I'm not using the car much just now (because of gearbox problem see other thread) and if I leave it for just two weeks, it won't start.

The battery is not completely flat, but the engine won't turn over. I start it with the battery booster pack. Is this a weak battery or is the current consumed when everything is 'off' enough to flatten the battery in just two weeks?

I remember my XJ6 Series One, in (about) 1977, I came over to the UK from Australia on a three month holiday. Returned home to Oz and fully expected the Jag to have a flat battery, but she started immediately as if I had never been away.

 

I am afraid you have a weak battery that needs replacement. After 2 weeks any good battery should have start your car. Sometimes you check the battery and shows even full, but still not able to star tthe car..!
Invest in a new one.There is nothing better for a car than a new battery.Everything works better after.

 

12.6 is a fully charged battery, as it goes into the 12< volt range the security module senses the voltage drop. wakes up and runs the battery down to nothing. Comparing your series 1 with virtually no computer to todays car that have more power than the computers that put the first man on the moon is not a comparison. No end up with all kinds of electrical issues cause of issufficient voltages....in other words ALWAYS maintain the battery. You odviously have lost reserve power capacity, so rpelace the battery and realise that a car will start with only 11 someodd volts and that driving it doesnt neccessarily bring the battery to a proper long lasting charge. you comtrol modules will thank you for it. Anytime you dont drive the car weekly I would reccommend putting the battery on a battery tender, theyre $30-50. with proper maintenance theres no reason a battery in the trunk cant last a good 6 or more years.

 

Thanks for the replies.

Any issues with changing the battery.

Will I lose anything (key fob, radio code) when the battery is disconnected?

 

yes radio code, time. windows setting and sunroof programming