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Author Topic: 12 volt car better life  (Read 274 times)
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« on: November 16, 2020, 01:48:27 PM »

Our 11 year old diesel estate car AGM 12v battery is failing I think. As someone who dreams of installing a domestic battery backup I am not doing very well.

It needs charging about once a month, it goes down to 11.4v and charges up to 12.8v. It still starts and runs OK. The in-car charging tests OK and at the end of a 100 mile run is is 12.8v but within two days it is down to 11.4v.

Advice please! Given its age and performance is it time to fit a new one! Thank you.

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Alan D
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« Reply #1 on: November 16, 2020, 02:11:13 PM »

Hello David

Ref " The in-car charging tests OK and at the end of a 100 mile run is is 12.8v but within two days it is down to 11.4v. "

When the engine is running the voltage across the battery should be a minimum of 13.8 volts with headlights on . If its not there is an alternator problem. Have you checked the belt tension.
How often is the car used. If not often it should be charged once a month with a high capacity charger to 14.4 volts and held at that voltage for one hour.

My van has to have the battery disconnected if not used for a few days. So many electronic bits / bobs inside new type cars / vans
Older vans would sit there for months and still start
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« Reply #2 on: November 16, 2020, 02:17:40 PM »

The lifespan of these lead-acid starter batteries is much more to do with how it is treated than how old it is. They like being held at 100% all the time, any time it is discharged below 80% it will be damaged.

Many cars draw significant amounts of power while the car is sitting doing nothing. Batteries also "self discharge" to sone extent. If you leave the car more than a week or two without being used, it'll discharge to a level where the battery will be damaged. Car alternators generate very little power at low revs, and take some time to recharge the power used starting the car, this is doubly so in winter when lights, heater-fan, heated window, and other large consumers of electricity are used. As such many car batteries have a lifespan of 3-5 years.

You've clearly treated yours well if it has lasted 11 years. It is time to buy a new battery!

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« Reply #3 on: November 16, 2020, 06:30:20 PM »

As others have said, if alternator is ok then put your hand in your pocket and buy a new battery, 11 years is good going!

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« Reply #4 on: November 16, 2020, 06:58:04 PM »

Difficult call. As others have said, 11yrs on a lead acid car battery must be a near record so it has served it's time, and could be a sign of one cell dead or dying.

On the other hand, as AlanD said, the charge voltage is wrong, implying either a faulty regulator or two phases dead in the alternator rectifier. But this could also be created by the faulty battery.

The easiest way is to fit another known good battery but please then test the voltage on load as Alan said to make sure the alternator is working correctly before you drive any distance. Otherwise you will just knacker the good battery in short time (and the warranty on a new battery will be void as it was not charged correctly).

Also check alternator, main battery and engine earth strap connections for corrosion as this is another source of alternator not charging at correct voltage even though the starter load can sometimes 'short through' the corrosion giving the impression connections are OK. Check for voltage between engine earth and battery -ve with engine running and lights on.

« Last Edit: November 16, 2020, 07:01:34 PM by Antman » Logged

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