Joolz,
it would help if you let us know some more details about the alternator.
To be specific; is it the one on your BMW engine and if so, does it have a remotely mounted regulator?

If it is the bike alternator, I find it surprising that it's rated at 50 amps, as some of the kit car crew who have fitted BMW engines complain about marginal charging when running with the headlights on.

ken

Measure the Resistance from "Batt +" to "Headlamp" then compare to a new bit of cable(similar length).
As cables + connectors + headlamp switch will also consume amps if not in good condition.

I see we're back at the crappy wiring/connections,a case of cleaning up all the connections earth points and maybe adding another good clean earth connection. I have halogen head lights a pair of driving spots,heated rear window radio and various other electronic items.I have never had problems with slow churning of the starter, even in the winter months. You can hear the tickover drop a few revs when you load the altenator,but it has never been a problem on the charging.

One thing with he colder weather is apart from you asking more of you electrics,you have to remember that as the lead acid battery is based on a chemical reaction,this is less efficent when cold,so it follows the better the connectionsthe better the electrics.

Pete, Not sure I follow, if there are any high resistances in the switches or wires it could be dropping voltage across them, but any increase in resistance will lower the current. I haven't tried measuring the voltage at different points around the wiring yet, the display I'm relying on is pluged into the cigarette lighter which is fed from the acc. position of the ignition switch via a fuse it shares with the radio.

Ken, yes standard BMW alt. There are two different ones found on the bikes, mine and a physically smaller one made by denso which I think has a lower amps rating.

This is the one fitted in my car at the moment,





No remote regulator or anything like that, it's exactly as a modern car alt and seems to be a very common bosch type. The idea I had for increasing it's output was to swap it's internals for those from a higher rated unit of the same type. I found these on ebay and the similarity in appearance suggested to me that parts would be interchangable, it would just be a case of using the front casing from my existing one, so that it still mounted on the engine in the same way.

Sorry I can't find an easy way to copy pictures from ebay.

70amp
http://www.ebay.co.uk/itm/VW-AUDI-SEAT- ... 27cb67f7eb

90amp
http://www.ebay.co.uk/itm/Saab-9000-90A ... 2576c007f5

120amp
http://www.ebay.co.uk/itm/OPEL-OMEGA-V6 ... 483e1c97ae

I've even seen a 150amp version from a range rover.

headlamps 2 x 45w
position lights 2x5w
rearlights 2x 5w
dashboard 2w

radio on? 30w

thats about it I guess

Makes 142 watt / 12 thats about 12 amp, so must be easy for your alternator, I guess theres something else wrong, maybee the internal voltage regulator is off or one of the blockdiodes is gone, (then it has only 1/2 or less capacity.....
or (one of) the brushes, making 14 volt when idling but less under load..
Is the belt tight enough , not slipping.
so enough things to check

That's what I thought, about 12amps plus whatever it takes to run the engine, should be easy for a 50amp alternator. I changed the brush/regulator block before because the previous one only gave me @13.5 volts, I've got the spare alternator that it came from so I could try fitting that and seeing if it makes a difference.

Tried the other alternator, but fitted with the same regulator and it gives the same results. I don't think it's a bady connection, the wiring's all pretty clean, it comes apart quite often. I tried measuring the voltage at the alternator with the lights on, but the reading was jumping around so much it wasn't making any sense. I'll try a different meter.

The current should already be rectified and voltage should be quite steady, maybe parts of the bridge are shot? As lpgo wrote (rectifier diodes). Changin the bridge for a new one is fairly easy in modern alternators (unlike some 2CV ones which require the whole alternator to be taken apart). It sits under that plastic cover on the back.

Analog?

Make sense (rectifier diodes). When one or more diodes are gone you have spikes all the time. (2 times in 1 revolution of the alternator, so I would check and or replace them, probably this will be it. Normally you must have a steady reading around 14,6 (3000 rpm) volt even with a digital voltmeter......