I have an -hp- 403b voltmeter that used a string of Ni-Cd rechargeable batteries with an internal charger.
It could be run from the AC line or on the battery power, but the batteries were required to derive the internal supply voltages on AC-power mode.
The batteries were dead, so I replaced them with a series string of several TL431 shunt regulator circuits (with fixed resistors) to replace the battery sections between voltage taps, operating from the original charging circuit.
There was plenty of space in the battery compartment to hold the TL431s on perfboard.