For breastpumping in a motor vehicle use the Medela Vehicle Lighter Adapter.
The 9 Volt Adapter powers all Medela Pump In Style Breastpumps from standard vehicle lighters.
The 12 Volt Adapter powers the Medela Freestyle Breastpump and Pump In Style Breastpumps manufactured prior to 2008.
Just plug small end of the adapter into the breastpump and larger end into vehicle lighter. It has a 6-foot long cord.