All batteries, including Range Rover batteries, have a date stamped on them showing the date of manufacture. This is not to be confused with the card on the battery showing the warranty period when determining life expectancy. A lead acid battery's life expectancy depends on the thickness and amount of plates used, but cannot be lengthened as easily as shortened. If a battery is purchased that sat on the shelf for a length of time with sulphuric acid and a full charge, the life expectancy began at manufacture. If a battery is bought dry and the acid must be added and then charged, longer service can be expected. If it is a 4-year battery and the date of manufacture shows it is 3 years and 7 months old, even though it was in service only 2 years and 6 months it is about worn out.
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Check the date on the battery to see how much service life is left. Start the vehicle and check the alternator output. If the battery is too dead to start the vehicle, then the battery must be charged in order to check it. It is impossible to check a dead battery. The alternator should be charging at 14.5 volts. If it is showing lower and the battery was dead, charge the battery.
Check the voltage drop on the battery to see if there is a bad cell. Check the voltage across the battery terminals while a helper starts the car (put the positive lead of the voltmeter on the battery positive and the negative lead of the voltmeter on the battery negative). The voltage should be 12.75. When the starter is activated, it should not drop under 10.5 volts. If it does, the battery is bad.
Check the voltage again with the engine running. There should be 14 to 14.5 volts with the headlights and air conditioning on. If the battery passed the first test or was replaced, and the voltage is lower than specified, the alternator needs to be replaced.