When might a battery appear fully charged yet still not start the engine?

A battery may appear fully charged yet still fail to start the engine due to shorted cells. When one or more cells in a battery experience a short circuit, it disrupts the normal operation of the battery. This situation can lead to a condition where the battery shows full voltage on a tester, giving the illusion that it is fully charged. However, the shorted cells prevent the battery from delivering the necessary current needed to start the engine.

This can be particularly deceptive because the battery may not exhibit visible signs of failure like corrosion or deformation, leading one to mistakenly believe that it is functioning correctly. Conversely, other issues such as temperature fluctuations or bad spark plugs might affect starting conditions, but they do not typically cause the battery to appear fully charged while still being non-functional. Improper voltage readings could stem from testing methods or faulty meters, rather than an inherent issue with the battery itself.