Dodd preamp battery plays 40 minutes

I have a Dodd battery operated preamp. It sounds very good, but the batteries (four square units well seated in their alloted places) do not play more than about 40 minutes. At that time, the preamp starts beeping to be recharged. It recharges, but then again will not play through a whole CD. Any suggestions?

The Dodd has a switch on the front to select between two sets of batteries, but I understand that this is a holdover from an earlier design and that in fact the preamp automatically uses all the batteries before it asks for a recharge. Changing the switch position to the other set of batteries does not change things. The preamp is plugged in via its adaptor, and is supposed to disconnect the wall power when it is turned on, and then to reconnect to wall power for recharging when turned off. I realize the batteries may be faulty, but it's a new preamp and it seems unlikely. Any guidance is appreciated.