Lithium batteries contain a PTC protection device to protect against short circuiting the battery. This PTC device has a low initial resistance that rises rapidly once it reaches it's trip temperature range due to high current. When the PTC cools, the resistance drops back down to a slightly higher initial value than before it tripped. Multiple trips will increase the initial resistance to the point where the PTC limits maximum current to an abnormally low value.
When this occurs, a digital volt meter will still show a good battery based on voltage; however, a battery tester, which measures voltage under load, will indicate a bad battery.
How can this happen?
1) The battery can be overloaded by a short circuit or excessive load which will cause the PTC to cycle until the load is removed.
2) The battery can be exposed to external thermal cycling, i.e. a battery in a black flashlight on a car's dashboard in the sun. Thus the battery is cycled once per day.
Hope this helps.
Bruce
