Battery drains

paul lambers · 9th January 2020, 13:27

My battery drains within a couple of days. The battery is brand new, and I fully loaded it. There are no lights on anywhere in the car after closing the car, satnav shuts down properly etc. Testing with a voltmeter shows normal charging of the battery. My garage searched for leakage of currents and also did a 12 hrs measurement and couldn´t locate any unwanted, unusual currents.

I searched the forum and found that a failing alternator diode can drain the battery after shutting down the engine. I checked this with a voltmeter and when the car is running, it shows about 27 volts when put on AC. So now I think it is a failing diode. I also notice that the lights of my dashboard are and stay very dim when I put on the head lights while driving.

My garage doesn´t believe the alternator is the cause, as they didn´t measure any unwanted current and the alternator charges the battery properly.
So, can the alternator be broken (diode) and the current it draws back not being detectable the way they measure, does anyone know?

Paul

COLVERT · 9th January 2020, 17:27

1/ What's the voltage at the battery terminals with the negative lead disconnected ?---( 12.6 volts+ )

2/ What is the voltage at the battery terminals with the engine ticking over ??---( 14.4 volts or close. )

3/ If a diode is failing and is leaking current to earth it will be detectable especially if the battery drains in two days.

4/ Does the regulator for the dash lights work ??

MarkS · 9th January 2020, 18:40

The following does NOT apply to diesels.

Make sure the engine is NOT running.

Remove the 150A (alternator) fuse in the engine fusebox.
Put a 10A DC ammeter (& a 10A fuse in series if possible) in the 150A fuse circuit. Be very very careful to not drop probes or let wires short to the chassis.
(With the engine still NOT running), any leakage due to diode failure will show up on the ammeter.

Do NOT try to start the car without reinstalling the 150A fuse, otherwise the alternator may be damaged.

Finally, if you are not sure what you are doing, take advice or don't do it. 12V lead acid batteries, ammeters and not enough knowledge generally leads to sparks, melted wires, damaged meters and blown fuses. Ammeters can be very dangerous if misused.

paul lambers · 9th January 2020, 19:14

Thanks for the help.

The regulator of the dash works.
I just checked the voltage, it dropped from 12.12V at around 15.00 to 11.87V at 21.04, not good i.m.o. The accu still has a green dot, not informative in this case I suppose. I measured the voltage of the battery, engine not running, with the multimeter put on AC and it shows 25.5V?? I first will disconnect the battery, to see if it can keep its voltage at all, overnight. Then for tomorrow morning I think of disconnecting the regulator, to see what happens then and later also do the tests you proposed.

MarkS · 9th January 2020, 19:40

AC voltmeters will give differing results depending on how they work.

Give them a non-sinusoidal waveform, or a waveform with a DC component, and anything may happen.

Take no notice of what an AC voltmeter reads with anything but a pure AC 50Hz sinewave.

If you must use an AC meter, know how your meter works, or prepare to be led up a blind alley !

COLVERT · 9th January 2020, 20:08

The green dot shows the condition of that one cell only.

There could be a problem with any of the other five cells.--

Disconnect battery from the car. ( negative lead. ) and charge over night. Remove the charger and let the battery rest for an hour and then take a voltage reading.

It should read around the 12.5 voltage mark.--( DC )

If possible leave it unconnected for a few hours then take another reading. It should be almost identical to the first reading. If it is then this removes the battery from the list of problems.

Let us know A.S.A.P.

9th January 2020, 13:27	#1
paul lambers Regular poster rover 75 2.5 saloon Join Date: Nov 2007 Location: Gelderland Posts: 55 Thanks: 0 Thanked 1 Time in 1 Post	Battery drains My battery drains within a couple of days. The battery is brand new, and I fully loaded it. There are no lights on anywhere in the car after closing the car, satnav shuts down properly etc. Testing with a voltmeter shows normal charging of the battery. My garage searched for leakage of currents and also did a 12 hrs measurement and couldn´t locate any unwanted, unusual currents. I searched the forum and found that a failing alternator diode can drain the battery after shutting down the engine. I checked this with a voltmeter and when the car is running, it shows about 27 volts when put on AC. So now I think it is a failing diode. I also notice that the lights of my dashboard are and stay very dim when I put on the head lights while driving. My garage doesn´t believe the alternator is the cause, as they didn´t measure any unwanted current and the alternator charges the battery properly. So, can the alternator be broken (diode) and the current it draws back not being detectable the way they measure, does anyone know? Paul Last edited by paul lambers; 9th January 2020 at 14:06..

