with series charging you need a high watt charer or you will have a low charge rate. if you have a 50w charger the charge rate on 6s will be around 2.3amps if you are charging 5000mah it will take over 2 hours . if you have a 400w charger the charge rate on 6s will be around 18amps if you are running 5c charge rate packs if you are charging a 5000mah it will take about 20min.
!!! I AM NOT SAYING YOU SHOULD POUND YOUR PACKS AT 5c ALL THE TIME!!!!