WMI判断电池电量、笔记本、台式机

Imports System.Management
Public Class Form1
    '获得电池剩余电量
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim mc As New ManagementClass("Win32_Battery")
        For Each obj As ManagementObject In mc.GetInstances()
            TextBox1.Text = obj("EstimatedChargeRemaining").ToString
        Next
    End Sub

    '获得计算机类型，Desptop(3),Notebook(10)
    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Dim mc As New ManagementClass("Win32_SystemEnclosure")
        For Each obj As ManagementObject In mc.GetInstances()
            For Each i As UShort In obj("ChassisTypes")
                TextBox1.Text = CInt(i)
            Next
        Next
    End Sub
End Class

    '获得主板序列号
    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        Dim mc As New ManagementClass("Win32_BIOS")
        For Each obj As ManagementObject In mc.GetInstances()
            TextBox1.Text = (obj("SerialNumber").ToString.Replace(" ", "").ToUpper & "TOREDSAPTOOL").Substring(0, 12)
        Next
    End Sub

 部分取不到主板序列号，是什么原因？