So I tried using sed via terminal with no success. I also tried using a .zip file with these lines in the script:
set_perm(0,0,0755,"/sys/devices/platform/cpcap_battery/power_supply/battery");
set_perm(0,0,0755,"/sys/devices/platform/cpcap_battery/power_supply/battery/charge_full_design");
run_program("/sbin/busybox", "sh", "sed -i 's/1390/3500' /sys/devices/platform/cpcap_battery/power_supply/battery/charge_full_design");
run_program("/sbin/busybox", "sh", "cat /sys/devices/platform/cpcap_battery/power_supply/battery/charge_full_design");