Android - How can I force Dropbox to upload even if the charge is low?

Currently the official Dropbox app doesn't support automatic camera upload if you have charge less than 30%. You can do it manually though or through a browser.

Only 3rd party apps like DropSync can upload when it is at low charge.

I don't know of a way to cause the DropBox app to do it, but if you use DropSync (an third-party app that syncs Android<->DropBox), you can control the sync vs. battery behavior.

Go to apps and start dropbox. From the hamburg menu select settings (the wheel icon). Scrolldown until you see "upload when". Select and slide to when the charge is greater than ... whatever value, 5%. That should do.