I wrote in a previous post that the standard for a dead battery — at which point it should be replaced — is 80% of its original capacity as a newly manufactured battery. Why is that? Is this a random value or is it based on a valid rationale?
At the surface of it, this question may seem benign, but it is not and its consequences can be quite severe.
The cycle life specification of a lithium-ion battery is defined as the number of charge-discharge cycles this particular battery can support until it reaches 80% of its original capacity. The capacity of the battery fades (decreases) with every charge and discharge cycle. This cycle life performance (number of cycles) is ultimately what determines whether a battery, and its host mobile device, is subject to a warranty return.
One attempt at cheating around this specification — and consequently the warranty claim — is to reduce this threshold down to say 70%…in other words, to deem the battery dead when it reaches 70% of its original capacity, not 80%! This attempts to artificially add a few extra cycles to the life of the battery and to push out the burden of a warranty claim merited at the higher threshold. Therefore, the question becomes: What happens to the capacity loss between 80% and 70% (or even lower)?
As it turns out, tests have shown that the battery capacity loss tends to accelerate past 80%. In other words, the battery loses its capacity at a very fast rate once it passes the 80% threshold. This renders the battery quite useless once the capacity drops past 80%. The cycle life fade in the following chart illustrates an extreme case where the battery loses its capacity at a very fast rate past 80%. In this particular case, the battery appears to hold its capacity well but right around 400 cycles, the performance turns very poor and the battery loses its capacity quite rapidly.
While this rate of capacity loss may be an extreme case, most batteries seem to accelerate their capacity loss past 80%. Furthermore, this rapid capacity loss may be accompanied by an increased likelihood of lithium metal plating. In other words, a dead battery with its capacity past 80% becomes a serious fire hazard !
Additionally, with this rapid loss of capacity (and increasing age), the battery begins to swell — its thickness rapidly increases and can cause serious physical damage to your mobile device.
So in summary, if your battery capacity drops to near 80%, you should seriously consider changing it. If it happens within the 2-year warranty window (in some locations, it is sadly one year), return the phone and the battery to its manufacturer.