Most like the battery.
For the Li-ion, which is one of the best energy densities, no memory effect, and a slow loss of charge when not in use.
The lifespan of your Lithium-ion battery is about 3 yrs in textbook, but I will say more than that. I have one canon 20D (this is one of backup camera) still works well.
A Li-ion battery can also fail short than that;
1) undercharge, if a Li-ion pack is stored in a discharged condition for long period of time will lose its charging capacity.
2) discharge/charge cycles, you can have as much as 4,700 discharge cycles if you recharge it at every 10% drop in capacity; 1,500 discharge cycles if you wait until 50% drop; and only 500 discharge cycles for 100% full recharge.
You can find more info from
batteryuniversity.com
howstuffworks.com