How to tell how much battery cranking power is left?

[79SuperVert]

Can a battery (Optima Red Top) that has reached the end of its useful life still measure full voltage but not be able to start a car? My battery is probably around 5 or more years old and measures 6.3 volts but won't start the car. A spare battery (never used - about two years old) measures 6.09 volts but starts my car up right away. Is the old battery toast?
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[Ace]

A bad battery can look good voltage wise then drop to the floor the moment you put any load on it.

[Bruce]

Is this a 6V car?

Take it to a battery shop and get it load tested. My 4 year old optima tested out at 787A a couple of weeks ago.
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[djkeev]

You can have voltage readings but not have good amperage readings.
Just like you can put (4) 1.5 volt AAA batteries together and get 6 volts but you'll have very little amperage. Voltage doesn't indicate actual strength.

Take it to a shop that can load test a battery and you'll probably find in an instant that it is toast.

Keep in mind that a load test simply mimics a starter draw under a controlled and measured condition. By trying to start the car, it already failed the load test. If it won't take a charge and then start the car there is very little (especially with an optima) that can done to revive it.
You know the starter and wiring are good by using a 2nd battery to test it.
You are dead, time to move on to the new one.

Good Luck
Dave
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