Mk5, 55plate.

Had my MOT a week ago last tuesday and car was fine. 11 days later I go to start it and its won't even turn on dashboard/lights/radio, or unlock automatically. Tested the battery and it's at 4.5v. I'm guessing that even though its been cold, the weather couldn't deplete it from working and starting the car to that low a level - I.e. there must have been something draining it?

Am I also right that if the battery has gone that low its a goner and needs replacing? I tried taking it out and neither of the battery clamp bolts will come loose (one is just turning its hole, the other just rusted locked and bending the plastic/metal around it when turned)

The cold weather is a sure way to find out your battery isn’t in the best of health. At 4.5V I’d think you’ll have a job to even attempt “reconditioning it”. If the battery is 3+ years old then it’s worth investing in a new one. 

Thanks, it's actually only 2.5 years old, and I don't do many miles either. That's what makes me think there must have been something draining it, rather than just the weather, in order for it to drop that low in such a short amount of time? 

If you’re only doing low miles do you have a battery charger that you could top the battery up with?

At 4.5V starting point you may need to connect another car / battery to it in the first instance to raise the voltage for a modern battery charger to even detect that it’s connected to a battery and to commence charging. 

 

I have a battery charger but its dropped so low/is deteriorated so much that the charger wont charge it (just get the icon for bad battery)

1 minute ago, Abra said:

I have a battery charger but its dropped so low/is deteriorated so much that the charger wont charge it (just get the icon for bad battery)

Do you have any jump leads so you can connect another car or battery for 15 mins or so to raise the voltage?

For the battery to get that low one or two cells must have gone.

47 minutes ago, Aldridge Andy said:

Do you have any jump leads so you can connect another car or battery for 15 mins or so to raise the voltage?

Sadly not, plus how it is parked would make it quite difficult to do so. 

I just tried connecting the ammeter between the battery and the negative lead, went from the 20A settng down to 200mA and no reading. I'm guessing the battery is just too dead for anything to draw any current from it (hence why not even the dashboard coming on)

 

Though I did test with the voltmeter and its now saying 8.5v, so no idea why its changed so much from earlier (it was just below zero earlier and now is a balmy 2C). 

Because drawing 20 Amps from it has raised the internal temperature and once it is drawing a trickle of current, the voltage will rise but it doesn't mean that the battery will be able to provide current.

The ammeter isn't drawing any amps though is it? It's just measuring whats being drawn from the car's electronics?

Also the voltmeter test was before the ammeter one. 

Just buy a new battery. That one is finished.

I've managed to get the old one out now but only by cutting through the retaining bar as one bolt just spins in its housing, and the other is just twisting and cracking the plastic. 

halfords sells a 'universal' battery clamp but doesn't look suitable, so i think i might need to find a replacement battery tray which sounds a pain

Okay new battery in. Thickest cable ties i could find around the tray and battery to hold it down. Car starting up fine. I got a voltmeter that sticks into the 12v socket  and it says 14v+ when engine running, so alternator seems to be working fine.

I'll check tomorrow for current drain when the car is off. Am I right in thinking it should just be about 50-60mA normally? 

Decided to just check tonight instead. It spikes to a couple of amps when connected and the door lock sounds, then drops to 0.3 amps and slowly down 0.18A after about 25 seconds, and then sits at that for 10 minutes while the oedometer screen is still lit up. Then screen turns off and it drops to 0.00A (which seems a little low if anything? i don't think there is any way to connect my ammeter at the 200mA scale because the initial spike will blow the fuse)

So I guess it must have just been the battery. Though still suspicious that it went from starting the car fine to not even being able to light up the display at all over just 11 days. 

Thanks for everyones help

11 hours ago, Abra said:

So I guess it must have just been the battery. Though still suspicious that it went from starting the car fine to not even being able to light up the display at all over just 11 days. 

Some years back I had a similar problem.   Started the car, went to the shops and when I got back to the car the battery was dead.   The RAC man got me a new battery and fitted it and said that it wasn't uncommon and the the cause was one or two cells failing.

Well that is a bit reassuring at least, thanks. 

Is there a general recommended distance and frequency to drive to keep the battery in good shape if most of my journeys are short (though only one or two a week)

3 hours ago, Abra said:

Is there a general recommended distance and frequency to drive to keep the battery in good shape if most of my journeys are short (though only one or two a week)

I'm in the same position since lockdown Abra.   I use a trickle charger once a week for a few hours.