Bad Battery Or Bad Alternator?

[Scoobs]

2.2D4D, 92K miles - I've had a couple of flat Battery moments in the last couple of weeks. On both occasions the Battery has seemed to recharge ok and gone for a week without problems.

Testing with a multimeter I get 13.8v across the Battery with the engine started. When I bring the revs up to 2500rpm, it's still 13.8v. When I turn the headlights on full beam and put the air-con on, it's still 13.8v.

Is this correct for the Avensis? I recall doing similar testing on a Focus many years ago and the voltage went up as and down as I revved the engine.

Thx

[alan333]

You should get about 14v with the engine on, so your alternator is probably good. Test the Battery voltage with the engine off, it should still be towards 14v, even after sitting for a few days. If it's down a bit your batty is probably on the way out, or there's something draining it.

[avensisd4d786]

if your in the UK Halfords do a free Battery check, they check its cranking and charging capabilities.

but do not buy a Battery from them unless you have a trade card www.eurocarparts.com is cheaper

my guess is Battery, all my toyotas are on original alternators

[jimtaxi]

This can be difficult to find the fault but I had numerous Battery changes and slow cranking problems and tests,the alternator always checked out ok but one morning the alternator failed when it was replaced the car no longer had low speed cranking problems in fact its had the same Battery now for three years with no problems.

[avensisd4d786]

This can be difficult to find the fault but I had numerous battery changes and slow cranking problems and tests,the alternator always checked out ok but one morning the alternator failed when it was replaced the car no longer had low speed cranking problems in fact its had the same battery now for three years with no problems.

good to know!

[Exvec]

My money would be on the Alternator,sounds to me like voltage regulator has failed as said Halfords will test it FOC just watch there price on a new one if it is there not the cheapest.

[Konrad C]

I would look at the Battery as the cheaper easier option.

Age. How long is the Battery? I recently changed a Battery that had done over 8.5 years service. It was fine when the car is in use but dropped the charge over night.

Sealed battery or cells can be topped up with distilled water. Check the sealed battery indicator, or with the standard battery the electrolyte levels and condition. As mentioned earlier, Halfords can do these checks.

My 1998 Mk1 is still on the original alternator, but 3rd battery.

[Scoobs]

So...I took the car to my local Battery specialist (not Halfords) who tested the Battery and the alternator. Battery was only managing 230 CCA and was definitely gone (it was the original battery). The alternator test was showing 14.1 volts, which was different to the 13.8 volts I was seeing with the old battery in. The new battery is a Varta Silver Dynamic and has been in for a week now. So far so good - the car starts very quickly and it is now quite evident that the old battery was on its last legs. I'm hopeful it was just the battery and there's nothing more to it.

[Druid Boy]

Glad you are sorted. I hope it was just the Battery - it usually is and they are a whole lot cheaper to replace!

[Haribo]

I had the same problem, I changed my Battery last Tuesday, and the car has been running fine all week

Just got into my car and now the batteries dead, so do I need a new alternator or is there a fault with my new Battery (sorry for the questions but I don't know anything about cars )

[alan333]

It could be either, or it could also be a fault in the electrics draining your Battery. It needs more testing to be sure.