If you do buy and install new batteries you have to reset the AIR or you will get inaccurate readings, get a manual. the only thing to know about an AIR that isn't covered in the manual is that when filling from empty do it slowly...or you will be replacing the cone O ring (flash fill) also when you are filling the tank it is good practise to turn the adjuster screw out to turn the airflow off, this will stop the sometime problem of the AIR venting from the over pressure plate on the regulators side.
The Maplin charger to buy ...there are several but the one with the required output and regulated and good high enough amps is the Palstar PS-04
Charging is once for 4 hours and then again for 4...double charge it is the subsequent charges that will be just an hour...unless the battery is real low.
Serial number is under the breach there isn't an electronic one...although the manual has a place to enter that info...a few questions on other forums have asked this same question and other than the breach number there haven't any others been found as yet.