I know mine charges very nicely on the 2A setting with the open circuit voltage of 11.5V.
Here is a link that might help.
https://www.electronics-tutorials.ws...s-voltage.html
Your VOM is probably reading something close to the RMS value of the rectified AC. With 2 diodes(I had to replace the ones in mine), it is very possible it is only a half wave value so even lower. But there is some time the output voltage is higher then the average. That is when the battery gets some charge. The diodes prevent the battery from discharging back into the transformer when the voltage is lower then then battery.