Club 80-90 Forums

2.5mm^2 copper cable has a resistance of 0.01ohm per metre. So at 10A eg headlamps it will drop 0.1V per metre.
6mm^2 copper cable at 50A will drop 0.3V per metre. Anything more than this is due to poor connections.

CovKid wrote:... but it does prove your point that an alternator upgrade may not be the answer at all - particularly on stock wiring.

Yebut... I'm still on stock wiring, apart from the run between the alternator and the starter, and my calculations (backed up by Marlinowner's post above) show that that wasn't responsible for the old voltage drop (maybe my new clean connections play a part here though).

The fact is that the bigger alternator DOES provide a better voltage under heavy load in my case. So, just guessing here, when the draw on the alternator is at or near the limit of what it can supply (I wonder what the output profile looks like as a function of load), then the starter battery starts to supply the amps, so its internal resistance comes into play and the voltage drops. Providing more current from the alternator stops this happening, so the resistance of the cable/connections dominates.

ghost123uk wrote:So as not to pee Chris off though.....

Don't worry, I'm not peed off

ghost123uk wrote: = 72 Amps = Chris, you have a point !!!!!!!!!!!!!!!!!!!!!!

See.

Ah - alternator standard ratings seem to apply to a rotational speed of 6000rpm. I think I'd need to measure pulley diameters to relate that to engine speed, but below that speed the current produced is less - see below.



Conversely, as engine speed rises I assume the draw from the ignition circuit increases (more sparks per second). So unless the pulley sizes are optimised then it may be that the maximum output from the alternator (after subtracting the ignition circuit draw) isn't as much as you'd expect.

Using a rough and ready measurement of the pulleys I reckon the alternator pulley will turn about three times as fast as the engine, so it will reach it's 'rated' rpm at an engine speed of about 2000rpm. So maybe my point above isn't so significant after all.

Definately a bit of both then Chris - cables and alternator. Voltage drops per metre and ohms law aside, the volt meter says things have changed quite a bit. May be tidying up battery connections has also helped. I did wonder if going heavier on headlight harness (ceramic H4 plugs, thicker wire etc) was worthwhile but think that will do for now. You should get on well with the 90amp alternator but I definately recommend supplementing the capacity to dash. Cable, lugs and junction boxes - perhaps £30? Took me about an hour - hardest bit really was scrabbling underneath to route the cable.

Oh, and its starts a bit easier now too.

As you would across any connection to check resistance (represented by a voltage drop between two points).

Martin

CJH,

I am also looking to fit the larger 90 amp alternator from a 45 amp one and was just wandering if I need a different i.e. longer alternator belt or is the alternator physically the same size?

Just want to be prepared for arrival.

Many thanks, Mark.

I didn't have to change the belt. I didn't make a note of the old versus new adjustment point I'm afraid - should have done - but fortunately I have a photograph of where the adjustment bolt was with my old alternator from this thread, and it doesn't look any different now.

I have a Brickwerks 1075 belt and one Keith's one-size-fits-all adjuster brackets from the thread I linked above (now with an added dog-leg), and the adjustment bolt is nicely in the middle.

E D I T: I should have said - the geometry of the mounting points to the pulley centre is slightly different between the two alternators as far as I can tell, but the net effect is that the adjuster seems to be in about the same place.