4AA battery holder doesn't match with ND2500.
About external battery, you can find it at this site : http://www.24traders.net/oxid.php/sid/cc780eb672e760439cfce2605c981f4c/cl/details/cnid/-/anid/e7d4488b09c19b5c9.15149545/NEXTO-CF-ND-2500---ohne-HDD/