New car, flat battery: causes?

I would suggest either you have a lamp on or other current drain somewhere, OR the alternator has a dud diode and its discharging through that.

When you said the battery was dead- do you mean dead dead (nothing at all) or just discharged enough to fail to start?

If its been flattened completely a number of times, you'll need to reaplce it anyhow as well as fixing the fault.

Tim..

Some people don't half talk some s**te.

[...]

Quite the reverse!

On many cars, locking the doors starts the alarm monitoring. This will draw extra current from the battery.

On some VAG cars the alarm is automatically turned off after a set period in order to avoid battery drain.

Chris

Thanks.

I noted that the previous owner must have switched off the interior light and the boot light --- but these go off properly: I checked.

Umm - dead dead? It will unlock the door, then it will cough once (or rather clear its throat: a cough would imply it's turned he engine over, which it does not. After that one spasm, it's definitely dead dead.

... or rather "they" will!

my dads yaris has doen it since new.

strange. its ok for 3 days,

ISTR some modern cars start the fuel pump when you unlock the doors to get things up to pressure sooner.

I may STR wrongly ;-)

You STR correctly; VAG being amongst those that do.

Chris

For this question though it's what happens after you turn it off that matters - does the pump stop when you take the key out or when you lock it?

Yes, otherwise it'll flat your batteryvery quickly.

Noooo.... I meant: since the pump starts when you unlock it, does the pump stop

(a) when you take the key out

OR

(b) when you lock the doors

??

IIRC When you turn the ignition off.

Buy yourself a hygrometer (a couple of euros), read the instructions and measure the density of the electrolyte in your battery. If the reading is close to 1.00, after the several full discharges your battery is probably ready to be replaced. Remember, new batteries are also only about 50% good, you'll see when you try your new hygrometer on them.

Remember also that almost no batteries can be tested in this way nowadays, they're sealed.

Steve

Also remember that it's hydrometer. A hygrometer is for measuring humidity.

Steve

Sorry, mea culpa, it is hydrometer. If his battery is a sealed one (no unscrewable stops on the top), I am afraid, John should ask an auto electrician to check his battery with a battery tester. But I repeat, in my experience, most batteries, be they 'wet' or sealed, bought from shops/garages/etc., are only about 50% good.

Beg to differ... Anyway, these days, a multimeter can be gotten for about the same price as a hygrometer, and provides much more information as to what might be up. For example, voltage on charge, voltage while cranking, current flow with ignition off, ...

This depends on the state of health of the battery and the amount of current drained during starting. These days manufacturers tend towards smaller batteries for weight saving and space saving so if your battery was slightly down on its health AND you abused it with short hops you've probably just pushed it over the edge. No amount of modern design can get over the laws of physics :)

It does sound like you have either a load on somewhere you cant see or a duff battery. If the lights werent dimming your alternator seems to be healthy on the surface. Duff batteries can appear normal but be internally shorted and self-drain. Simple option would be to replace the battery - after you have drained it to flat 3 times and left it drained (holiday) it will have picked up significant damage.

Thats rubbish. You should always lock cars anyway if you like your insurance to pay out but there is no mechanism that 'ensures all things are turned off'. This occurs when you take the key out of the ignition. J

My apologies. This thread - which I started on Mar 31 - died a natural death after 3rd April; it resurrected itself a few days ago. Perhaps I should have wrung its neck properly by reporting that:

I did take the car to the dealers on April 3rd as stated; they tested the battery overnight and said Yes, the battery was dead, and put in a new one.

Naturally (having experienced bullshit from professionals over some years by now[1]) I took this with a small pinch of salt, and I've had my fingers crossed since 4th April, in the hope that it was indeed the battery and not, for example, the alternator, regulator, or owt else.

So far things are looking good, but I'm keeping the fingers crossed for three months from the date of installation.

Cheers for all the feedback, which has been very useful.

John

[1] Yes, I agree that an approved Dealership has far more to lose than an average garage who might replace this, that and then the other, each with a large dollop of bullshit to ensure payment. So I have no real reason to suspect they weren't just fobbing me off with "Yes it was indeed the battery".

Misset charge voltage can fairly rapidly to gradually fry a battery. The battery may have been killed, rather than died naturally. The best measure of this is a voltmeter over the battery on charge.