The power house of di2 is the long life 74v li ion shimano 7970 dura ace 6770 ultegra di2 battery that is both compact and reliable taking shifting effort away from you excellent sealing and reliability have been proven through repeated field tests in various riding conditions including extreme temperatures rain mud and cobble stone roads using a hard wired battery system which is both light weight and accurate compared to current wireless technology as a wireless system would require