Hopefully all the cells are working. Each cell should provide between 1.94 volts fully discharged to 2.20 fully charged. You should be looking for all the cells to be more or less the same, any variation is a bad sign. The same goes for the SG readings. These should also be checked. If any cells are noticeably lower/higher than others this is also a bad sign. Ideally both these measurements should be taken when the battery is fully charged and fully discharged to give you an accurate picture of the batteries condition. You still need to know the age of the battery you are purchasing, the serial number on the battery data plate should have this on and if you are lucky the installation date. If you only have the serial number, ring up your local Deka dealer and they will be able to date the battery for you.