Voltage Levels To Look Or In Good Battery

February 23, 2011

My car alarm has started going off and it seems a common problem on this model (Avensis) when the battery's on the way out. It's marked with a sticker dated 2004 so I've checked it with a multimeter and got the following:

No load, engine off: 12.9v

With headlamps on only, engine off: 11.5v

Does it look okay? I'd have expected a dead Battery to have a lower voltage on both counts

February 23, 2011

Seven years from a Battery isn't bad at all, it depends how you use it. The reading with engine off, assuming the car has been left for some while unused, is quite high & can often occur when "sealed for life" batteries are on their way out. If headlights are put on for about 30seconds, engine not running, then about 12volts is normal. I have just changed my Battery, original from 2002, sitting on the bench it shows 12.9 on the multimeter, but would struggle to start the car. As mine is an automatic, I wouldn't take the chance. If you go to a Battery specialist, they can do a heavy discharge test (normally free) which will confirm the state of it.

February 23, 2011

The readings look OK at first glance.

Off load and you only get a true reading after several hours.

On load and it should support headlights/foglights/heated window all on together. No cells in the Battery should be gassing doing this test, if they are it's faulty. That should give a current draw of around 20 to 30 amps which a new healthy Battery should have no trouble supplying for 1 to 2 hours depending on capacity. Try that for 5 or 10 minutes and see what happens to the voltage. If it drops below around 10.8v then abandon the test and start the engine.

February 23, 2011

A batteries capacity isn't measured in volts but in amp/hours. After all if you joined 8 AA cells end to end it would give a voltage of 12 volts but we all know it wouldn't power car electrics.

Get the Battery load tested by a Battery supplier who has the correct test equipment and ensure they do at least 3 consecutive tests. If there is a week cell it'll show up.

Personally I wouldn't recommend any of the so-called fast-fit outlets - it's rare for them to have either the right kit or skills. Try and find a good auto-spark or specialised Battery supplier.

See attached picture of a proper battery tester.

February 23, 2011

Great. Thanks guys. It dropped to 10.5 volts within a few minutes so I think the battery's the problem.

February 23, 2011

Great. Thanks guys. It dropped to 10.5 volts within a few minutes so I think the battery's the problem.

If it dropped to 10.5v after only a few minutes then the Battery is certainly suspect as regards its capacity.

10.8 is considered the "fully discharged" voltage for a healthy Battery (1.8v per cell).

Whether the Battery is the cause of the alarm problem I wouldn't like to say... although you say it is a well documented issue. Perhaps the alarm monitors its supply voltage as one of its trigger conditions. Another possibility is that the alarm is sensitive to rapid voltage fluctuations of a few 10's of millivolts... and strangely that can occur with a lead acid battery particularly in the hours after charging stops (engine stops).

Lets hope a new one fixes it :)