Alternator output ?

[phib]

Been trying to help a friend with a BMW 840ci who has been having battery issues.

Had flat battery and then replaced with a new one two and a half weeks ago, since then the car has been running perfectly we thought it was solved. Used every other day long runs short runs all good until today .....

Went to start it and flat battery again it had been left since Sunday, so its either alternator or battery drain,

The battery had 11.6v, but the ignition clicked. So jumped it all good battery reading 12.4 v with engine running, lights and qc on, with engine running no lights not ac 13.6v

Am I right in thinking alternator is working ok ?

Any advice welcome

Thanks
Phib

[chrisbirrell]

Sounds like alternator is fine but there is standing battery drain. If the alternator was knackered, the car would slowly grind to a halt as you drove along and the battery drained. As you have said it drives fine when used regularly and the battery is only flattened when it has been left for a while, this would lead me to the above conclusion.

No idea what could be causing the drain off the top of my head, I shall leave that to the more knowledgeable here.

[phib]

Thanks that was my thought but electrics not my strong point !!

I think the alternators are loads of £'s so hopefully thats good news, now trying to find drain !!! how many electrics !!

My theory is either, parrot blue tooth kit or car not going into sleep mode

Phib

[MGJohn]

phib said:

So jumped it all good battery reading 12.4 v with engine running, lights and qc on, with engine running no lights not ac 13.6v

Am I right in thinking alternator is working ok ?

Any advice welcome

Thanks
Phib

With lights and other stuff on on my Rover 620ti a healthy Alternator reading should be something like this at idle :~

[ShaunTheSheep]

phib said:

battery reading 12.4 v with engine running, lights and qc on, with engine running no lights not ac 13.6v

The alternator's goosed :-(

Only 1 battery? I've got it in my mind there's 2 batterys on these, there's one in the boot, right hand side behind a panel, can't picture where the other one is. Maybe there is only one battery.

[Huff]

OP - via google find how to unlock the onboard computer ('OBC unlock' or similar) - it is a simple procedure involving a few button presses and the date. Then you can tap in the right test code, I think it's test 3 - and the OBC will show battery charging voltage as you drive around - very neat

Anyway, the post above is correct. Alt voltage should not really drop below about 13.6v at any point (except momentarily) and when the battery is fully charge / on a cruise (motorway) it should show something in excess of 14.0v (14.0- 14.4v or so depending on temperature)

If not - check the alternator ( I think the reg block/brushes are replaceabl); check all major wiring inc the engine- body ground wire for security and corrosion, check the 80a fuselink near teh battery if its in the back of the car like the E32 / E34 of similar vintage. HTH.

[phib]

Ok makes sense, before I give him the potential bad news, would it make any difference that we had only just jump started a flat battery ? I.e. The low reading

It was dark this evening so I will check again in the morning
Thanks
Phib

[chrisbirrell]

Yes, it would make a difference that it was run flat and then jump started. The lead acid battery chemistry has quite a steep voltage/discharge curve and jump starting it does not instantly recharge the battery, but the actual measured voltage across the terminals depends on battery voltage, current and internal resistance, which means it can be higher or lower than the 12.6V nominal value depending on conditions. To check this, I would start it, go for a decent drive (30+ minutes) to allow the alternator to (hopefully) charge the battery and then measure the voltage under load (electrics on), minimal load (electrics off) and with the ignition off.

I know nothing about checking computer codes on a BMW but if it works then why not?

[M3333]

I am an auto electrician.

You need a multimeter that reads AMPS. Disconnect the battery. Put the Ammeter in series with the connections and see what current is being pulled. Make sure this is done with interior lights off, doors shut, locked etc so how the car would be stood. Anything more than a few miliamps is not normal.

If you establish a drain then pull each fuse out one at a time from the fuse board until the drain dissapears. You then know which circuit the drain is on and can work from there.

Advice given at your own risk!

[Mr Dave]

There are pleanty of 540s about and its not too hard to get an alternator for them, I picked one up for my dads 840 which is similar enough to work and a few other bits and pieces for 80 quid.

Looks easy enough to swap anyway. Easy cars to work on really although some parts can be dear and a lot of places charge more for 840 parts than 540 parts because they can get away with it.

[PhillipM]

Sounds like the regulator/sense circuit on the alternator is goosed, it's not upping the output when the voltage drops.
Just double check the connections because it wouldn't be the first time I've seen it and it's merely been a poor crimp on the wire or abraded through from rubbing against something!

[phib]

Right quick update

Flat battery again this morning 4.5V

Put it on charge ( only for 20 mins) started first time

Initially 12.6V when running at idle .6k revs

Then 13.9v under load 3k revs

Drove it 20 mins, all good apart from the car saying it had a side light issue, think its a loose connection banged it it works ( can sort that out later) maybe this is the issue doubtful though

So after 20 mins its reading 13.9v at idle .6k revs and when switched off the battery now registers 12.6V

I am going to sort this one way or other ....

Thanks for help Phib

[MGJohn]

phib said:

Right quick update

Flat battery again this morning 4.5V

Put it on charge ( only for 20 mins) started first time

Initially 12.6V when running at idle .6k revs

Then 13.9v under load 3k revs

Drove it 20 mins, all good apart from the car saying it had a side light issue, think its a loose connection banged it it works ( can sort that out later) maybe this is the issue doubtful though

So after 20 mins its reading 13.9v at idle .6k revs and when switched off the battery now registers 12.6V

I am going to sort this one way or other ....

Thanks for help Phib

Those are better readings. Could be both Alternator at fault AND something draining batteries when parked up for a period.....

[phib]

Well in 2.5 hours its gone from 12.6V to 12.04V , surely this must be battery drain ?

Phib

[dvance]

M3333 said:

I am an auto electrician.

You need a multimeter that reads AMPS. Disconnect the battery. Put the Ammeter in series with the connections and see what current is being pulled. Make sure this is done with interior lights off, doors shut, locked etc so how the car would be stood. Anything more than a few miliamps is not normal.

If you establish a drain then pull each fuse out one at a time from the fuse board until the drain dissapears. You then know which circuit the drain is on and can work from there.

Advice given at your own risk!

This. Follow it, and you should know if there is battery drain.

Alternatively, checking the reading of the battery at a set interval of a few hours when the car is not being used for a while should also give you a clue if there is battery drain. Yet another method would be to disconnect the battery for a period of time which would otherwise render the car dead, and then reconnect and try to start. If all works, then the problem is not with the battery, and you definitely have a drain issue. However, I recommend the quoted technique above. Will give you an easy answer in 15 minutes of your time.

EDIT: You should be using the DCA setting (or the 10A) setting on your multimeter if you go for the above technique. Can't make out the writing out for the different modes, but you should start from the one that supports the highest current to make sure you don't burn out your multimeter. If you get no reading, go down through the settings to make sure there really is no current being drawn.

Edited by dvance on Thursday 18th August 11:51

[E30M3SE]

ShaunTheSheep said:

Only 1 battery? I've got it in my mind there's 2 batterys on these, ....................

Think you are right on that.

[MGJohn]

phib said:

Well in 2.5 hours its gone from 12.6V to 12.04V , surely this must be battery drain ?

Phib

A healthy Battery should read around 12.6 Volts after that time.

Try the above suggested disconnect test. It is possible for a faulty Alternator to drain a battery when car is not in use. There is always a drain on a battery in many modern cars for the alarm and radio code support. Not a big drain but, a drain even so.

Systematic pulling of each fuse in turn will show which item is responsible if a battery drain is identified.

[g3org3y]

If the battery has drained all the way down to 4.5v I'd be concerned about permanent damage and not being able to retain charge.