Ignition left on for two hours

[Ian.V]

Good evening electrical wizards ( and everyone else)

I inadvertently left the ignition on for two hours on a Disvcovery 3.

Started ok but battery light now permanently on.

I presume something very expensive has burnt out?

Any help much appreciated.

 

[lynall]

Most likely need codes read.

You could try a hard reset, which is remove both battery leads, then hold the batteyr leads together for a few seconds, then reconnect them to the battery.

Might be worth checking if its charging with a voltmeter across the battery terminals.

 

[Ian.V]

Thanks Lynall
I'll try those options tomorrow

 

[Nodge68]

I'm surprised the battery isn't flat, but it'll definitely need a charge after keeping the car systems running for 2 hours.

 

[Ian.V]

I was surprised myself, particularly as I had to make an urgent journey of around seven miles with the battery light on.
Fired up ok this morning but testing with a volt meter confirms it’s not charging.
I was hoping it was the alternator fuse but of course we’re never that lucky are we!!
Anyone know of a reliable auto electrician in the Notingham area?
Thanks

 

[kermit_rr]

I know it doesn't help, but if i leave the ignition switched on, on the bmw, it gets bored and switches itself off after a while

 

[Ian.V]

I can't believe that this has damaged the alternator.
Maybe I should buy a BMW

 

[Nodge68]

I can't believe that this has damaged the alternator.
Maybe I should buy a BMW

It can damage the alternator, as the rotor windings are active at full current, but the alternator isn't turning, which means no cooling.

 

[kermit_rr]

I can't believe that this has damaged the alternator.
Maybe I should buy a BMW

I'm selling my bmw and keeping my RR

 

[flat]

I can't believe that this has damaged the alternator.
Maybe I should buy a BMW

i was behind a bmw 120D in traffic the other day that had a window sticker that said ‘life is to short for mundane cars’ - I presume the driver had stuck it on a mundane car by mistake....

Don’t go mundane stick with the adventure that is a LR product

 

[kermit_rr]

i was behind a bmw 120D in traffic the other day that had a window sticker that said ‘life is to short for mundane cars’ - I presume the driver had stuck it on a mundane car by mistake....

Don’t go mundane stick with the adventure that is a LR product

They are mundane now. Run of the mill. Mass market. Safe.
They used to be the ultimate driving machine, according to bmw.
That said, anything made in the last ten years is a mundane tech unit. You don't even have a throttle anymore, but a power request pedal - request X and the onboard computers manage the throttle, injectors, turbo and timing to deliver precisely the power you requested

 

[NPG]

I can't believe that this has damaged the alternator.

Although one could theorise how something like that could happen with the ignition on and engine off, I'd say it's extremely unlikely. I don't have a D3 but at the time when the D3 came out, smart charging and battery management systems were becoming mainstream. Maybe it's an area that you need to look at - sometimes a reset would work just fine - either with an OBD tool or simply disconnect the battery for 10 mins or so.

 

[kermit_rr]

It's not trying to alert to a high state of discharge, is it? Have you checked the voltage at the battery with engine off and engine on?

 

[Nodge68]

It's perfectly possible for the alternator to fail, if the ignition is left on, without the engine running.

 

[Ian.V]

Problem solved by going back to basics!!
I think I was over-thinking the possible causes due to the scenario.

I decided to clean the battery terminals even though there was very little sign of corrosion.
Vaselined and replaced and tightened.
Light goes out!!

Incidentally to answer Kermit’s point the reading without engine running before was 12.6 and with engine running 12.2. This is why I presumed it must be the alternator.

Since cleaning the terminals the readings are 12.6 – 14.4
I’m inclined to think it was more a case of a reset as mentioned by NPG as opposed to dirty terminals although as the first voltage readings suggested it wasn’t charging, was I actually reading the voltage going into the battery as opposed to the voltage coming out of the alternator?
in which case I presume it would have been due to the terminals.
Hope this question makes sense!!

 

[NPG]

Problem solved by going back to basics!!

Good to hear that although I'm not 100% sure of what went on in here. It looks like the regulator wasn't energising the field windings on the rotor but like I said, I'm not familiar with the charging system on the D3. What I know from modern passenger vehicles is that smart charging and battery management systems have "memory" and determine states based on current and historical data. This is why I suggested resetting the system and start from scratch, as if you're installing a new battery.

BTW, @Nodge68 does have a point when he says that an alternator can fail if the field windings are drawing a current (ignition on) for a significant amount of time without cooling (engine off). The current needed to excite a magnetic field can vary from 3A to 10A but the initial voltage is usually quite small and an alternator typically needs less than 10% of the output to maintain a strong magnetic field. It really all depends on the type of alternator you have installed - it could be that yours is controlled by the ECU or has an internal inertia switch that allows the field windings to energise only when the rotor starts to turn. Both these scenarios are common in modern vehicles as a means to do load shedding when there is a high current demand by a critical component such as the starter motor and also to conserve energy while the engine is running. In both these instances, the alternator draws zero current when only the ignition is on.

 

[Ian.V]

Blimey my brain hurts after all that NPG, but thanks for such a comprehensive reply.

Also thanks to all who contributed to this thread.

As a last and probably silly question

Under what circumstances, other than a short, would the alternator 5amp fuse have blown?

 

[NPG]

Under what circumstances, other than a short, would the alternator 5amp fuse have blown?

I don't have the circuit schematics for the D3 so I'm using this as a reference : http://rave.stringsandints.com/LRL183853501(LR3-Wiring).pdf

I'm assuming we're talking about Fuse F20E located in the BJB (Page 142). It seems that circuit is always live and connects the internal regulator to the battery so the alternator can monitor the presence of the battery and its state of charge. Can't say for sure why such a fuse could have blown except like you say, in the event of a short circuit. Perhaps there's an issue with the regulator or a charge with too high a voltage was applied to the battery without being disconnected. But I'd replace the fuse for now and monitor the situation before investigating further.

 

[Ian.V]

Thanks for all that plus the wiring diagrams.
I think maybe I wasn't clear regarding the fuse.
Initially I was hoping it was a blown fuse but as I said things are rarely that simple, the fuse was ok but I was just wondering what would have blown it if it had.