Magnetek converter/chargers are, at least in some cases, two devices in one box. ONE is a non-regulated converter that puts out not very good DC (Rectified AC) at about 12 volt average (18 volt peak) not recommended for electronics but great for lights, pumps and the like.
The other side is a very low current battery charger. Sometimes in the single digit.
IF the batteries are very very low, it will show very very low voltage, If you have no battery, the type of meter you use will make a big difference in the reading (Since the meter thinks it is AC it will show low on a good DC meter).
Replacement with say a Progressive Dynamics 9200.. highly recommended.