The BA B737-400's were all delivered with two batteries. One was the APU battery, but when standby power was on, this battery was coupled to the main battery to give better time. The rest of the time (all the time) it was isolated.
Then we acquired the B737-400's from Dan Air. They only had one battery.
So we had 10 734s with one battery, and 25 734s with two. But all batteries had to be serviceable, we cannot MEL the APU battery, even though the other B734's did not have one!