The Lapotron Pack is a device added by the GregTech mod. The Lapotron Pack can be used to store 10 million EU, as much as an MFSU. The Lapotron Pack can be placed in a player's chest slot to keep all power tools charged, or it can be placed in any of GregTech's energy storage devices (Lapotronic Energy Storage Unit, Adjustable Energy Storage Unit, or Inter-Dimensional Storage Unit; the MFE and MFSU are not high-tier enough to charge it or be charged by it) to charge them or to be charged. The Lapotron Pack is equivalent to the Ultimate LapPack from GraviSuite; however, the Lapotron Pack costs considerably more in both Iridium Plates and other expensive materials, e.g. Diamonds and Lapis Lazuli.
Here is a video that shows how to use all of the GregTech storage devices, including the Lapotron Pack.