Proper voltage setting for laptop car adapter?

I'd measure the actual output voltage of the existing laptop adapter, under both light and heavy (charging) load conditions and then see which setting of the multi-output adapter results in the closest match. The checks need to be under load, since the regulation of the power supplies won't be perfect.

A "breakout box" can be made with a plug/receptacle pair. Don't try to get live readings by sticking wires into the laptop or cutting the insulation.

You may not hit exactly the same readings but close enough is probably OK. There is a DC-DC converter inside the laptop which should have enough headroom in either direction to manage within 0.5 V.

Thank you but as I said I know nothing of electricity, how to measure it, or what a breakout box is.

OK, that sounds promising. Which would be better, to underrun it by 0.5V or overrun it by 0.5V? Or, to put it another way, which way is less likely to damage the machine?

I've been unable to find any car adapters that output 18.5 V.

Thanks again.

Captain Infinity

Well, starting from scratch and given that you're without tools or prior knowledge, the best thing that I'd recommend is to carry them both back to that store and get the salesman to (a) state which setting is correct and (b) state that he warrants the laptop if his recommended setting causes any damage.

For my own in-car use, I've always gone with a DC-to-AC inverter, into which the laptop's own, normal power cord can be plugged. There are some losses in efficiency by introducing the extra conversion but there are gains in flex ability -- other stuff could be used, including a later generation laptop someday.