Low voltage - thoughts please?

Iceweasel

Last week I had the yellow 'low battery' symbol light up when I switched on - before I started the car.

It did start as normal. But I did see the light another couple of times that day.

My Scanguage tells me the standing voltage is 12.3

With the engine running the voltage reads 14.2

I left the car to sit for a couple of hours and checked the voltage with a separate meter and it's agreeing with the Scanguage reading.

Date on the cars original battery is June 2011

I've charged the battery overnight and the voltage still reads 12.3 after a couple of hours sitting.

My deduction is that the problem lies with the battery and that the alternator is working correctly.

Am I right or have I missed something here?

It's never failed to start - yet.

Local dealer wants the car for a couple of days to see if there is any undue battery discharge.

Any comments or advice.

jaydeeuk1

I'm no expert but 12.3 volts doesn't sound too low for me for a standing voltage. What happens when the engine is running and you put air con on full ice cold, main beam on, and anything else that draws power, like heated seats, winding up and down all electric windows, does the symbol appear then?

Iceweasel

No - the warning light only comes on after the car has been standing idle for an hour or so.

With everything switched on as you suggest the voltage over 14 which is why I reckoned the alternator is fine.

Rolandtheroadie

I know our 2.5 diesels wouldn't start with a 12.3v battery unless we were very lucky.

forgotmyname

12.3 is less than 50% charged.

Either the battery is weak or the alternator has bouts of not charging it properly.

Test the voltage with the engine running and with the headlights on, Heater blower on full and rear heated window on.

System

The standing voltage should be around 12.6V for a battery in top condition.

As long as the standing voltage is greater than 12V then it shouldn't really be a problem apart from the annoying warning light

System

forgotmyname wrote: »

12.3 is less than 50% charged.

Explain please?

Iceweasel

Thanks for the replies so far.

I've never seen 12.6v with this AGM battery.

Tomorrow I'll check the battery terminal connections and the earth strap to body.

And I'll do another full load voltage check, with the engine running at around 2000rpm rather than idling.

I'm now wondering if the alternator output is erratic.

Back in the day we had voltage regulators and dynamos - this modern electrickery is almost beyond me.

nobbysn*ts

Diesel I assume?

Iceweasel

nobbysn*ts wrote: »

Diesel I assume?

Oh yes.

2 litre diesel.

forgotmyname

!!!!!! wrote: »

Explain please?

A 12v battery is actually around 12.8 volts when fully charged.

12.4 volts = 50% charged and when its at 12v its actually flat. The lower the voltage the shorter its lifespan. Especially if stored at a lower voltage.

Voltages will vary with temperature and the accuracy of the test meter. The cars onboard diagnostics will usually show a lower voltage than a decent meter directly across the battery with the ignition off.


No need to raise the revs to 2000rpm to do the test i suggested. Just make sure the aircon and heated front screen is off if you have them.

Aircon increases the fuelling and revs so distorts the figures and heated front screens consume lots of power so drag the voltage down anyway.