Low voltage - thoughts please?

nobbysn*ts

Iceweasel wrote: »

Oh yes.

2 litre diesel.

12.3v is about 75% charged, which isn't too bad. Voltage off load though, so ideally with the battery lead off, or at least with a meter and the key out of the ignition.
At 20c, Fully charged the battery could be 12.6, goes down to 12v at about 40 to 50% charge, and considered flat at 10%, about 11.7v for a standard 6 cell 12v battery. Voltages with no load.
The numbers go up by about .1v for every 5c below 20c though. So a hot day, your battery will show less voltage than a cold day. And you're also never going to manage to get the battery 100% charged, the chemical reaction tapers off at about 90%, even for a new battery.

With a diesel, when you turn on the ignition, the car loads up. Pre heater, ecu, fan, bulb checks, various relays, so the battery voltage dips. On mine, with the technology in it, if the key is cycled a few times, sometimes it's enough to convince the car the battery is running flat. It goes into energy saving mode, shuts down the air con, fans, heated screen, etc, and lights up the battery light. The car actually starts and runs ok, and when the battery starts to charge it relents and turns them back on. (usually within a second or two)

I would guess yours does the same, when the battery drops to the magic level, the car thinks it's discharging too much, and tries to preserve enough charge to restart.
Could be the temperature throwing the sensor off, could be the temperature affecting the battery, could be a slightly off sensor, or a slightly weak (but still good) cell in the battery.

I wouldn't give it the garage for a couple of days, unless it becomes a regular occurrence, and the car will let you know before the battery's getting to a point where it won't start anyway.

Iceweasel

OK here are the results I took this morning.

After standing all night voltage was 12.3v
Swiched on ignition, radio, headlights and rear screen - voltage dropped to 11.9v yellow low battery light came on too.

Started car no problems - voltage immediately went to 14.9v

After 2 or 3 minutes running the voltage started to drop to around 14.5v

Switched off radio,headlights and rear screen and the voltage dropped further to 14.2v

I'm still inclined to think that it's the battery which is faulty, which is a surprise for an AGM battery at 3 years old.

reeac

nobbysn*ts wrote: »

12.3v is about 75% charged, which isn't too bad. Voltage off load though, so ideally with the battery lead off, or at least with a meter and the key out of the ignition.
At 20c, Fully charged the battery could be 12.6, goes down to 12v at about 40 to 50% charge, and considered flat at 10%, about 11.7v for a standard 6 cell 12v battery.

The old adage is: "12.7, you're in Heaven, 12.5 still alive, 12.2 lots to do" [that's off-charge and off-load].

nobbysn*ts

Iceweasel wrote: »

OK here are the results I took this morning.

After standing all night voltage was 12.3v
Swiched on ignition, radio, headlights and rear screen - voltage dropped to 11.9v yellow low battery light came on too.

Started car no problems - voltage immediately went to 14.9v

After 2 or 3 minutes running the voltage started to drop to around 14.5v

Switched off radio,headlights and rear screen and the voltage dropped further to 14.2v

I'm still inclined to think that it's the battery which is faulty, which is a surprise for an AGM battery at 3 years old.

I would say you have a cell in the battery going, so I'm amending the "still good" to think about replacing the battery.

forgotmyname

Thats the problem when the battery is placed under load it cannot cope with the demand.

New battery time.

Slowly_does_it..

As forgotmyname says, sorry, but new battery time.

Agree with "12.3 is less than 50% charged."

Whilst that battery MIGHT keep going for an indeterminate time, it will choose to fail completely when it is raining, and you are late, and there is no-one around to give you a jump. - Or that is just my luck.

Iceweasel

Thanks for all the advice folks.

The car is now booked in next Wednesday at the main dealer in Aberdeen.

It's under Approved Used Car Warranty which includes the battery (I checked)

They agree it's a toss up between the battery or the alternator - most likely the battery though.

They say they will:
Charge the battery off the car, leave it overnight and see if there is any drop.
Then charge again if necessary and leave it connected to the car for 24 hours and see if there are any unexpected 'current leakages'.

Sounds good to me - they seem to be wanting to do a thorough job.

I just hope it doesn't fail and leave me stranded in the next few days.

I often have the same luck as you, slowly does it..

nobbysn*ts

I reckon the readings will be exactly the same as you saw.

Iceweasel

nobbysn*ts wrote: »

I reckon the readings will be exactly the same as you saw.

I agree - but they're not going to give me a new £200 AGM battery on the strength of my say so.

I get a free courtesy car so no problems with them taking their time.

nobbysn*ts

Post back the outcome.