Could it be the power converter in the vehicle that changes the 12v DC to 120VAC, has an
issue or gets interrupted? Some vehicles have a cigarette lighter socket that only works if
the key is turned to Accessory, or to On.
The power conversion device usually requires much more electrical draw from the 12V.
vehicle battery, in the way of amperage, which is partially lost due to heat generated by
the conversion transformer process. Prior to MagSafe power adapters with the magnet
to connect adapter to the MacBook series, older Apple portable mac cord was widely
adopted and not proprietary to Apple; so makers of power adapters were able to sell
DC to DC power adapters, for boats, airplanes, motor vehicles, etc. these worked well
since they only needed to convert 12 VDC to 24VDC, not to AC then back to DC.
An example, used to sell adapter DC to DC, replaced PowerBook/iBook AC adapter in
their day, ten years ago:
https://www.lindelectronics.com/fuzzysearch/results/dc%20to%20dc%20adapter
(Sadly, you cannot buy the DC to DC adapter from any of the makers of good product
to use with the MacBook series, since the proprietary MagSafe isn't widely available.)
So you may need to check the power converter and how it is supplied by the vehicle
if the computer is left plugged into a power source controlled by the ignition key switch.
There may be an interruption, and sometimes these devices overheat or fail.
What is the MagSafe power adapter's specification? Is it a 65Watt or 85Watt?
There is too much unknown that an electrical multi-meter would needed to tell,
before someone could guess at the issues behind your situation. Troubleshoot
the main components in the vehicle including converter and accessible 12VDC.
I've had some electrical troubleshooting experience, and some things are a mystery
as to why they work at all. Especially since DC current can run backwards, in a quest
to find a proper ground. Things such as relays, and loose wires on an aging vehicle
may also present a challenge. My vehicles are rather old, yet mostly predictable so far.
Which MacBook Pro model are you referring to? There's several build year differences.
You may try to find a third-party battery charger such as OWC macsales used to offer
for portable Apple computer batteries, that stands alone & just charges the batteries.
http://eshop.macsales.com/item/Newer%20Technology/BATCAPUMB15/ like this.
In any event...
Good luck & happy computing! 🙂
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