How to write recursive function from the loop?

//for loop on all serial no.
foreach (var item in usersGroupSrNo.ToList())
{
    //group by date of one serno
    var obj1 = item.GroupBy(x => x.timestamp.Date).ToList();

    //finding decreasing bettary level upto lowest so 4 variable for computing uses

    decimal? firstBatteryLevel = -1;
    DateTime? firstTimeStamp = null;
    decimal? lastBatteryLevel = -1;
    DateTime? lastTimeStamp = null;

    // ---------------------------------
    var orderedItems = item.OrderBy(x => x.timestamp).ToList();
    firstBatteryLevel = orderedItems[0].batteryLevel;
    firstTimeStamp = orderedItems[0].timestamp;
    for (var i = 1; i < orderedItems.Count(); i++)
    {
        if (orderedItems[i].batteryLevel > orderedItems[i - 1].batteryLevel)
        break;
        lastBatteryLevel = orderedItems[i].batteryLevel;
        lastTimeStamp = orderedItems[i].timestamp;
    }
    if(lastBatteryLevel == -1)
    {
        result.Add(new clsReturn()
        {
            srno = orderedItems[0].serialNumber,
            dailyavg = "",
            sstatus = "Unknown"
        });
        continue;
    }