Battery death after about a month

XK* 2001 convertible battery runs down after about a month of non-use. Is this typical? Any way to prevent it?

That doesn’t sound typical. When it is shut down check for battery drain. Allow several minutes for the electronics to shut down. How old is the battery?

It is almost mandatory for these cars to be connected to a battery tender if they are not used almost daily.

I measured the quiescent current on my car (which is a '97 and has the early module set). It turns out to be 30mA. Theoretically, with a new, fully charged 90AH battery it would take about 125 days to fully drain the battery at this discharge rate but of course there are other factors which affect the useful power in the battery and therefore reduce the time before you can’t start the car.

One factor is that the drain isn’t linear. Also, at least with the early modules, things stop working properly long before the battery is fully exhausted. IRL I found that it was about a month of standing before the car wouldn’t start, maybe up to a week less in winter.

What can you do about it? Either a battery tender (trickle charger) as baxtor suggests or what I have done which is to fit a battery switch. One of those that fits on the battery terminal with knob that screws down to make the connection. This does mean that I have to reset the windows, clock and radio code each time I use the car. The radio remembers the stations, fortunately, and it means that the car will always start even if I haven’t used it for a while.

Shropshire, UK

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~30 mA quiescent drain is typical for X300/308 and presumably the later models also. If it is higher I would check for lights staying on in the glove box, boot etc.

If you set the alarm when you leave the car, it will drain the battery after awhile.

Thanks everybody for all of the comments. I had heard before on another forum several years ago that the problem may be that the car was searching for the key, and so was always on and kept draining the battery. The battery is a couple years old , maybe 3 and always starts up even in cold weather.Any other possible battery drains? In searching for battery tenders i found that they all seem to connect to the battery terminals, Which brings us to the crux of my problem. I really hate tearing the floor of the trunk apart to get to the battery to jump start it! I can never get all of the panels back in correctly, (and neither can the non-jaguar garage that I use for service.) Any kind of tender that would connect to cigar lighter? Any other suggestions?

Most tenders are supplied with a short fly lead. The projector brand unit I have fitted uses a short lead with one end permanently connected to the terminals, the other end which is fitted with small plug exits the boot/trunk floor at the rear corner near cd changer. Simple procedure to open boot plug in tender, good to go.

I’m liking it. I can snake the lead up between the floor panels to the tender and run the cable out the trunk lid to an outlet?

Check that the cigar lighter is not switched along with ignition, it generally seems to be. Also check the quiescent current with an ammeter for the level of the drain.

Michael, If the battery is 4 years or older, its time for change.
THE BIG QUESTION. Did you use a OPTIMA battery?
They are the WORST batteries for MODERN cars with computer drains.
Unless the car is on a constant trickel charger/tender, the car will die either way.
Go back to an original old fashioned acid battery with at least 800 plus cold cranking amps and leave on a charger.
Goop batteries will give you plenty of headaches on this car…been there done that.
Good luck!

Joey, why? I thought they were supposed to be best?

Michael, pic of tender plug access. Tender itself has around 2 metre lead plugs into the socket in pic. Very convenient.

They don’t hold…
Ford GT was the first to use it then ASTON MARTIN on all replacements.
The thought was it was smaller,lighter and more advanced.
The goop does not respond well for sitting long periods of time AND small draws such as clocks and computer brains, make a constant draw.
They have to be on a trickle charge al the time or else.
You can put one in a 1955 chevy and it will run forever but a constant drain car, the reaction to the goop in the battery does not hold up well .
Many web threads of this.
Either way 4 years and the battery goes these days, keep it on a charger, even in sleep mode.
Good luck

Michael, If my '99 XK8 sits for a week with no charger, it’ll start, Two weeks, maybe. Three weeks, probably not. These cars need to be driven REGULARLY, or kept on a charger.

Very important to use a modern “float” or “smart” charger, not just an old-style trickle charger. The older ones will overcharge and shorten battery life.

I’ve given up on Optima batteries, and the local pros seem to agree that they are no longer the best for most uses.

Bob Frisby
Boise, Idaho