  The battery passport is proposed as a method to make the use and remaining
value of batteries more transparent. The future EU Battery Directive requests
this passport to contain the round-trip energy efficiency and its fade. In this
paper, an algorithm is presented and demonstrated that estimates the round-trip
energy efficiency of a battery pack. The algorithm identifies round trips based
on battery current and SoC and characterizes these round trips based on certain
conditions. 2D efficiency maps are created as a function of the conditions
`temperature' and `RMS C-rate'. The maps are parameterized using multiple
linear regression, which allows comparison of the efficiency under the same
conditions. Analyzing data from three battery-electric buses over a period of
3.5 years reveals an efficiency fade of up to 0.86 percent point.
