Some time ago I bought a DVR to my car. I decided to buy DVR build-in into rear mirror to make it as compact as it could be. I found Media-Tech U-DRIVE MIRROR BT MT4046 (more spec under the link).
This DVR has a battery build-in which is pretty small (3.7 V 230 mAh and dimensions are 35x20x2.7 mm) and if I don't use car for more than two days (one day during winter time) battery run out of power and all settings (time and date especially) are lost. Another thing is that without power it works only couple of seconds. So I decided to upgrade battery.
I asked manufacturer about it and I was told than I can upgrade it to 420 mAh and dimensions are 42x30x3.5 mm. It is more but no as much as I wanted to, so I decided to check how big battery could be :)
Before you begin I ask you to read this whole instructables to avoid errors. You have also know that by opening DVR your warranty is void any more and I am not taking any responsibility if you make something wrong and your DVR will be unusable.
Step 1: Slip Inside
Opening is quite ease but needs to be done gentle.
Before start you need to prepare few tools and buy new battery.
I found new battery in shop near my house It is Li-Pol, 3.7V and has 1500 mAh. Dimmensions are 60x39x4 mm. It has also embedded security PCM chip (protection against overcharging, over-discharging and short circuits). It cost me... +/- 5 EUR :) On first picture you can check model.
Tools which you will need are: Philips 00 screwdriver, plastic prying tool or something similar, soldering iron.
First remove two screws (red circles on the second picture). Then use something thin (I used plastic prying tool) to detach plastic latches around camera. Be careful! Plastic is not the best quality and it is easy to damage it and it is also easy to damage mic cables. If you are lucky and did it gently you can open it like a seashell.
Step 2: Main Meal
As I wrote before my new battery have dimensions: 60x39x4 mm. As you can see it is a bit to big but ... IMHO Bluetooth is completely useless so I decided to remove speaker and make space for battery.
First I remove foam (pink oval). After that desolder old battery (yellow circle) and speaker (red circle). I removed also mic (green circle).
Now it is time to solder new battery and close DVR. Remember about quality of plastic during closing :) and ... it's done. You can test it if it is charging or not.
My advice is to charge battery completely using any mobile charger before mounting camera in a car.