Car won't start after a week, but battery 'seems' ok?

[lambysdad]

Hi all,

I've got a 2005 Mazda 6 2ltr petrol. If i leave it for more than a week it won't start, this has been happening for about the last 6 months.
I have a battery conditioner and as soon as i connect it all the lights say the battery is fine, and literally saconds after connecting it the car will start.
I've even had a garage look at the battery and they also said it was fine.
So what is the problem? Is it going to be an easy fix or will it be one of those where i send the car to a garage,pay hundreds for them to look at it, only for them not to find a fault?

Any advice / help much appreciated

[XG332]

Bad earth?

[Ozzie Osmond]

2005 car. How old's the battery? Over 2 years and I'd start by buying a new one.

When you say a garage "had a look at it" the only way to check a battery properly is to do a high current drain test with a discharge tester. Did they do that?

As a DIY assessment you can remove and charge the battery. Leave it to stand for about 8 hours (at room temperature) and then check the voltage on the terminals. The voltage should be close to 12.6 volts. Anything less than 12.4 and your battery is knackered.

[lambysdad]

Right that's me back from swimming, time to have a look.
Ozzie, just to confirm. Are you saying i fully charge the battery, then leave it to stand for 8 hours, then check the voltage?

[varsas]

Turn on the headlighs as you try and start it. Do the lights dim? If so the battery is at fault.

As said above, testing the battery with no load on it doesn't mean a thing.

[Ozzie Osmond]

lambysdad said:

Ozzie, just to confirm. Are you saying i fully charge the battery, then leave it to stand for 8 hours, then check the voltage?

Yes. And remember it needs to be at room temperature for correct readings so best to remove from car in this cooler weather and stand it indoors.

[Cost Captain]

varsas said:

Turn on the headlighs as you try and start it. Do the lights dim? If so the battery is at fault.

As said above, testing the battery with no load on it doesn't mean a thing.

Every car in the world will dim the headlights when you start the engine! That's why things like interior fans and stereos turn off whilst starting a car, because you want to send all power to the starter motor.

[lambysdad]

I think i might just buy a new battery, it's had the same one since new so is probably due anyway.
Was just about to take the battery off and charge it but i don't have any way of measuring the volts/amps. And i'm away in the morning (i'm away a lot!), so not sure i have time anyway.
Thanks for your comments and if i still have the same problem after putting in a new battery i'll be posting again.

[Puddenchucker]

I had a similar problem.

Left the car for a week, and when I tried to start it, not enought juice in the battery to turn the engine over. Attached the battery conditioner and after about 15 minutes, it indicated that the battery was OK and fully charged. Took the car for a 20 mile drive, and then put it back in the garage.

Next day, tried to start the car and had the same problem. Again, 15 minutes connected to the battery conditioner and the car would start.

Turned out to be a duff cell on the battery. New battery required.

[varsas]

Cost Captain said:

varsas said:

Turn on the headlighs as you try and start it. Do the lights dim? If so the battery is at fault.

As said above, testing the battery with no load on it doesn't mean a thing.

Every car in the world will dim the headlights when you start the engine! That's why things like interior fans and stereos turn off whilst starting a car, because you want to send all power to the starter motor.

Yes, they will dim but it should be obvious if the battery is really struggling. If the lights go out completly accompanied by the starter struggling it's a good sign the battery is toast. You need some 'spare' current to run the ECU or the car won't fire.

I should have been more clear.