Hopefully I'm asking the obvious; but my computer is important so I would like more opinions. According to the question Laptop battery: is voltage really important to respect? It seemed their consensus was that modern laptops will accept a wide range of voltages into their charging ports. And second, I have a Lenovo T420. According to their currently suggested universal charger, it may output anywhere from 11.5 to 18 volts; and the original charger I have outputs 20 volts. So rather than this nonsense of inverting 12 VDC (nominal) from the car to 120 VAC and then rectifying it back down to 18 VDC, would there be any harm in interfacing my computers charging port directly with the car's DC wiring?
In short: Don't do that. The laptops are rather picky about the input voltages and a 20V Lenovo laptop will only work with a 20V Lenovo power supply. And the car 12V can go down to 6V while starting the engine and up to 15V while charging in high revs. It'll fluctuate wildly, have all kinds of EMI and spikes in it. That's all bad for sensitive electronics.
In addition, the both the round and rectangular Lenovo charge connectors have 3 contacts. Outer barrel surface is ground, inner barrel surface is +20V, pin is data. The data pin is used to identify the power brick to the laptop, such that the laptop knows how much current it can draw.
The easy solution is a Lenovo car adaptor. It's not too expensive, does a direct DC/DC conversion from 12V to 20V, and filters and regulates the voltage, also providing the proper information on the data pin. Non-Lenovo, "universal" car adapters may work, depending on how close they get to providing what the laptop expects.
According to the question Laptop battery: is voltage really important to respect? It seemed their consensus was that modern laptops will accept a wide range of voltages into their charging ports.
There may be some truth in that but what I expect you will find is that you will need the input voltage to be higher than the battery voltage for things to work correctly (i.e. for the battery to be charged and for the laptop to draw it's power from the input instead of from the battery).
According to their currently suggested universal charger, it may output anywhere from 11.5 to 18 volts
While I don't know for sure I strongly suspect that the charger outputs different voltages depending on which laptop you connect to it.
So rather than this nonsense of inverting 12 VDC (nominal) from the car to 120 VAC and then rectifying it back down to 18 VDC, would there be any harm in interfacing my computers charging port directly with the car's DC wiring?
Car electrics are known for being spiky, personally I wouldn't connect any device to car wiring that was not specifically designed for the job. Even if you think you will only use it with the car not running sooner or later you are likely to forget and start the car with it still connected.
Thank you for the insight. I'm starting to investigate methods of filtering to provide a nearly constant voltage. Maybe with a switch mode style regulator.
While with sufficient electronics experience you could build such a thing yourself I don't really see the point when it's readily available off the shelf.
Yes, there could be harm. The car's "12 volts DC" is very unstable. It can easily reach almost 15 volts during charging and may have far higher brief spikes. While the car is running it will also have a strong AC component (this shows up in poorly designed audio circuits as "alternator whine").
This is not what you want to feed to your laptop.
By the way, I don't for a second believe the specs you quoted on that universal adapter. I strongly suspect that the "11.5 to 18 volts" rating is the allowable DC input spec, not the output.