Battery aging

Contrary to the common beliefs battery aging is not caused by fully draining a battery.

Battery manufacturers make battery safe range from 2800mV up-to 4300mV, while it's considered empty at 3200mV on Android (that 40% remaining capacity). So going there won't hurt any battery (well, no guarantees obviously).

There are 2 factors that degrade batteries:

1. Batteries have limited cycles, so the more cycles the sooner the battery will age, deteriorate or even die.
2. Charging and heat are not welcome either for batteries, Past 70%, heat accelerates battery aging.

The second factor is often dealt with by manufacturers by offering an option to limit the maximum charge level while plugged.

This is what the Battery Life eXtender mod from XDA member Ezekeel is all about!  And Android Tuner allows you to configure it when available in your custom kernel, see here for more information.

For more information on how to prolong your battery life, here is an interesting Wiki (see battery life chapter). 

The important points are those 2:

-Keep battery cool, and avoid charging past 70% while using the device and creating over-heating.

-You can discharge your battery fully (voltage going down to 3200mV) it won't hurt as 3200mV actual means 40% of battery capacity is left, which is the safe threshold mentioned in the Wiki above!