This might help ?
http://www.uboatarchive.net/Manual.htm
A damaged balast tank would mean one taking in water ... you'd have to pump water out constantly in order to stay at the same depth. It would thus also drain your compressed air.
They have 5, according to the manual. 2 of which are reserves and used for fuel oil reserves as well. See page 37 and on. That manual contains a wealth of info for RSD btw
