You're right; at least in theory, a regulated external 3.3 V power source should always win the "battle" with the VCC_RTC PMIC outputs on the SOPine modules, so you might go without the previously described Schottky diode. However, having even the smallest possibility for the current to start flowing into the external power supply would be unacceptable to me. Adding a Schottky diode, as already described, should be easy and very cheap.
Edit: Another problem with the AA batteries is that they get rather rapidly discharged when the Clusterboard is powered on, much faster than when the Clusterboard is turned off. That just makes the first problem, the batteries becoming charged by the PMICs, more pronounced.
Edit #2: According to pages 33 and 34 of the A64 datasheet, these are the electrical characteristics for the VCC-RTC power input of the A64 SoC:
Thus, you'd still be fine without the Schottky diode voltage-wise, but I really would't recommend it, as described above.
Edit: Another problem with the AA batteries is that they get rather rapidly discharged when the Clusterboard is powered on, much faster than when the Clusterboard is turned off. That just makes the first problem, the batteries becoming charged by the PMICs, more pronounced.
Edit #2: According to pages 33 and 34 of the A64 datasheet, these are the electrical characteristics for the VCC-RTC power input of the A64 SoC:
- Absolute maximum voltage: 3.6 V
- Recommended maximum voltage: 3.3 V
- Recommended typical voltage: 3.0 V
Thus, you'd still be fine without the Schottky diode voltage-wise, but I really would't recommend it, as described above.