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Charging System Dilemma
#11
According to my old neighbour who was an auto electrician 12.5v is low for a battery and 12v is a flat battery. He was very good at his job and I was sorry to lose him as a neighbour he helped me out many times with problems I had been scratching my head over.
So where does this bit go then ?
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#12
(29-08-2019, 06:46 AM)ffrenchie Wrote:  According to my old neighbour who was an auto electrician 12.5v is low for a battery and 12v is a flat battery. He was very good at his job and I was sorry to lose him as a neighbour he helped me out many times with problems I had been scratching my head over.


Your Auto electrician was right of course 12.5v you have a very low battery at 12v your battery is ready to put at the end of the drive for the tat men.

I do a lot of short journeys and my battery is an Exide, they used to be a very good brand but this one is not the best at 3 years old. In the winter my voltage can go down to 12.5v when it does i can tell by the cranking speed. I have to put it on charge (just a 4A charger) and it will be okay for a few weeks. 

The alternator is fine its just a rubbish battery and short journeys that cause the problem.
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#13
(26-08-2019, 11:50 PM)David_R Wrote:  
(03-08-2019, 01:04 PM)iano Wrote:  They diagnosed a failed drop test on the battery and low output on the alternator, and recommending to renew both at a cost of £600 ish. Ouch.
I'm sceptical

You should be.

Battery condition and Alternator output are very easy to check.  Connect a multimeter to battery voltage, ideally directly to the battery, but it doesn't have to be, off the cig-lighter socket or interior lamp will do.  Put it somewhere you can check it while driving.

When you come to the vehicle in the morning, the battery voltage should be nearly 13V.  12.5V is ok, 12V is a bit low.
Glow-plugs on, BV should not drop below 11.5V  If it gets to 11V there is something wrong.
Cranking.  Volts should drop to about 11V.  Lower and you have an Issue.

Immediately the engine starts, BV should rise.  Very quickly it should be 13.5V-14.4V.  Lower than this indicates a weak alternator.  Higher indicates a failed voltage stabiliser in the alternator and it will be cooking you battery.

Now drive for a bit.  BV should hold at somewhere between 14V and 14.4V.

Now turn EVERYTHING on.  lights, wipers, interior fan, etc.  BV should drop a bit, but not below 13.5V.  If it does, the alternator is weak.

Now turn it all off again.  BV should again be 14-14.4V.

Turn off and allow the vehicle to sit with no electrical load for at least 30 mins.  BV should stabilise back at it's "resting" voltage.  As per above this should be somewhere close to 13V.  If it drops back towards 12V, the battery is tired.

Example:  My car (actually a Merc, but that's irrelevant)  sits at 12.2V when parked.  Glowplugs pulls this down to 11v, cranking sees 9.5.
Driving sees the voltage a very stable 14v.  Big electrical load only drops this to 13.9v
Engine off and the volts drop back to 12.5 within a few minutes and 12.2 after 30 mins.

Conclusion:  Alternator output is good.  Battery is tired.  (which it is.. you get about 15 seconds cranking before it's dead)

Apologies for the tardy reply, been away on holiday. Thanks very much David, that's helpful !
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