One of the PRAM battery voltages that you quoted is under the requirement. A Mac II needs that both batteries be adequate. As a generality, 3.3V in circuit is the point at which to replace a lithium half-AA cell in any old Mac, and 2.8V is about the point at which a battery may become a hindrance rather than a help. The AIO Classics can (just) get away with no battery, but the more (than just time) that is stored in PRAM, the more critical is the lack of a properly functional battery. This can be fatally true if startup of video is affected by a faded/dying/kaput battery.

Are your batteries soldered-in, or on a little board of their own for easier replacement?

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