batch file calling to function fail

been trying to figure that out but batch file looks quite complicated compare to web languages.

i'm looking to generate new _RndAlphaNum for each for loop.

any idea?

@Echo Off
Setlocal EnableDelayedExpansion
Set _RNDLength=6
Set _Alphanumeric=ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789
Set _Str=%_Alphanumeric%987654321
:_LenLoop
IF NOT "%_Str:~18%"=="" SET _Str=%_Str:~9%& SET /A _Len+=9& GOTO :_LenLoop
SET _tmp=%_Str:~9,1%
SET /A _Len=_Len+_tmp
Set _count=0
SET _RndAlphaNum=
:_loop
Set /a _count+=1
SET _RND=%Random%
Set /A _RND=_RND%%%_Len%
SET _RndAlphaNum=!_RndAlphaNum!!_Alphanumeric:~%_RND%,1!
If !_count! lss %_RNDLength% goto _loop



for %%a in (C:\folder\*.png) do (
set /a count+=1
set "fname=%%~a"
setlocal enabledelayedexpansion
ren "!fname!" img_%date:~10,4%-%date:~4,2%-%date:~7,2%_%HR%%time:~3,2%-!_RndAlphaNum!.png
endlocal
)