Flat battery puzzle

[Quietlybonkers]

Not had any battery issues ever.....until this morning. Oh, except when I had the 100 amp fuse go on a 140 mile journey last summer, which of course flattened the battery completely.
used the car yesterday, two 40 mile trips with headlights on (foggy), then two ten mile trips after dark, again with lights on. No issues. Parked the car, locked and alarmed as normal.

This morning my daughter noticed that the driver's side rear indicator was glowing dimly.....not flashing, glowing. Went to the car, pressed the alarm fob (standard Meta), central locking worked just about, sluggishly. Tried locking and unlocking, no beeping on locking. Door release button finally activated the door solenoid at the third attempt, so got into the car and put the key into the ignition. Deactivated the immobiliser, battery definitely virtually flat - fan came on but engine wouldn't turn over. Connected up and plugged the (proper, not trickle) battery charger in, left it to charge for a couple of hours. Just tried it. battery has taken some charge, but not enough to start it. Central locking now works, four beeps on locking.

I have had mouse problems in the last three months, well documented on here, involving a number of chewed wires behind the radio and in the centre console, and some chewed carpets. All wiring soldered or replaced, car was working fully and normally yesterday, during daylight hours. I am not ruling out a mouse generated short last night, after I locked the car, but I did have the sonic mouse deterrent in the car.

I am suspecting the 100 amp fuse, but frankly I was concentrating on the road not the speedo last night (last time it went, the speedo read 45 when i was doing 70), and with oncoming headlights, and a deep river just to my left, I wouldn't have noticed if the speedo was reading wrongly. But thinking about it, on the way home in the dark I did think the headlights were dim, even on full beam - my four candles seemed to be a bit short on wick, but I just put it down to dirt on the glass at the time.

Anyone got any other thoughts before i crawl underneath the car tomorrow morning with my multimeter?

Edited by Quietlybonkers on Monday 7th January 10:37

[LuckyLucky]

Put your multi meter on the big positive terminal on the back of the alternator and to earth.

A reading means fuse OK as circuit isn't broken.

[rev-erend]

If the indicator was glowing then it would suggest that there is a earth fault or trapped wire near that point in the loom.

[Quietlybonkers]

Thanks both. I will investigate both options tomorrow (had to work today).
Tonight I am testing whether I have a cataclysmic battery short/drain or not.
Last night I locked the car at 6.30 pm, apparently working fine. 9 am battery flat.
I charged the battery up during the day today, went out to the car at 6.45 this evening and it started first time. Yay!!
So i have disconnected the charger and will see if I have a flat battery in the morning.
If I do, then I am looking for a short or similar. If I have a working battery, then the first thing I will check is the 100 amp fuse, followed by your suggestions. Clearly a non-working alternator would also cause the same effect as the 100 amp fuse.....

Any more ideas?

[Quietlybonkers]

Battery fine this morning. Car started first time. So that rules out major battery earthing or drain.
When the 100 amp fuses turn up (we never get any post on a Tuesday) I can move to the next step, as now I suspect 100 amp fuse or alternator.

[rev-erend]

Never ignore the symptom.

Only 1 way to turn on a light and that is to give it an earth..

[Quietlybonkers]

rev-erend said:

Never ignore the symptom.

Only 1 way to turn on a light and that is to give it an earth..

Thanks - I am being dim, like my headlights.......please explain a bit more. Much appreciated.

[LuckyLucky]

Quietlybonkers said:

Battery fine this morning. Car started first time. So that rules out major battery earthing or drain.
When the 100 amp fuses turn up (we never get any post on a Tuesday) I can move to the next step, as now I suspect 100 amp fuse or alternator.

Check the main power wire on the back of the alternator, saves grovelling under te car to check the fuse.

[Quietlybonkers]

LuckyLucky said:

Quietlybonkers said:

Battery fine this morning. Car started first time. So that rules out major battery earthing or drain.
When the 100 amp fuses turn up (we never get any post on a Tuesday) I can move to the next step, as now I suspect 100 amp fuse or alternator.

Check the main power wire on the back of the alternator, saves grovelling under te car to check the fuse.

Will do. Multimeters at dawn.........presumably I start the engine first?

[GasMunkey]

Yeah start the engine, should be looking at 14volts+

If it is the problem, get the part or have it refurbished, and I will fit it for you

[LuckyLucky]

Engine off.

Just think of the big nut on the back of the alternator with the red cable attached as an extension of your battery positive terminal with a 100 amp fuse in line.

Just open bonnet, put multimeter from that terminal to engine for a good earth.

12 volts = fuse OK
No reading = fuse blown.

[Quietlybonkers]

Many thanks for the kind offer .

[Quietlybonkers]

So if I test the back of the alternator and get something, the alternator is working, with or without a working 100 amp fuse. If no output, it's the alternator, not the fuse?

[LuckyLucky]

Test engine off to see if fuse has died.

Check with engine on and the alternator should show 14 volts or thereabouts.

If the reading slowly drops from 12. Something when the engine runs then your alt isn't putting out any current, easy peasy.

[Quietlybonkers]

LuckyLucky said:

Engine off.

Just think of the big nut on the back of the alternator with the red cable attached as an extension of your battery positive terminal with a 100 amp fuse in line.

Just open bonnet, put multimeter from that terminal to engine for a good earth.

12 volts = fuse OK
No reading = fuse blown.

Right - got it. Thanks

[GasMunkey]

Quietlybonkers said:

So if I test the back of the alternator and get something, the alternator is working, with or without a working 100 amp fuse. If no output, it's the alternator, not the fuse?

14+ volts at alternator = alternator ok

13 volts or less = alternator not well

14+ volts at battery terminal = alternator and fuse ok (most likely a knackered battery)

No volts at battery terminal = fuse gone (if alternator was checked out to be ok)

Edit, And what Peter said

[Quietlybonkers]

Ok. Been out in the dark with an inspection lamp and multimeter. Stuck the red lead into the thick brown cable near the alternator (back of alternator has black plastic cover, so went for the same place as the RAC man used last summer when the 100 amp blew). Got 12 volts. So 100 amp fuse ok.

Will start the engine and do part 2 in the morning, as it's not fair to my neighbours to start it ths time of night.

Thanks so far, Peter.

[Quietlybonkers]

Thanks for your last post, Alex. I will try that tomorrow at a more reasonable time of the evening (will be out from 6.45 to 6.45, like you), and with a torch bearer to help. The battery has taken and held charge, so my number one suspect, my dear Watson, is now the alternator.

[Quietlybonkers]

Right.
Engine off, multimeter black to earth, red to the brown thick cable going from alternator to 100 amp fuse. 12.4 volts. So on the LuckyLucky method of deduction, the 100 amp fuse is fine.

Engine on, multimeter probes in same place. Rev engine, 14.4 volts. So alternator is working.

So, chaps, why did I get a fully discharged battery in 24 hours?

Any thoughts would be most welcome. Please repeat any that are above and I still haven't tried - I am a bit dim at times.

[Hedgehopper]

If your battery is discharging that quickly is something getting warm, wiring or fuses perhaps? Might be worth a check.