Here's the thing about these batteries, you need to understand the discharge curve of the batteries to understand what you can safely use. The chart I provided doesn't contain any voltages because it can represent any voltage pack, I am using it to demonstrate my point. Also, even though the diagram says it's for Ni-CAD batteries the same discharge curve applies for Ni-MH's as well.

If you look at the chart you will see that at the beginning of the discharge the battery pretty quickly falls off to the Mid-point voltage. At that point it stays at that voltage for a large portion of the batteries capacity, and then it drops off quickly after that. For a 4 cell battery that mid-point voltage will usually be the 4.8 volt level. So if you are monitoring you battery and stop using it when the battery hits 4.8v you will be wasting a large portion of the batteries capacity (60%-80% of the battery capacity!!!).

So where does this leave you?? It means that you can use the battery while it reads 4.8v, but when it drops below that it's time to stop using the battery. Pretty simple. Follow this and you'll get a lot of usage out of that pack, but if you stop using it at 4.8v you are wasting a lot that you could be flying on. The best way to know how much time you have to fly on is to cycle the battery with a cycler that can measure the capicity of the battery. This will tell you exactly what you can us that battery for. The other advantage of a cycler is that you can tell when a pack is starting to fail. When you first get a pack you need to cycle it to establish it's capacity. Then throughout the life of the battery you should cycle it and refer to the original capacity of the battery. When the pack gets down to 80% of it's original capacity it's time to chuck the pack and get a new one.

Hope this helps

Ken