what's the 3rd conductor for on Dell laptop P/S?

I can't directly answer your question but will share something with you. If you take apart a modern Li-Ion battery pack (for a notebook computer) you will find several integrated circuits and lots of other stuff. Those darned things are complicated!

all i can say us DONT mess with the charge terminal. most laptop batteries have highly integrated charge/drain monitoring circuitry that will get TOASTED if you try to defeat it.

u are far better off stocking up on ten or so extra correct battery packs. or get a bonified ac adapter that inserts in same place as battery pack.

they are not just batteries anymore, they are power systems all inside the plastic.

alternately, use a few charged 12vdc car batteries, a 12vdc to 120 vac inverter and a standard dell 120vac adapter. yes, its ugly but you might get a few good weeks of performance from the setup :-))

I think I once read somewhere that the power supply contains a Dallas chip in a transistor-style TO-92 case, that contains a serial number, to prevent power supplies with the wrong profit margin^H^H^H^H^H^H^H^H^H^H^H^H^H specifications from being used. Better get yourself a logic analyser, or a DC-AC inverter.

Chris

Yes - it's a dallas chip to identify the PSU type, in particular whether or not it can supply enough current for the device it's connected to - there are different Dell PSUs with different current capabilities with the same plug. These Dallas chips can be emulated fairly easily with a PIC/AVR etc. as long as you don't need the

1-wire powering facility, which you don't in this case as you have other power available.

also, a good alternative may be a correctly sized solar panel (or two) with circuitry to charge /offsett battery drain.

batteries

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Finally I rigged up a cable to send the signal to the microphone jack (through a resistor of course!) of the same computer, and used it as an o-scope with Audacity. All it does is send a few pulses, not even remotely resembling ASCII as I was hoping. The line stays at 3.1 to

3.2VDC when there are no pulses (as seen with a VOM), and I can't tell whether they are talking back and forth or just the P/S is sending the pulses. I believe the computer is generating the 3.2 because the 3rd line was at 0VDC until I plugged the power cable into the computer; so that would presumably make it the bus master? I only bothered to capture a few seconds because after about 8 seconds the pulses ended. I guess it might be sending at 140Kbits rather than 14, which would account for why I can't see any data at the 44K capture speed of the sound card. Well, thinking out loud... just wanted to share what little I'd found out.

Perhaps it's an IIC type signal that identifies the type of battery to the laptop? There might be an ID chip of some sort in the battery pack. Got an old one you can cut open?