Hi Vic,
Do not get discouraged.. At the beginning, no one knows what and who to ask, and where.. Now, that you have joined - let the people know your experience with the particular problem at hand in as much detail as possible..
To charge a battery from the car, you will need some kind of a device that will take control over the supply voltage/current delivered to your device’s battery. It is called a power supply regulator. It will keep its output at the level your battery will accept without the damage, and independent of the changes of voltage of the battery/alternator of your car.
Please let us know what is the power consumption of the device when it is in the ON state and in the STANDBY state.. and.. if you want to just charge the battery.. or, charge and operate the device at the same time..
Let us know what is the capacity of your battery as far as the voltage is concerned, and the current that is being drawn from it when your device is operating.. And let us know of everything I may have not mentioned - but you know it from experience that brought you here..
We need to have bit more information from you before we can figure out what the next right step should be..
Regards,
xanadunow