The idea of a backup battery (connected under the hood) to maintain settings when disconnecting the main battery (to replace or charge) sounds good.
Since I'm not trying to start the car, are there good alternatives to a full size car battery? A plug in product that provides reliable 12v? Too bad the local Radio Shack
disappeared many years ago. Thanks

 

If its for a short time a 7 AH Gell battery would be fine ..clip the positive onto one of those live bolts under there near the fuse boxes. Or one of the small starter chargers with crock clips you can get...May be easier to recharge than a gel cell. I have a cheap charger which gives out 14.3v so I put a diode in the line which reduces it to 13.5v ideal for maintenance .I leave it connected whenever i am not using the car . (13.5 v will not cause it to gas so no problem).

Why do you want to disconnect it for charging ..its no problem with a domestic charger to leave it connected.

 

Thanks for the suggests. There was a thread in here about the 12v connect to the fusebox to maintain settings like radio, drive cycles, etc. Sounds like disconnecting the battery to charge isn't a must do. Still learning about Jag electronics.

My 1 year old Bosch AGM battery (bought by previous owner) keeps losing voltage. Worked fine when I got the car 6 months ago. I've driven 1,000 miles so far. Charged it up to 12.8. Down to 12.6 the next day. One week later (not driven) it's 12.0 Figured I'd disconnect the battery and charge separately to see if the problem is the battery or parasitic drain. Are there any standard things to check (radio on for example)? Does it make any difference if the car is locked or not? Recently, alternator output suddenly dropped to 12.0 then slowly climbed up to 13.0 This lasted for the 3 miles it took to get home. So far, it's happened just once. Always a steady 13.9-14.5 before and after that.

 

Charging to 12.8 is not enough .it needs to go to 14.5 or if its been abused up to 15v and that will rejuvenate it . You must keep the car locked if its standing or the computers are waiting for you to start up.

 

Just did a 15 mi test drive. Alternator is running lowest since I bought the car [13.3 - 14.0]. Battery is 12.62 Have the charger hooked up (no disconnect) and I'll keep connected until highest voltage. Also HVAC just stopped working. Nothing. It's always been ice cold. Thanks for the suggests.

 

When you remove the key from the ignition - make sure the shutter in the keyhole closes to 3/4 position so the car can go into sleep mode. If it stays fully open, you will get drop in battery voltage

 

If the battery needs a recharge the alternator will no do it you need a mains charger and get it up to 15v for a few hours . the alternator is fitted to supply the electrical loads of the vehicle and to replenish the very small amount of discharge caused by starting not to recharge a discharged battery . Old wife's tale that the alternator will recharge a drained battery.

 

Hi there: I just had a replacement battery in my 2005 XK8 and the technician at "Batteries and Bulbs" attached a battery pack before he disconnected the main battery in the trunk. It eliminated the need to for ny resets after the battery was changed out - VERY Helpful!

And, they installed it for free while I waited for about 15 minutes... Great service - just thought I would mention this.

 

Thanks. Also I keep reading about about having to "reset the windows". No idea what that means or how to do it.

 

Resetting the windows: For each window, press the button to lower the window all the way down, and hold the button down until you hear a distinctive click before you release the button. Then, press the button to raise the window all the way up, and hold the button down until you hear a distinctive click before you release the button. Done....

 

CA Jag,

In the past, I too had a very similar situation where I noted that, while running, the battery charging voltage was down around 13.0V. I managed this situation for a while, but basically the car wasn't recharging the draw down on the battery caused by start ups. Pistnbroke is right when he said it should be around 14.0V and running the car won't recharge a dead/low battery (unless you drive approx 8 hours straight at speed on a freeway - easier to just put the battery on a charger.)

As I have a battery tender/microcharger I used to plug it in on the Jag, but in the end I accepted that basically my old oil encrusted alternator/generator was dying. It was the original unit in the car so it needed to be replaced/repaired. Here in the Arabian Gulf it was easier for me to get a new identical Denso replacement otherwise I would have got it repaired (always the better option IMHO if you can find someone to do it). Perfectly DIY-able swap out job.

You may also have a combined situation ie bad alternator and a parasitic draw. There lots of info on checking the fuses to see if there's current draw - check Youtube for videos etc. All perfectly do-able DIY. When you're checking the fuses, make sure the car is asleep. From my experience parasitic draws are rarely engine related, normal suspects are any post-mods additions, radio/stereo, climate control etc. One case on this forum was the battery backup to the Jag's alarm (located in front right wheel arch).

Best of luck, and let us know what you find.

 

Even 8 hours on a freeway will not fully charge it as it will not go above 14v that in fact only takes it to 80% of its full potential but it does not gas which means the battery does not need any top up screw vents.
Bringing it to 15v brakes up any sulphate on the palates but it gasses so dont leave it too long.

 

Thanks for the feedback. Battery charging voltage (OBD/cell phone) has dropped just once to 12.0 (and ambient temp was 105). Next time I started the car, back to 13.9-14.2 If it starts happening on a regular basis, time to replace the alternator. Drove 30 mi today and it was a steady 14.0-14.1 I charged the Bosch battery overnight. Have an old 6 amp charger that's putting out 13.75 - 14.0 (it cycles quickly between those 2 numbers). Highest voltage I've gotten is 13.2 When I measured the battery (disconnected from car, no charger), it dropped (fairly quickly) to 12.94 and stayed there. Had it load tested at the local auto shop. They told me it's good. After a 30 mi drive today, it's 12.9 For the past 6 months, I've never locked the car. Is that enough to cause problems? CD may have been on too.

 

Yes shutter closes. Thanks for the suggest. This forum is just great.

 

Follow up. After car sat for 2 days, battery voltage was 12.3 Docks locked. Started fine. Drove for 15 miles. Getting 13.9-14.1 charging
Got home. At idle w/ max load (idle, AC, heated seat, high beams) charging dropped to 13.2
Day after is again 12.3 Highest charge is 13.2
Battery?
Old alternator wearing out?
Hopefully not both.

 

Back to the original question I was also wondering about some gizmo to retain settings while the main battery was disconnected. I think I recall seeing in Pep Boys or similar places things that plug into the cigarette lighter and retain the settings but never investigated. Does such a thing exist?

Doug

 

Yes, but only works on cars where the lighter is permanently live - not sure if the XK8 is or not.

 

its not live on my 2002 Jaguar XKR once the ignition is turned off.


Z

 

BE VERY careful that the positive battery cable does NOT touch the body when disconnected from the main battery with an AUX battery connected. (it will be LIVE)