My 1998 Jaguar Sovereign Has a problem with its battery retention. Even though the battery is new if I don't drive the car at least once a week it...

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This could have several possible causes:

1) The battery itself is failing

2) The alternator is faulty

3) The false bulkhead connector under the bonnet is faulty or intermittent. This supply line is critical as it supplies both the high current to the starter, and also the charging current from the alternator back to the battery. If this connection corrodes or fails, then it will prevent the starter from obtaining the high current needed & will also prevent the alternator from properly charging the battery.

 You need to measure the voltage at the battery with the engine at a fast idle speed-above 1000rpm. You should see around 14v with no heavy electrical loads switched on.

Progressively switch on the heavy electrical loads like the headlights (main beam), foglights & heated rear windscreen. Raise the engine speed to around 1500rpm & check the battery voltage again-it should be well above 13V & should really be around 13.5v or higher.

If you're getting below 13v or have dropped down to around 12v with a heavy electrical load at a fast engine idle speed, then either the alternator is faulty or the false bulkhead connection is failing & preventing the proper charging current from reaching the battery from the alternator-this is quite a common fault as the connector can corrode.

Also, don't rule out a faulty battery-they should never be left in a discharged state as they can sulphate & won't recharge properly.

Another possibility is that the battery is gradually losing charge to 'residual/parasitic current drain'. This is where some of the car's electrical circuits do not shut down completely when the car is locked for the night & continue to draw a small current which can flatten the battery over a period of a few days. The usual suspects for this are the alarm circuits, glovebox switches leaving the light on, security systems & phone circuits etc.

Please have the alternator tested first to see if it is charging as it should be please.

Any local autoparts store in your area can test the alternator while running in your car for free.

Please let me know when you find the cause of the problem using the steps below please.

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