In series you add the voltage and look at a series battery as a single battery, so a two cell pack is a 7.4 volt battery and thats how the charger sees it. If they are 2000 mah cells then the charger sees a single 7.4 volt 2000 mah pack.

In parallel the voltage stays the same, you add up the capacity, so 2 2000 mah batteries in parallel is seen as a 3.7 volt single battery of 4000 mah capacity.


So a 2s2p in my example above, you add up the voltage on the series, you get 7.4, you add up the capacity of the parallel bats and get a 7.4 volt 4000mah pack, which is how a charger sees it (unless we are talking a balancing charger that sees each sell individually but thats usually just lipo packs)