If you are doing mostly email and only surf the web occasionally on your BlackJack, then you may want to consider this: by turning off HSDPA on your phone battery life can increase dramatically. This article from Mobility Today explains how to do it.

EDGE (2.75G) provides about 115Kbps, which is fast enough for push email, downloading a few attachments and browsing the web. Only if you are a bit impatient when browsing, download many attachments or intend to stream audio or video to your phone the HSDPA (36) speed of 750Kbps are really important.