Session timeout problem

See more:

ASP.NET

My project is online shopping site.But my problem is after adding to cart, after some times session will be cleared(not more then 5 min,but the default time is 20 min).So all the cart details will lost.So i use following code to extend session time. But my problem is When i load site "Default.aspx" after 2 minutes alert message will display "Your session will expire in another 18 mins. Do you want to extend the session?". But i need this message only after i used any session variable.

<script language="javascript" type="text/javascript">
        var sessionTimeoutWarning = "<%= System.Configuration.ConfigurationSettings.AppSettings["SessionWarning"].ToString()%>";
        var sessionTimeout = "<%= Session.Timeout %>";


        var timeOnPageLoad = new Date();
        var sessionWarningTimer = null;
        var redirectToWelcomePageTimer = null;
        //For warning
        var sessionWarningTimer = setTimeout('SessionWarning()', parseInt(sessionTimeoutWarning) * 60 * 1000);
        //To redirect to the welcome page
        var redirectToWelcomePageTimer = setTimeout('RedirectToWelcomePage()',parseInt(sessionTimeout) * 60 * 1000);

        //Session Warning
        function SessionWarning() {
            //minutes left for expiry
            var minutesForExpiry =  (parseInt(sessionTimeout) - parseInt(sessionTimeoutWarning));
            var message = "Your session will expire in another " + minutesForExpiry + " mins. Do you want to extend the session?";

            //Confirm the user if he wants to extend the session
            answer = confirm(message);

            //if yes, extend the session.

            if(answer)
            {
                var img = new Image(1, 1);

                img.src = '/KeepAlive.aspx?date=' + escape(new Date());

                //Clear the RedirectToWelcomePage method
                if (redirectToWelcomePageTimer != null) {
                    clearTimeout(redirectToWelcomePageTimer);
                }
                timeOnPageLoad =  new Date();
                sessionWarningTimer = setTimeout('SessionWarning()', parseInt(sessionTimeoutWarning) * 60 * 1000);
                //To redirect to the welcome page
                redirectToWelcomePageTimer = setTimeout('RedirectToWelcomePage()',parseInt(sessionTimeout) * 60 * 1000);
            }

            //*************************
            //Even after clicking ok(extending session) or cancel button, if the session time is over. Then exit the session.
            var currentTime = new Date();
            //time for expiry
            var timeForExpiry = timeOnPageLoad.setMinutes(timeOnPageLoad.getMinutes() + parseInt(sessionTimeout));

            //Current time is greater than the expiry time
            if(Date.parse(currentTime) > timeForExpiry)
            {
                alert("Session expired. You will be redirected to welcome page");
                window.location = "/Default.aspx";
            }
            //**************************
        }

        //Session timeout
        function RedirectToWelcomePage(){
            alert("Session expired. You will be redirected to welcome page");
            window.location = "/Default.aspx";
        }
    </script>

and in webconfig

<code><add key="SessionWarning" value="2">

<sessionstate mode="InProc" stateconnectionstring="tcpip=127.0.0.1:42424" cookieless="false" timeout="120">