Place the PB5xx flat on a table in normal orientation, screen open and up, but facing away from you so you can see the rear connections. With the negative of an ohmmeter connected to the exposed ground metal around the serial, ADB, or ethernet connections measure low range ohms to the power connector bottom two male pins. The bottom one farthest to the left is pin 3 and is connected directly to ground at the logic board. The bottom one closer to the serial port is pin 4; it is not clear if this has a function as I cannot measure it going anywhere obvious. Using the AC power adapter pin 4 is at 0 V, but the 12V auto adapter does not connect to it at all (missing female connector metal in position 4). The upper left side pin above 3 is pin 2, and the remaining upper right side pin above pin 4 is pin 1, as numbered on the logic board soldermask, as viewed from the rear of the PB5xx. Pins 2 and 1 each get +16V from my AC adapter and +14.2V from my auto adapter, with respect to ground pin 3. I'll leave it to you to figure which is main vs battery power. I do not know what happens if you power one but not the other, nor whether it is ok to power both tied together from a single higher amp supply, but at least here is a guide to the polarity part of the puzzle.