Get a portable "instant" charger pack online for about $40. Charge it and take it along. It can also charge your cell phone or video equipment. Trickle charging from a vehicle outlet is just not going to do the job for many items.
Based in MD, near DC
Shopping from the Carolinas to New York
Have video cam; will travel
You need a portable "Laptop Car Charger".
This is a peculiar issue that most of the professionals face. I was also having same type of issue few months back with my Asus Chromebook. It was also discharging in 4-5 hours until i bought laptop car charger from PapaChina. They have some of the high quality laptop car charger at wholesale price.
I use a power inverter and just keep the computer plugged in. I strongly suggest getting one labeled as "pure sine wave" as they don't get power spikes that can damage electronics. If not, at least use a surge protector.
The bonus with these is you can use them to plug in anything you would normally plug into a wall outlet so long as your inverter puts out enough watts to power the item.
You definitely want an inverter that produces a sine wave or you risk burning out your laptop power supply. Many cheap inverters (designed for things like immersion heaters and simple irons) are switching transformers and generally produce square waves. An inverter which produces a modified sine wave will probably work and are available on Amazon beginning at about $17. Inverter which produce a true sine wave start at about $120 on Amazon.
Shopping Southeast Pennsylvania, Delaware above the canal, and southwestern NJ since 2008