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morris 19th July 2015 10:37

Alternative alternators
I've been looking at alternatives to the current arrangement of my alternator being down on the off side of the engine next to the exhaust. You can buy small 60A alternators that should fit in the correct place on the M50 engine and then allow me to have a proper belt tensioner rather than my current best guess fixed tension.

There's a helpful formula at the link above that tells you what output you need. I've been round the car working out the power draw on various components but the ones I'm not sure on are

fuel pumps
Engine ignition, injection and ECU.

I know what fuses I have fitted but these will be typically much higher than the normal power draw.

Does anyone have any ideas, especially the engine?

MartinClan 19th July 2015 18:06

Horns you can ignore as hopefully they are not on often.
Fans vary but 10amps is agood allowance. But again it should be on infrequently so you can probably ignore.
Fuel pumps. 3amps or less for the lp. 5ish for the Hp.
Engine that is ecu injectors and coil -bit of a guess but arround 7amps.

A 60 amp alternator should be more than adequate. The big consumers are the headlights specially if you have additional driving lights and use the bmw system of both main and dipped on together.

Cheers Robin

Grey V8 Pete 20th July 2015 10:11

Also worth checking the heater motor (on full) and the wiper motor as they are often on together and continuously. Peter.

NigelB 20th July 2015 20:54


My memory is a bit vague here but I checked the current drain in all ignition switch positions before I connected the battery for the first time.

I seem to remember 9.4amps in the 'ignition on' position which compare favourably with Robin's 7amps. So if you assume 10 amps I don't think you'll be far off.


morris 20th July 2015 20:59

yeah, my gut instinct was that 60A should be more than enough for the sportster. I have quad main beams with the headlamps and spots though I was going to ditch the spots. I only bought them as I didn't realise that my headlamps did side, dipped and main beams. I'm not sure if my dipped and mains stay on at the same time or not. The engine definitely dips revs slightly when you flash which is a bit odd.

Pete, no problem with wipers and heater, I have none of those luxury items :)

In fact judging by the temperature of my feet and knees during the IVA test, staying warm won't be an issue, more heat shielding is needed if anything.

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