Flat battery

[roachw]

Hi, I have some readings and additional info:

• Engine off (with no chargeconditioner for just over 1 week)  = 12.4v
• Minimum seen during cranking = 11.4v
• 3,000 rpm = 13.75

The issue only seems to occur with extended use, so drive the car for an hour and during that time the battery, instead of charging, discharges (down to 11.4v last time around). I bought a new battery some 5 months ago believing that to be the problem. At that time, the supplier put their diagnostic equipment onto the car, found no issues and so assumed it must be a 'sulfated' battery that would not hold charge properly.

Also, its worth re-highlighting that when the car is in the 'fault state', and revving at 3,000 Rpm the mult-meter shows the volts across the battery actually decreasing.

I have carried out some of the other checks too:

The alternator is fixed firmly and the alternator belt is in new condition and very tight. All connections look good, tight and clean. The car is a 2015 sigma with only 3k miles so I don't think wear is too much of a concern in this case. However, there is a cable that goes from the alternator to the starter motor and there appears to be a kinkcrack just below where it attaches to the alternator, I'll post a picture shortly. I was trying to disconnect it from the alternator to check further but there is a silicon 'blob' on top of the nut and I'm struggling to undo the nut without ripping the silicon 'blob' from the nut. What do you guys think?

Thanks,

Will

 

 

 

[Jonathan Kay]

Those voltages look OK.

Also, its worth re-highlighting that when the car is in the 'fault state', and revving at 3,000 Rpm the mult-meter shows the volts across the battery actually decreasing.

What voltage have you seen?

And does the warning light come on (assuming it works normally when starting)?

It would be interesting to measure the voltage at the live terminal of the alternator at the same time that it's low at the battery.

However, there is a cable that goes from the alternator to the starter motor and there appears to be a kinkcrack just below where it attaches to the alternator, I'll post a picture shortly. I was trying to disconnect it from the alternator to check further but there is a silicon 'blob' on top of the nut and I'm struggling to undo the nut without ripping the silicon 'blob' from the nut.

Definitely needs investigating. A thick cable? It's often wired that way to save a run on the high current live side. I'd get it off if you can't get a look all the way round it in situ. The silicone may be an attempt at waterproofing or a bodge for a bad connection. Remove it so that you can have a good look at the wire and connections.

Jonathan

 

[roachw]

thanks for the super prompt response :-)

here is the suspect wire:

[Jonathan Kay]

Do you think the conductor is damaged? You might be able to detect some crepitus on flexing. If you aren't sure take it off and strip a bit to get a good look. (It will be easy to reinsulate: I'd probably use tape and then heatshrink sleeving if you can get it over the connector.)

Jonathan

[roachw]

Thanks Jonathan, I'll do that. Not sure how this issue fits with the intermittent nature of the fault but I'll do as you suggest and see how it goes. I'll keep you posted.

 

Thanks again,

Regards,

WILL

[John Vine]

...crepitus...

I love it!  You'd only ever get that word from a medical man!

I had to look it up:

"a grating sound or sensation produced by friction between bone and cartilage or the fractured parts of a bone."

That pictured brown cable supplies the alt output to the battery, so poor conductivity there would match your symptoms.  

JV

[SM25T]

I remember that sensation when I broke my collar bone in 2005. Proper break rather than just a fracture. The gravity was particularly strong that day !

[roachw]

Hi chaps, I've ordered a replacement cable from CC.

any tips on how to  do this "It would be interesting to measure the voltage at the live terminal of the alternator at the same time that it's low at the battery."

Whilst I have a multi-meter I am a novice with it.

 

thanks,

Will

[SM25T]

Put the black lead on to the -ve termal of the battery .... or a clean metal part of the engine. Put the red lead on to the alternator terminal bolt. Set the meter to say 20V dc range. Be aware of hot parts like exhaust. Be aware of moving parts like alternator belt.

[Jonathan Kay]

I've ordered a replacement cable from CC.

What did you find when you had a look at the other one? Was the conductor damaged?

Jonathan

PS:

Proper break rather than just a fracture.

Those mean the same to me...

[Jonathan Kay]

There are still lots of possible causes but you're making progress.

