Takes energy for electrolysis, and swapping electrons. Then there's heat. Measuring battery resistance is sort of tricky - valid for some things, wildly inaccurate for calculations. Any chemical reaction is not linear. Damned tricky to express efficiency in a simple formula.
The most EFFICIENT charge regimen energy wise relies almost totally on charge rejection. The least efficient charging comes during the last 20% of the recharging cycle. The latticed sulfation department laughs and says it doesn't care what the charge efficiency department thinks.