I recently acquired a sony DCR-TRV730 camcorder and replaced ther battery for a NP-FM 50 which purports to give me about 70 mins of recording time . In fact I have used it a max of about 20- 30 mins and have a flat battery???Is short battery life normal with these things? At over £50 they are expensive things to replace although I noitice there are companies that make replacements which arent Sony but which presumably do the job. Does anyone know anything about these and could you please make suggestions for efficient battery use? Would an NP-FM91 (or equivalent) be a better idea?