can't mix new and old batteries. how are people building battery packs out of old laptop batteries?
If you put 1 old battery in parallel with 1 new battery (of the same voltage), it will work fine. Similarly, if you always add batteries in parallel to a series of batteries, it'll be fine.

The problem arises when you try and place old and new batteries in series. The new one tends to drive the weak/old one backwards. That can be hazardous to your health. Many batteries will leak fluid or become unstable after they've taken an inverse cycle. Once they unstable, when charging, they might explode or vent.