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Okay. I found a schematic, and some manuals - some in the Files section, some on Manualslib.
So far, I haven't found anything about a low battery warning, but I have found a couple of hints that it shouldn't say "?attery !", or similar, on startup - maybe someone could verify what it does say?
Most of what I found was about one fitted with a Grey Matter E! board, so I don't know how much is relevant - e.g. this thread:
https://yamahamusicians.com/forum/viewtopic.php?f=35&t=10376&p=61495&hilit=DX7+IIFD#p61495
245mV at pin 11 of IC20 appears to just be a normal logic low level, from the reset circuit (pin 6 of IC7), pulled up by a 1k resistor to 5V, which suggests IC7 has open collector outputs - I haven't found where to look up the ICs, yet. IC20 appears to just be inverters, e.g. a hex inverter chip.
The part I'd suspect first, after the diode, would be C38 (4.7uF, 16V) which is meant to smooth the battery backup voltage, after the diode - probably meant to stop it glitching as the power goes off. Capacitors can fail in different ways... maybe that one's more or less shorting to ground?
I'd suggest measuring the voltage at that pojnt - across that capacitor or at the cathode of the diode, with and without the power turned on, if possible, then trace it to the input of the microcontroller / CPU that reads it. On the schematic, near the 3V battery and the diode, there's an arrow labelled "Backup"... I haven't seen where that goes, yet.
- Andy