Heres a few questions I have had from beginners:
1. Whats the diff. between 4.8v and 6.0v batteries?
2. Why is my voltage always more than 4.8 or 6.0 when fully charged on a pack labeled 4.8 or 6.0?
3. What voltage is it safe to go down to on a 4.8 pack or a 6.0 pack before I need to recharge?

Here's my :twocents: on what little I know about batteries.

Talking about NiCad bat pacs:

Four cell pacs are 4.8 volts.
Five cell pacs are 6.0 volts.
1.2 volts per cell.

Fully charged NiCad batteries will have top voltage over their rated capacity till some discharge/use takes place. I don't know why, just one of those things. :unsure:

The bat pac voltage will fairly rapidly go down to rated voltage under load/use, where it will hold that voltage most of it's discharge cycle.

When voltage under load falls below 4.8 (for four cell pac) or 6.0 (for five cell pac) volts, it will then rapidly reduce in voltage till discharged.

If a battery checker shows say 4.8 volts for a four cell pac under load, you are probably good for one more flight unless pac is used under high discharge condition (such as more than four servos, mechanical load on servo, servos under near stalled condition, other onboard electrical items, or any combination of these conditions) and/or flight is overly long.

My rule of thumb:
If in any doubt about battery charge, fast field charge or don't fly.
If I had just made a flight where the starting voltage under load was 4.8, I would charge again.
I know others will put in multiple flights at 4.8 or 6.0 volts and is fine if you know past history of how much actual time was left in pac from practical experience.

Yeah!!! What Alvin said!!! :D :D :P