While reading this question, where a user is having difficulties with his dashboard camera battery, I noticed that one of the answers suggests (or seems to suggest) to replace the existing battery with a converter attached to the car battery.
Another way is to use the un-switched (i.e. always-on) 12V from your car, regulate it to 3V, and put this into your cam. But make sure you do not drain the battery and also use fuses, as there's a lot of power behind the 12V in a car.
This seems like a good idea - why bother replacing the battery with a capacitor (or another battery) if you have a huge one just laying there - the car battery. I happen to have the same dash camera at hand (G1W) and also have a 12V to 3V DC-DC converter just lying around.
Hence, the question. Can the replacement be done in-place or would an additional circuit be required to prevent the device attempting to charge the car battery through the converter. This is what happens during normal camera operation when attached to switched-on 12V car power supply - it only uses the battery to shut down safely and maintain its memory for settings. The original battery is a 150mAh 3.7V rechargeable battery.