Carputing sent me the following details. Essentially, powering from an outlet (the laptop) is best as long as the car is not connected to a battery charger too. If you have a battery charger on the car, best then to power the laptop from its own battery or an inverter (but some laptops use a different ground and will fry the pcm... so you need to check this).

Definitely disable screen savers, sleep mode, power saver mode, virus scans, etc...

Quote:

A few combinations are completely safe:

1) power the laptop form an AC outlet with an extension cord. This is very safe if the car is not plugged into a battery charger that is plugged into an AC outlet also.

The only question for this setup is if you have a very old battery.
The battery needs to work for a few minutes (with the engine NOT running, but with the ignition turned on) without fading.

2) Another choice that probably is fine is to power the car from a
battery charger, but then it would be safest if the laptop was not
plugged into AC


The only problem is in laptop chargers that connect their negative
output (to the laptop) to the battery +12. We have found at least one that does this (and it is called an auto/air laptop power supply).

For an AC power invertor, you would have to check it's output. Just measure the voltage on the negative output (that connects to the laptop) compared to the negative input. They should both be at zero volts.

The only time a problem occurs is when laptop negative side is not at the same voltage as the vehicle negative side. (since our cable uses these negative sides as ground, connecting them together).

Laptops in general work better when powered, because they tend to not go into power saving modes. Of course you should be able to disable this under the Power setup in the Control Panel.