Max dc volt for boozhound jfet phono stage

Good day

I need some guidance re above. On the boozhound website it says the volts is from 12 to 24v. I would like to connect 2 x 12v batteries in series in order to get 24v.

However, once I have charged the one 12v battery, my volt meter reflects 14v.

I suppose the above is normal, this will mean in the end I might sit with 28v total feeding into the phono stage which is to much. Can I perhaps use some type of resistor to rectify the total voltage?

I appreciate the assistance!