the output of that curl is a complete xml file with a rather long content. I did some other tries and managed to get it to work.
I changed the way my script gives back the response of the curl call. Originaly I directly used the output of grep -c, which did not work in case of 0. Now I added a if statement to check if it’s 1, then echo 1 and else echo 0.
In case someone else needs the script, here is it in complete:

#!/bin/bash case "$1" in unten) IP=192.168.38.102 ;; Bad) IP=192.168.38.109 ;; esac case "$2" in an) curl --silent -d "p0=play&player=${IP}" http://192.168.38.21:9000/status.xml >/dev/null ;; aus) curl --silent -d "p0=power&p1=0&player=${IP}" http://192.168.38.21:9000/status.xml >/dev/null ;; ping) detect=$( curl --silent -d "p0=power&p1=?&player=${IP}" http://192.168.38.21:9000/status.xml |grep -c "<b>Ein") if [ "$detect" = 1 ]; then echo "1" else echo "0" fi ;; esac

Battery attribute is offered in graph page for sensors which are not battery devices