9th January 2020, 18:40	#3
MarkS Gets stuck in Rover 75 Diesel Tourer, called Perseverance Join Date: Nov 2011 Location: Bournemouth Posts: 664 Thanks: 189 Thanked 157 Times in 108 Posts	The following does NOT apply to diesels. Make sure the engine is NOT running. Remove the 150A (alternator) fuse in the engine fusebox. Put a 10A DC ammeter (& a 10A fuse in series if possible) in the 150A fuse circuit. Be very very careful to not drop probes or let wires short to the chassis. (With the engine still NOT running), any leakage due to diode failure will show up on the ammeter. Do NOT try to start the car without reinstalling the 150A fuse, otherwise the alternator may be damaged. Finally, if you are not sure what you are doing, take advice or don't do it. 12V lead acid batteries, ammeters and not enough knowledge generally leads to sparks, melted wires, damaged meters and blown fuses. Ammeters can be very dangerous if misused. Last edited by MarkS; 9th January 2020 at 20:06.. Reason: Finally.....

9th January 2020, 19:14	#4
paul lambers Regular poster rover 75 2.5 saloon Join Date: Nov 2007 Location: Gelderland Posts: 55 Thanks: 0 Thanked 1 Time in 1 Post	Thanks for the help. The regulator of the dash works. I just checked the voltage, it dropped from 12.12V at around 15.00 to 11.87V at 21.04, not good i.m.o. The accu still has a green dot, not informative in this case I suppose. I measured the voltage of the battery, engine not running, with the multimeter put on AC and it shows 25.5V?? I first will disconnect the battery, to see if it can keep its voltage at all, overnight. Then for tomorrow morning I think of disconnecting the regulator, to see what happens then and later also do the tests you proposed. Last edited by paul lambers; 9th January 2020 at 19:20..

9th January 2020, 19:40	#5
MarkS Gets stuck in Rover 75 Diesel Tourer, called Perseverance Join Date: Nov 2011 Location: Bournemouth Posts: 664 Thanks: 189 Thanked 157 Times in 108 Posts	AC voltmeters will give differing results depending on how they work. Give them a non-sinusoidal waveform, or a waveform with a DC component, and anything may happen. Take no notice of what an AC voltmeter reads with anything but a pure AC 50Hz sinewave. If you must use an AC meter, know how your meter works, or prepare to be led up a blind alley ! Last edited by MarkS; 9th January 2020 at 20:07..

9th January 2020, 17:27	#2
COLVERT This is my second home R75 Saloon. Join Date: Feb 2009 Location: France/or Devon. Posts: 14,003 Thanks: 3,851 Thanked 2,167 Times in 1,816 Posts	1/ What's the voltage at the battery terminals with the negative lead disconnected ?---( 12.6 volts+ ) 2/ What is the voltage at the battery terminals with the engine ticking over ??---( 14.4 volts or close. ) 3/ If a diode is failing and is leaking current to earth it will be detectable especially if the battery drains in two days. 4/ Does the regulator for the dash lights work ??

9th January 2020, 20:08	#6
COLVERT This is my second home R75 Saloon. Join Date: Feb 2009 Location: France/or Devon. Posts: 14,003 Thanks: 3,851 Thanked 2,167 Times in 1,816 Posts	The green dot shows the condition of that one cell only. There could be a problem with any of the other five cells.-- Disconnect battery from the car. ( negative lead. ) and charge over night. Remove the charger and let the battery rest for an hour and then take a voltage reading. It should read around the 12.5 voltage mark.--( DC ) If possible leave it unconnected for a few hours then take another reading. It should be almost identical to the first reading. If it is then this removes the battery from the list of problems. Let us know A.S.A.P.