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Hi all

After a month or two of trouble-free driving, this morning I went to start the green Forester SF and it sounded like the battery was low. The car started, I ran it for 5 minutes, no warning lights etc,  stopped the engine, cranked it over again and it started as it should. I left it parked until this evening, 7-8 hour's later, and it wouldn’t start, obviously with a drained battery.

The battery has been reliable and strong, it is a two year old OPTIMA red-top battery. 
This evening I jumped it with the other Forester, and after one minute of running I could stop and start the engine on its battery as normal.

I then pulled all of the fuses one by one with the ignition off and measured voltage. There was  voltage drop only across the 10A ALT-S alternator fuse connectors in the engine bay fuse box. All other fuses showed no voltage.

No lights left on, key out of the ignition, but voltage across the alternator, is that correct?

Reading during a few minutes showed voltage dropping by 0.01V every ten seconds, I imagine until the battery is drained.      

I tested the voltage across the battery whilst running the engine and read a healthy charging current from the alternator.

 The only thing I have done to the car in the last days is try a cheapish OBDII reader to find an intermittent CEL ,  but this only showed error when connected; “can’t connect with vehicle”.

Could a faulty code reader possibly set off a battery drain in the charging circuit? Impossible coincidence?

Am I barking up the wrong tree, and does the alternator circuit always carry battery voltage?

 Stumped here, as auto electrics are not my cup of tea!

Thanks in advance for any ideas or starting points.

From the owner’s manual. The highlighted fuse showed voltage with the key out:

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Edited by Mitchy

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After some research I see how I should progress- parasitic battery drain tests. 
Fascinating.

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Normally the alternator output is connected to the battery through the fusible link, so it will show voltage. I think they call it the Main Fuse on yours.

I would disconnect one of the battery posts and let it sit for 7-10 hours, randomly checking the voltage. If it goes below 12 volts it is weak.

If you have an AMP Meter you can hook it between the battery post and the cable you removed from that post and see what current is being drawn from the battery. When you first connect it will show a spike and then should settle down to almost nothing.

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any chance there's a CD stuck in the radio? that has caused problems on some cars before. stuck in some EJECT cycle.

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Years ago I had a battery mysteriously draining overnight. Replaced the battery as it was pretty shot, still happened. Everything seemed to check out initially. I even took it to NAPA (where I got the battery - small town) and they hooked it up to their fancy battery and charging system tester. They gave it the thumbs up.

After much searching and advice from online I ended up finding it was a leaking diode on the alternator. I can't remember the various tests I did but this rings a bell:

Hook up a load tested, or multimeter should work, to the battery terminals then start up the car.

The charging voltage should look good but give it a minute and if the voltage slowly drops below what is acceptable that can indicate a bad diode.

The more knowledgeable members can certainly correct me on this and maybe I'm way off base with the issue but thought I mention this.

 

Could you swap the alternator from the other Forester into this one and see if the issue persists?

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3 hours ago, potter2010 said:

Years ago I had a battery mysteriously draining overnight. Replaced the battery as it was pretty shot, still happened. Everything seemed to check out initially. I even took it to NAPA (where I got the battery - small town) and they hooked it up to their fancy battery and charging system tester. They gave it the thumbs up.

After much searching and advice from online I ended up finding it was a leaking diode on the alternator. I can't remember the various tests I did but this rings a bell:

Hook up a load tested, or multimeter should work, to the battery terminals then start up the car.

The charging voltage should look good but give it a minute and if the voltage slowly drops below what is acceptable that can indicate a bad diode.

The more knowledgeable members can certainly correct me on this and maybe I'm way off base with the issue but thought I mention this.

 

Could you swap the alternator from the other Forester into this one and see if the issue persists?

I had the same (leaky-diode) problem on another car (Suzuki) several years ago.

I fixed it temporarily by wiring a 12V relay into the alternator circuit.  The relay contacts were Normally-Open, so that the alternator-diodes were disconnected when the ignition was off (0V), and the contacts changed to Closed when the ignition was activated (12V).

I ran with this temporary-relay for several months, until I eventually found a good used alternator.

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Thanks guys.

@1 Lucky Texan:
I have no CD player. Therefore no stuck CD!

Following internet advice on these common leaks, I diligently turned everything off and removed the key, doors closed etc. and did a few tests.

 One expects a small current (20-50mA I understand), for the immobilizer, clock etc, testing for current in the mA scale. My reading today was 20mA, and no fuses other than ‘Clock’, when removed, changed that.

I made the same test on the other Forester and got the same results.

seems to be normal then...

Testing for AC in the mV scale across the battery whilst running the engine showed 3.2mV, apparently a sign that the alternator diode is faulty, as this reading should be zero. I tested the other Forester and got the same reading, 3mV.  ?

I have been starting and running the car today, and the battery voltage seems to be holding steady now at 12.35V.

I have left the battery connected tonight, and in 8 hours I will know if this has just been ‘one of those things’ or if the problem persists.

I hate vehicle electrics, but at leastI have learned some more about this.   

Thanks all for the input, and I will report back as to whether this was  just due to a badly closed tailgate or some other gremlin.

 

Edited by Mitchy

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As far as I know the battery voltage suppose to be around 12.5 volts with engine not running and somewhere around 14 volts (13.8 to 14.2) with the engine running.

 

Good luck,

Sam

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20 hours ago, Mitchy said:

Testing for AC in the mV scale across the battery whilst running the engine showed 3.2mV, apparently a sign that the alternator diode is faulty, as this reading should be zero. I tested the other Forester and got the same reading, 3mV.  ?

For AC ripple testing, 3mV is good, 50-100mV when the car is loaded at 1.5-2k rpm (all lights on, accessories running) is concerning but acceptable.  100mV+ is a failing alternator.  I prefer testing ac directly from the alternator red wire and alternator case ground.

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Thanks again. 
The car cranked over fine again this morning (05:45am, aargh!) got to work with heater and lights on.

In a few hours I’ll be back in the driver’s seat after a 16 hour day, so ‘fingers crossed’.

This problem appears perhaps to be cured, but unsolved....

 

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6 hours ago, somick said:

As far as I know the battery voltage suppose to be around 12.5 volts with engine not running and somewhere around 14 volts (13.8 to 14.2) with the engine running.

 

Good luck,

Sam

That sounds about right.

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