What SM25T says about testing. The question is whether that dropping voltage you see at the battery is caused by dropping output at the alternator or by resistance in the wires and connectors. Measuring directly at the output should tell those apart.

Jonathan

Experts: Do you prefer to do it this way or by looking at the voltage drop between points of interest along the live side? Thanks.

[revilla]

Measuring voltage drops is generally preferable in circuits which pass higher currents. Very small resistances can lead to large voltage drops in the presence of large currents (Ohm's law in words!). It's much easier to measure a large voltage than a small resistance accurately.

[Jonathan Kay]

Thanks, Andrew. I've learnt that one!

But I was thinking of measuring PD between different points on the live side as opposed to a series of measurements of PD from each of those points to earth...

Jonathan

[revilla]

If you're looking for largish voltage drops it makes little difference. However if you've got a series of small voltage drops they are easier to measure when not superimposed on a large (and possibly fluctuating) background voltage, so measuring along the circuit can be easier.

[Jonathan Kay]

Thanks

Jonathan

[roachw]

Many thanks for the info guys, working away ATM but will progress when I get home.

regards,

Will

[roachw]

Just an update - I've had the alternator off and tested, it's 100% fine.

So I think it's a wiring fault....

[Tom_Arundel]

Just a thought, are you driving with headlights on? Sigma alternator struggles to keep up charge unless you keep engine revs high, i.e. slow traffic/ low revs = flat battery with headlights on.

[roachw]

Thanks Tom, no it goes flat on a long run within lights on.

I'm taking it to an auto electric place tomorrow as I'd really like to have it resolved for any Christmas blast.

TTFN

[roachw]

Sorry -  without*

[roachw]

So, just to close this one off. Having confirmed the battery & alternator were both good, I took the car back to Bristol Auto Electrical for them to diagnose the fault. Thankfully they did find and fix the issue.

The problem was a faulty earth connection, possibly the one that goes to the chassis near the alternator. BAE said that they had carried out lots of voltage drop tests and resistance checks and noticed that when they simply put the car into gear, the charge level began to drop a little. So just that small engine movement was enough to trigger the poor earth connection. BAE's preferred fix was to introduce a new earth strap going from the battery earth to the starter motor. Job done, my temporary volt meter is now showing charging levels of around 14 volts, higher than I've ever seen!

Whilst reading some of the content on here recently I noticed a post by somebody that said "90% of electrical issues on 7s were caused by poor earth connections". I'll bear that in mind for the future and try to improve my own diagnostic & multi-meter skills.

Thanks again to all who've helped me with this :-)

[rockinroyston]

Get yourself one of these too..

https://www.amazon.co.uk/gp/product/B01BEZV0IA/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1

DBPOWER 600A Peak 18000mAh Portable Car Jump Starter Battery Booster Pack Smart Power Bank Charger with Compass, LCD Screen and LED Torch for Laptop Phone Tablet and More (Black/Red)

Saves getting the charger out... I have one that starts my Dodge Ram 5.7 litre v8 which quite often drains its battery in the winter if I don't drive it for a week or so, I just keep it in the glove box. You wouldn't believe something so small could turn the engine over... but it does! Unfortunately the truck doesn't fit in the garage & I have to park it down the road... so highly inconvenient to lug the battery around (I've now bought a new battery LOL, which took a while to track one down...).

[grumpy the 7th]

Hmmmm! new earth strap from battery direct to starter motor.

Now that's a good idea.

It cuts out the middle man, belt and bracers etc. I've already run a double +ve to the solenoid whilst fixing the 'dreaded click of death' with the relay mod, but a direct earth to the starter might be on the agenda for the next time I'm in the garage.

 

[Kensboats7]

Just a thought here, is there any possibility of having a backup battery permanently in linked in for the winter months to give more capacity? I have been keeping mine indoors after it dying in the garaged car, but it is a bit of a pain if the weather looks good.

 

[Jonathan Kay]

Wouldn't be hard to wire but of course it would take up space.

How would that compare with one of those amazing small light jump starter packs for your needs? One advantage would be that it wouldn't require any clipping on or removing the bonnet...

Jonathan