This has happened twice now. A totally dead car in two days in garage. When I check the voltage it is something like 3-4 volts. There should not be anything "on", I haven't found anything. Battery was professionally checked, it is good, charges normally. When I start charging the first thing that happens is car's alarm goes off, tells that trunk causes the alarm - I go in with a key, obviously.

What should I try to test or find?

 

Do you leave your Smartkey fob in the car when it's in the garage? If so, that will drain the battery because Convenience Mode doesn't shut off and the computers stay awake.

Do you lock the doors when your car is parked in the garage? That shuts off Convenience Mode.

Are any aftermarket electrical accessories (tracker, dash cam, etc.) installed in your car?

How old is the battery? When was it last replaced?

How often do you drive it and how far do you travel? Are you sure that you're driving enough for the alternator to fully charge the battery?

Do you use a battery maintainer, like a CTEK?

 

Once a car battery goes below 10 volts, it'll never be the same again. Do that several times, and it's a goner.

 

My '09 XK 4.2 Lt Coupe was left outside all winter.I do not use a battery maintainer of any type. I double click lock it, and it starts up without a problem, even in all of NY's winter, when the temps can fall below -10º F. It has a new Autocraft Battery, installed by the used car dealer, where I purchased it in November of 2017.
Maybe study the manual a bit more, and find the problem. Also it has been my experience that if I do use a battery maintainer, in my other cars, I need to disconnect the battery first. If you don't, it could make a mess of the electronic systems.

 

Additional devices have been the same a few months. No maintainer, has not been needed, I drive more than 20 miles every day. It was left in a garage for a week two months ago, started without a problem. Now it can't take one night. It is always double-locked.
Something discharges the battery. But is it really true, that a battery is gone if once it goes below 10 volts? Hard to believe, I have other experiences.

 

All the above suggestions are excellent. But if you're still stymied, and want to figure it out for yourself as a last resort before going into a shop or dealership, there are numerous DIY videos on the web. Basically they all involve pulling the neg battery cable and inserting an ammeter between the lead and the terminal. That will tell you how much current is still flowing, even after shutdown. Then you systematically begin pulling fuses, and you will locate the electrical zone, or even the specific module / item that is the culprit.

Most people are reluctant to mess with their electrical system... which is probably a good thing. My Jag Indy is a wizard at this stuff. Probably because he's so familiar w/ the systems, he knows all the common failures and goes through them first.

 

They don't all - you can also just measure the (tiny) voltage across each fuse. Use the mV range. There are tables showing the mV per fuse type.

If it used to be OK then something has changed, clearly. So you know it can be fixed

 

One additional factor to note; Our cars have a programmed shutdown sequence wherein it may take up to 45 minutes for everything to ramp down and go into a 'sleep' mode. Consequently, prior to that, some circuits may appear to draw more current than their final resting mode.

 

[QU Also it has been my experience that if I do use a battery maintainer, in my other cars, I need to disconnect the battery first. If you don't, it could make a mess of the electronic systems. [/QUOTE]

This is absolutely not true of the CTEK battery maintainer. It causes no malfunction of any kind in the electronics, and it maintains the battery at full charge indefinitely; it also analyses, desulphates, pulse charges....I have used these units on all my cars, often for months at a time (meaning as a storage device) as well as using them whenever the cars are not being driven. It takes about 4 seconds (if you are slow) to connect or disconnect the CTEK wire. The result has been much extended battery life, and absolutely zero electrical issues.

 

I agree with sov211.

 

I've got five cars, and six CTEK chargers. No problems. w/ any electronics.

 

So, we’re assuming, that the sixth charger is hooked up to you?

 

I charge some cars at the front and some at the rear; hence, more CTEK chargers than vehicles.

 

Macc; Sorry didn't mean to hijack your thread with a pretty picture... so lets get back to the reason for your post:
.
So you've got an unusual symptom of alarm going off suggesting an open trunk / hatch. I tend to agree with others that a weakened battery will cause all sorts of wonky behaviors w/ the electronics. My experience with this type of problem manifested itself over a longer period, like a couple of weeks,

But to drain the battery within a day or two suggests to me there's a ground fault or short somewhere. I'd say if you've checked off the obvious boxes, then its time to find a professional with the right background and equipment. Unless of course, you want to try to troubleshoot yourself, which is not all that difficult.... just sometimes intimidating considering the value of the car's electronics keeps most from experimenting on them.

gordo

PS: I'd change your sig quote until you get this problem fixed. ;-)

 

The fact that as the battery begins to charge, the alarm sounds....this does not inecessarily indicate a fault in the trunk latch...rather it shows that the battery is now at the stage of "almost dead" and the car systems are reacting exactly as they do when the battery voltage is too low. Look at it from the other direction: as the battery voltage drops below the threshhold needed to properly feed the various modules, strange electronic issues begin to appear in the XK. These can be in ANY system fed by the battery. Typical symptoms on the XK are: the touchscreen freezing or slow to boot up, seats, windows, door locks not functioning properly, the convertible top failing to latch, multiple and random warning lights for any system, and finally, in its death throes, the battery signals its demise by triggering the alarm, by flashing the outside running lights...I have seen all of this...the most recent example of an XK with a battery near the end of its life in my experience was the appearance of the tire monitoring system warning light ...despite the fact that the tires were at proper pressure...no other symptom. It was the 8-year-old battery giving a message. A new (and fully charged before installation) battery was installed. Result? No warning light, and it has not reappeared.
The alarm sounding is in a sense a red herring here. The battery may accept a charge but it is unable to maintain a charge over 48 hours. The first culprit to suspect is the battery itself (despite the testing done). A fully charged (and that is important) new battery is the first step before going further. Batteries can and do fail even when relatively young and as has been stated by others, once a battery has been fully depleted its chances of failing again get higher.

 

There are 2 ways to solve every problem: (1) the hard way, or (2) the easy way. The hard way is to troubleshoot the electrical circuits, which is time consuming and requires specialized equipment and knowledge. The easy way is quick - simply install a new, fully charged battery and hope for the best. Odds are that even if you went the hard way route, you'd still have to replace the battery. So why not replace the battery first? That might be all you needed.

 

Agreed. I replaced a brand new battery, only two months old, (under its purchase warranty) on this car because it failed to hold a charge and I was presented with some bizarre symptoms. Had I immediately began troubleshooting, instead of replacing the battery, I would have certainly found myself at the bottom of a lengthy and frustrating rabbit hole.

 

Right: Occam's Razor: The simplest answer is most often the correct one. For any electrical issue on these cars: battery first.

 

There is something funny going on with autoparts sold through shops.

I cant tell you how many parts I have bought that have been defective out of the box or failed 2 months later.

Replace my starter about 3 months ago. Last week while traveling x-country my truck wont start, I spend 3 hours in a parking lot examining relays, sanding down grounding points, wigging wires, begging folks for yet another jump, all to no avail.
Fed up I get under the truck and using a dead-blow hammer beat the daylights out of the new starter- starts right up.
The best thing I did was to not turn in my oem starter for their measly $20 core charge. I will have that rebuilt here in America and throw the fricking Mexican reconditioned one in the trash. The original lasted 250,000 miles.

 

Thanks for all the really good ideas! I went to a specialised battery shop. They tested the battery and said it is in good condition...so it is something else.
I will try the amp meter route. I do a lot of DIY electronics (synths) so this is easy. I do not know how much is "too much" current though: there is bound to be some systems which take several mAs even if the car is in rest mode.
The curious thing is, that I haven't changed anything for a long time. Something must be broken I guess. Or a loose wire or sth.