count length of filenames in batch

my problem is I want to count the length of multiple filenames and save this numbers into a file.

My approach is this:

@echo off
for %%i in (*.txt) do (
    set Datei=%%~ni
    call :strLen Datei strlen

    :strLen
    setlocal enabledelayedexpansion

    :strLen_Loop
    if not "!%1:~%len%!"=="" set /A len+=1 & goto :strLen_Loop
    (endlocal & set %2=%len%)

    echo.%strlen%>> tmp
)

The problem here is it only works for the first filename and after that it is stuck and does not go on to the next filename.

via Chebli Mohamed