De-charging issue.

[Jesus325iTouring]

My battery has now decided to de-charge.

Only de-charge's when running.

First discovered at night as the car has only be used during daytime for months.

Can the lights cause a de-charge,and if so how and what should I look for?

Usual story,Jesus does mechanic's but Jesus don't do leccy's

Thanks in advance.

PS I haven't got a multi meter.

[Brianmoooore]

PS I haven't got a multi meter.

Time to invest a tenner then!
The battery is a storage device.
If you fill it as fast as you empty it, then all will be OK. If the filling rate is limited for whatever reason, and you increase the emptying rate (heavy load, like headlamps), then it will go flat.
You need to measure the voltage across the battery terminals with the engine running.

[Jesus325iTouring]

Thanks Brian,I like your layman description,perfect

I'll have to go and get a meter I guess,been putting off for years!

[Jesus325iTouring]

Right,i've got multi meter.

What should I do with it Brian?

I've worked out the ON button so far

[Simon13]

set it to volts, DC current i think* and fire the engine up and put the prongs on each terminal, see what it reads!

Might be the alternator kaput, my electrical skills aren't too hot either.

[Brianmoooore]

DC current range will result in a rejected warranty claim for your meter!! Not a good idea at all!
DC voltage range, probably 20 volts, if it's not an autoranging meter.
Place the black probe on the battery negative, and the red probe on the battery positive, then read the meter.
Start the engine, measure the voltage again.
Hold the engine at about 2500 RPM and measure again.
Turn on the main beam headlamps, rear screen, fan and wipers, measure the voltage, and finally rev the engine to 2500 RPM again, and take one more reading.

[Jesus325iTouring]

Righto Brian,many thanks i'll be trying this in the morning,little dark out now, and i'll post up the results.

Si,cheers chap

[Jesus325iTouring]

Right,here goes.........

With the car running and nothing on 11.77

With the car running and everything on 10.86

With the car running and everything on EXCEPT the lights 11.15

Whats the prognosis then Brian?

[Morat]

Those are a bit low, I would imagine Brian is going to advise you to get either a new battery or start poking around the alternator (which charges the battery).

Over to the wiringmeister

[Jesus325iTouring]

Right,your probably right Morat

Different battery........

1st Running with nothing on 14.50

2nd Running half an hours driving later with nothing on 13.68

3rd Running with everything on 11.65

4th Running with nothing on 13.50 increased after 45 seconds to 14.59

[Jesus325iTouring]

Should also add,both batteries will charge and run in my R5 no problem.

[daimlerman]

Time for a re-con alternator,I reckon.Do not forget to re-fit the earth lead.

[rallye_turtle]

Could possibly be the following:

* Alternator regulator pack brushes worn
* Faulty alternator phase diodes (or capacitors, I think)
* Bad ground between alternator and mounting

Had this problem with mine as the charge voltage across the battery was only around 11 - 12.5v. My issue turned out to be a faulty alternator as some of the phase diodes where faulty. Are some of the warning cluster lights glowing faintly?

[Jesus325iTouring]

Which earth lead do you mean DaimlerChap?

The only glowing dash light is the battery one,this is glowing a little,gets brighter,goes off,glows again etc etc. Currently isn't on at all

[daimlerman]

Some M20 alternators are mounted in rubber bushes and therefore require an earth lead,I found out the expensive way that mine was one of them....

[Jesus325iTouring]

Some M20 alternators are mounted in rubber bushes and therefore require an earth lead,I found out the expensive way that mine was one of them....

I'll check in't morning,thanks

[Brianmoooore]

I think Mr Turtle has the answer. Probably diode trouble in the alternator, losing you a phase or two, and severely limiting the alternators maximum output.
Possibly worth unscrewing the regulator pack on the back of the alternator and checking the length of the brushes, and you could do one last check with your meter to check the alternators earthing - Switch the meter onto the voltage setting again, and with the engine running, put the red probe on the alloy part of the alternator, pushing the probe's tip in hard to make contact, and place the black probe on a good body earth (bracket by the fusebox that the bonnet hooks into). The meter needle shouldn't move.
If it doesn't, change the meter setting, one at a time to its lowest voltage range.

[Jesus325iTouring]

I think Mr Turtle has the answer. Probably diode trouble in the alternator, losing you a phase or two, and severely limiting the alternators maximum output.
Possibly worth unscrewing the regulator pack on the back of the alternator and checking the length of the brushes, and you could do one last check with your meter to check the alternators earthing - Switch the meter onto the voltage setting again, and with the engine running, put the red probe on the alloy part of the alternator, pushing the probe's tip in hard to make contact, and place the black probe on a good body earth (bracket by the fusebox that the bonnet hooks into). The meter needle shouldn't move.
If it doesn't, change the meter setting, one at a time to its lowest voltage range.

Thanks Brian,with the brushes,if they are worn are they worth replacing or is best just to put another alternator on?

And with your last suggestion above,if the meter needle does move,does this also suggest the alternator is shot?

Few more checks to do then,thanks for the advice,I hate leccy's

[Brianmoooore]

If the brushes are completely worn so that they are beginning to arc on their ends, then pulling gently on them to stretch the braided wires straight will give a few hundred miles more life from them, but as I said above, I think your problem is more likely diode failure in the alternator itself, not the regulator pack.
A reading on the meter, from alternator body to body earth, indicates an earthing problem.

[Jesus325iTouring]

If the brushes are completely worn so that they are beginning to arc on their ends, then pulling gently on them to stretch the braided wires straight will give a few hundred miles more life from them, but as I said above, I think your problem is more likely diode failure in the alternator itself, not the regulator pack.
A reading on the meter, from alternator body to body earth, indicates an earthing problem.

Champion,thanks again O' Wizard

[Jesus325iTouring]

Right then,altenator -> body zero reading,even on low setting,so earth ok.

Daimlerchaps suggestion of seperate earth strap...... not present on mine.

So Brian,i've taken some pictures..........

What I believe to be the brushes have plenty of life in them,this is the brushes yes?



Is this this bit the diode thing?



This is the bit that the brushes rub on,does it look like it should?



And finally a general picture,it looks a bit rusty in here is this usual?




Thanks for looking Chap

[beemerbird]

Brushes should be minimum 5mm long. If your battery light is glowing I'd agree that some of the diodes have died in the alternator and you'll need to put a new one on.

[Jesus325iTouring]

It doesn't always glow,thats whats bugging me Damn electrics!

[Brianmoooore]

There's not a lot of meat left on those brushes, but they look just long enough, unless one was stuck. If a brush is the problem, there will be signs of pitting from arcing on the end, instead of the polished surface their should be.
A lot of the grey dust inside the alternator is what used to be the rest of your brushes!
You can see three of the diodes in the end view

[bmwscorpion]

Hi Brian.
I have read the above as I have a similar problem. my altenator has a reading from casing to chassis earth. so I have swapped the altenator with another one I had here. still the same, where should the altenator earth to? it is a late altenator on an early engine, dont know if that makes a different, either that or the alternator from this other car is also shot.
I have on your instruction removed and refitted all earths, the temprature gauge jumps up when the head lights are turned on.
The voltage when the engine is running is only 12.4v @ 2500 rpm again suggesting altenator prob..
any help is appreciated.?

[Brianmoooore]

If you can measure a voltage of more than a few mV between the alternator casing and a earth point, there is a faulty earth connection or connections between the alternator and that point.