esc or 3 bad battery packs?
A 50mah TX charger will work for your 8 cell packs, but 15 hours will only be enough to charge your 600mah pack fully discharged. (actually, you'd want to go about 17 hours at that rate).
The bigger packs would take significantly longer. Use this formula to figure it out. You want to put 140% of the rated capacity in to a completely discharged pack for a slow charge at C/10 rate.
For your 1000mah battery, 140% would be 1400 mah. Divide this by your 50mah charger and you'll get your necessary charge time... 28 HOURS!
If you have a charger that will charge at C/10 (100mah for your 1000mah battery) it will save you some time.