When your battery pack drops below 4.8 volts, the voltage starts to drop dramatically over a short period of time. You can fly below 4.8 volts, but it can be very risky. Since you are going to recharge at some point, I suggest you do it when it's reading 4.8 volts or more. It's better to operate on the top of your battery charge.

When you travel, you probably refuel long before the gas guage is just barely above empty. 4.8 volts can indicate barely above empty, depending on the current draw of your airborne system.

Hope this helps