As with all rechargeable batteries, the day will come when it will not take a charge that lasts as long as desired.
The trick to extending the useful life of the battery is to shallow-cycle it, rather than deep-cycling it.
In other words, put it back on the charger after each use, rather than running it all the way down between charges. Better still, keep it plugged into external power while playing it, whenever possible. (Protect the battery, so that you can enjoy it when you don't have access to external power.)
Table 2 compares the number of discharge/charge cycles Li-ion can deliver at various DoD levels before the battery capacity drops to 70 percent. The number of discharge cycles depends on many conditions and includes charge voltage, temperature and load currents. Not all Li-ion systems behave the same.
Depth of discharge
Table 2: Cycle life as a function of
depth of discharge
A partial discharge reduces stress and prolongs battery life. Elevated temperature and high currents also affect cycle life.
300 – 500
1,200 – 1,500
2,000 – 2,500
3,750 – 4,700
Notice that by repeatedly taking the battery down to only a 50% depth of discharge with each cycle, you get a lot more than twice as many cycles as had with a repeated 100% depth of discharge. And if you repeatedly use only 25% depth of discharge, you get even more "free" cycles before the battery has to be replaced.
When convenient, I keep the X5 connected to an Anker Pro external battery during use:
This ensures that I'll have more than a couple of years service from the internal battery when I go lightweight - with just the X5 and headphones.