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There are three reasons why a battery goes flat.
1. Battery unservicable.
2. Charging system fault.
3. Electrical drain.
Check the charge rate by using a voltmeter. With all electrical loads on and with the engine running the charging voltage should be around 13.5 volts, and around 14.4 volts with all loads off.
Use an ammeter connected in line with the battery earth terminal and post and see if there is any current being drawn when the keys are out of the ignition and the car is locked. There should not be any more than 0.1amp. If current is being drawn then this will be the reason why your battery is discharging. You'll need to isolate circuits to find the cause.
Batteries fail for a number of different reasons. To simply say a battery is 'faulty' is no description at all. There will be a specific reason why it's faulty, and this is why the result from a battery tester cannot always be relied upon. You should be aware that car batteries are not deep cycle batteries, and often don't properly recover if allowed to completely discharge, especially in extreme hot or cold conditions.
If tests 1 & 2 prove good then you should replace the battery, regardless of what Halfords have told you.
Edited by Railroad. on 25/10/2016 at 09:37
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