When you have lots of battery packs laying around you want to use for an application that requires a lot more power, but your packs don't have a high enough cell count, connecting them in series can be a solution.
Of course you can merge the cells together in a permanent fashion, but when you have packs of matched cells you might be reluctant to do so.
Note
The positive pole of the first pack is connected to the negative pole of the other pack. The free connections go to the ESC.
Warning
Do this only with packs of the same type. Don't mix brands, capacities or cell types. If you do, chances are you will drain one pack so low you will damage it.
You can charge packs in series too. Just connect the open leads to the charger. Make sure your charger can handle the total cell count.
