system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it a really bad idea to put dead or mostly dead batteries in a device with fully charged batteries?
gpt
As battery charge is used up, resistance increases. Fresh batteries with low resistance, pushing voltage through dead batteries with high resistance may cause it to heat up and leak. It could also cause a short in the battery, which would rapidly discharge the battery and possibly damage the device. The best outcome is just reduced power.

Additionally: 

You shouldn't mix battery types, such as LiPo and Alkaline. The chemistry and voltage differ and may cause problems.

You shouldn't mix brands. The overall chemistry and voltage may differ enough to be a problem.