How can I determine if a battery starts to fail?
What indicates your battery’s fail is a lower performance, low voltage, no recharging, swelled container and acid corrosion.
All batteries fail eventually. A battery will typically provide less capacity as it begins to age, this is normal. Depending on the application where it is being used and what power is required for that application in may still have a useful a life. End of life is considered to be reached when a battery fails to deliver 80% of its rated capacity.