I got a Soyo 025J2 based on the SiS 85C471 / 85C407 chipset (with the 85C407 containing the RTC). I want to connect an external RTC battery to it.

It does have the common pin header for external RTC batteries (Vbat - NC - GND - GND), however, the board's manual does not state a voltage that should be applied to this connector. Also, no datasheet for the 85C407 exists out there that I know of, only one for the 85C471.

I wanted to go the 6V CR-P2 lithium route which has worked fine for me on XT systems, however, this is a much newer system, and it's not a MC146818-like RTC, but a rather highly integrated IC which also does other stuff (DMA and IRQ?).

As it stands right now, the only components between the ext. bat connector and the 85C407 are a 1k resistor and a diode. Once connected, 6V are fed directly into the 85C407 (6.5 V out of the CR-P2 battery pack over the diode voltage drop). I have my doubts that the IC is designed for such a voltage, as it normally just runs off 5V and also isn't JUST an RTC.

Can I safely risk this? Other boards using the same chipset do actually recommend 6V packs like the CR-P2 for use as an external RTC power source. But these might have a higher voltage drop between the battery and the IC... I can't know since I don't own any of those.