There are camera systems with built in rechargeable batteries but it's been my experience that they are usually dead or low on charge when I need the camera. I always have a supply of fully charged cordless tool batteries on the shelf, so it makes sense.
I use these handy tool battery adapters for powering a variety of projects. Be aware though that with some brands, the low voltage cutoff is in the tool, not in the battery. It is possible to over discharge using these adapters, which may damage the battery. Best to check the brand you use to be sure.
I particularly like the Milwaukee M12 batteries for their compact size.
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2 Packs Power Wheel Adapter for Milwaukee M12 Battery Adapter 12V Battery Conversion Kit with Switch, Fuse & Wire Terminals, 12AWG Wire, Power Connector for DIY Rc Car Toys and Robotics - Amazon.com
www.amazon.com
I use them for powering smaller tools, like this hand held fuel transfer pump:
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Unfortunately, Milwaukee, along with DeWalt, are among the brands that don't have a low voltage cutoff in the battery.