Cheapest battery back up for F770 in parking mode.

[TheBlondeFella]

Using 1 camera parking mode the camera recored all day, however, since installing a 2nd camera too my F770 the parking mode only lasts around 5 hours and I really need it to last all day when i'm at work.
Are there any affordable battery back up options out there that will extend the parking mode to around 10 hours or so?
The Cellink and Thinkware options are almost a much as the camera itself which is jus crazy!!!