nb Peter wrote:
I need a definitive answer.
The definitive answer is in nearly all cases you can't use anything other than powerful household current (or something similar like a portable generator) to charge and maintain the power requirements of a Mac laptop computer.
What most people do is attempt to use 12v to 110v step up transformers in their vehicles, then connect their Magsafe with it's step down transformer to 16.5 volts (or so) 65/85 watt to supply power to run the computer and/or charge the battery.
The problem is there isn't simply enough wattage at the 12v end, like in most cars with simple cigarette lighters not designed for heavy wattage needs.
If one does this on a system that doesn't have enough wattage, the best result is a slowing of the draining of the battery provided the battery was near fully charged (like via high wattage household current) earlier.
There also can cause overheating/premature death of the wires in a vehicle not designed to have such a high drain of electrons. Solving the electrical problems of a car is extremely expensive, usually all the wiring is replaced as that's still expensive but less expensive than trying to find out where the problem is in a mass of wires.
Emergency and police vehicles have beefed up wiring and power systems to power devices like computers and emergency equipment, however the typical passenger car usually isn't, unless it's a used police or emergency vehicle sold to the public.
Apple's Magsafe connector is proprietary, meaning only Apple can allow other companies to use their design which they rarely do because they can't trust other companies to maintain the proper quality to ensure no damage occurs to the Mac itself, which Apple would then have to repair under AppleCare or warranty.
The Mac also talks to the MagSafe I believe now to better ensure no other hardware hack is used, like cutting off the end of a Magsafe and wiring it to a 12v to 16.5 adapter, like a few were doing for profit.
Apple provides a Airline adapater that looks like a 12v car charger, but because airline voltage is higher like about 15v or so, it can be used to slow the drain of the battery, but again, still not enough wattage to charge the battery.
If your Mac is old, out of warranty or AppleCare, your certainly free to experiment, however if Apple could charge up/run a Mac properly in all passenger vehicles they would have included such a adapter integrated on the MagSafe itself long ago because people want to that so much.