
transistors that can switch this amount of current must be designed to get rid 
of the heat created by the transistor in the full ON state. The first step may 
be " ... converting the d. c. voltage into a square wave..."  That is one good 
way to go, but then "...us(ing) a Fourier transform to approximate it into a 
sine wave ..." -- well -- those Fourier transform blocks exist on paper only 
for the power you're talking about.  All you really need is a low-pass filter.

But you have the correct idea that ups (uninterruptible power supply) systems 
do exactly what you wish.   Design of such schemes is a specialty, quite beyond 
the ken of the average experimenter.  It requires knowledge not only of 
electronics, but magnetics and battery technology.  Such systems are rarely 
assembled from off-the-shelf components.

Sorry I can't be of more help.

Larry Skarin