The A320 uses NiCad batteries (main batteries are 2x23Ah, IIRC). Removal of the battery cover is to allow inspection of the battery cases (if the charging rate is too high, the cases can swell and even split from heat, allowing alkali (potassium hydroxide) escape ... very bad). I have no clue about simply loosening the case nuts.
When charging, "The battery cover is off during charging to allow visual monitoring and to increase ventilation."
https://www.faa.gov/documentLibrary/med ... 00-33A.pdfNote the FAA caution (Sec. 3, ln 3-5) about batteries charged directly from the DC bus... which is the system used in the A320.
http://www.airbusdriver.net/airbusnotes.pdf p.10