A fully charged battery should read 12.5 volts at least and with the engine running the terminal voltage should be 13+ volts indicating the alternator is charging the battery and even if you switch on dipped headlights the voltage should not drop below 13v. The alternator should only put out less than 13.4 volts to avoid popping bulbs ( at least that what used to be the case when I was repairing alternators) If even after a period of chaging the battery reads less than 12.5 then it is not holding its charge and should be replaced check what sort is fited to your car as there are varying sorts (i.e. capacities measured in ampere hours like 110 Ah). All these figures can be tested simply with a cheap multimeter connected across the battery terminals. The price you pay is up to you considering such things as guarantee period and capacity.
Last edited by MIKE4369; 6th October 2007 at 10:19 AM.