The Hobbywing ESC (the one it comes with) has an LVC (low-voltage cut-off), which should cut out when the cells reach ~3V per cell. This is a safe level. You can tell the cut-off has come on when it sometimes refuses to move.
The C rating is the level of discharge that the battery can sustain. E.G. a 5000mAh pack is rated for a continuous 25C. This means that the pack can discharge at a level of 25 x the packs capacity in Ah (divide the capacity in mAh by 1000). Meaning that 25x5 = 125A continuous. This is about the minimum level of discharge that I'd run on 4S. Therefore, the capacity and C rating must be 'balanced'. If you are only using 3000mAh packs, the C rating must be higher so it can discharge at a high rate.
Packs come in anything from 15C up to 45C, and with 1/8 it is best to go for at least 25C as a rule of thumb. The higher the C-rating, the longer the cells generally last (life cycles), the more useable capacity they have (so a 5000mAh 40C pack will actually give slightly longer run times than a 5000mAh 30C pack) and as they are not pushed as near to their limit anyway, they will last longer again.
__________________
|