Android - How can I take a photo when battery <20%?

Try downloading and using a third-camera app from the market. The restriction is apparently on the stock camera app, not on the hardware (the camera itself). I downloaded Retro Camera, and it is able to take a picture when my Galaxy S is on low battery. That might not be the app you want, so do a search on the market with the keyword "Camera".

This is, unfortunately, not adjustable in the stock version of Android. If you root there are 3rd party versions that modify this low battery level.

I think I've found a solution. It's pretty simple - just try to hold the Menu button. The info about battery disappears and allows you to use the camera.