Why does it have two batteries? Does it need 6V for some reason?
On the Mac II, the batteries supply the real-time clock (RTC) and the soft-power on/off circuit. The real-time clock is designed to operate with a single battery, so they just connect a wire to one of the batteries for that purpose. The soft power circuit uses HC logic series technology which runs from 2V to 6V. This should also be fine for running from a single battery. That suggests something else needs a higher voltage.
There are three other connections to the soft-power circuit:
1. The VIA chip that allows software to shut down the machine runs at 5V. Having a 5V signal going into an HC chip running at 3V is not good. But, they could always have used a diode or transistor to safely permit the low signal to power down.
2. The ADB power-on key signals power-up by grounding this pin. So, no problem there.
3. That leaves the power supply turn-on signal. It must need a higher voltage than a partially-used single lithium battery can supply after a transistor voltage drop. So, yes,
I would conclude the power supply itself must require the pair of batteries to turn on.
Apple improved the power-supply design in the IIcx (and future) by having the power supply produce a separate always-on +5V. This provides three benefits:
1. No batteries are needed to power on the computer. The always-on +5V powers the soft-power circuit.
2. Only a single battery is needed for powering the real-time clock when the computer is unplugged.
2. Current from the single battery for the real-time clock is not needed when the computer is plugged in, because the always-on +5V supplies power to the real-time clock. This allows the battery to last longer.
- David