Some metal detectors will only pick up iron/steel, while others will detect just about any metal (including aluminum, lead, etc.).
The most important part in regards to your concern is the amount of metal present. Most venues probably have it set fairly insensitive to avoid a huge number of alarms due to keys, zippers, etc. The main thing they're after is stopping any guns or knives, which have a much larger "footprint" than your battery box.