i have a hitec x4ac charger that has a fast charge option and i dont understand what it does differently than the balance charge mode. i recently charged 2 of the exact same battery (2s 5000mah 40c). one batter i put on fast charge mode at 5amps and the other battery i put on balance charge mode also at 5 amps. both batteries charged at the exact same rate and finished at the exact same time (about 45 minutes). i watched the the voltages during the charge on both batteries and they were the same for both batteries so i am confused on what fast charge mode does. you guys have any ideas? thanks

Fast charge mode means that the battery gets an unbalanced charge. I have no explanation how both of your batteries finished at the same time. It doesn't make sense. The one getting the balance charge should have taken longer. Were both batteries at the same state of discharge before charging?

Yes both batteries were at 3.4 per cell. I literally saw no difference in charge times. Also I thought fast charge won't fully charge the battery but both batteries were at 4.2 v per cell when they were finished. I'm confused

That does seem odd. What did the amps do on each charge after hitting full voltage?

Fast/quick charge modes generally skip the cell balancing process and should not IMO be used regularly. Depending on how well the cells in the pack were picked out (matched) the balancing process will vary, the better they are matched, the lesser the balancing time.
Only three things come to mind on your scenario:
You have perfectly matched cells (somewhat rare actually)
The balance charge mode in the charger used to balance was flawed
Or maybe you missed something and charged both packs with the same mode/total charge times were actually not the same.
Mind you that the difference in charge times between those modes won't always be drastically different. The times can be fairly close when the packs are matched well (while not necessarily perfect) and/or are new or near new. The balancing part of the charge cycle will take longer with packs that have cells that aren't as closely matched and/or have gone through several cycles.