Alternator Output

[Guest mcramsay]

One thing I have noticed when running my engine is that due to the fact I had to fit a smaller 40 amp nippon/denso alternator I have charging issues at idle. When the engine is cold and on fast tickover the battery is seeing over 13 volts, however as the engine heats up and idle drops to around 750 the battery spilt age is around 12 and starts to drop. If you give the engine a few more Rpms it jumps back up. I am guessing that the alternator is not spinning fast enough at idle to give sufficient output. The pulley diameter is 54mm with the stock alternator pulley having a diameter of 50mm and also being a 100amp unit.

 

Aside from machining a new pulley slightly smaller is there anything else I could do? I cannot raise the tickover at idle as it is all programmed in on the ecu which I cannot change.

[Jez Morton]

Smaller alt pulley or bigger drive pulley only way to make it spin faster, don't think there's any thing clever you can do with it to increase low rpm output.

[theduck]

Could you not just adjust the pedal stop to increase idle?

[Guest Ian & Carole]

750 rpm is a very low tick over surely it needs to be higher than that ??

[Guest mcramsay]

I did think of adjusting the pedal stop( well there is no pedal at the moment) but reading on the bmw forums the ecu has target rpms, which it will get to with the IACV and ignition timing adjustment so I'm not sure if I will start interfering with the ecu if I start messing around. On the bmw diagnostic software it states idle should be between 700-800 rpm.

[sj-bradley]

I wouldn't worry too much about the volts output from the alternator.

 

After all, how long do you REALLY spend at idle anyway?

 

As you say, the output jumps back up when you give it some revs which is fine, that's what it's supposed to do and the increased speed gives the internal fan some cooling air to the alternator. If the alternator was working hard at idle, the excess current it is generating would cause more (I squared) current heating which it might not be able to get rid of if running at idle.

 

Simon.

[Longboarder]

13v is a bit low when you rev it. A healthy alternator should give low 14s at anything over 1500rpm. Look to the alternator wiring. What alternator are you running. Sure it's getting enough excitment?

 

Nigel

[Guest mcramsay]

Alternator has a main battery feed, exiter wire which is def getting 12v and a warning light feed I think the max I saw on the volt meter was around 13.8 at 1500 rpm. It's only a 40 amp alternator, it might be struggling to cope

[sj-bradley]

Don't forget that alternator max voltage is limited by the internal electronic circuit to prevent overloading the external electrical devices.

 

If that regulator circuit is bypassed or fails, you could get much more voltage than the electronics (ECU etc) can cope with.

 

It may be that that alternator max's out at 13.8 volts. Have you tried it under full load - I.e Headlights, fog light and everything turned on (The wipers would produce pulses in the output voltage which may make reading it tricky)

 

If the voltage output stays above the (nominal) battery voltage, then it's coping with the load and still charging the battery.

 

You can estimate what the full load will be if you add up all the wattages of the lamps and other items and then divide the total by 12 (nominal battery voltage) and that should give you and approximate guess how many amps are required. That's the "Pig In Van" triangle ...

 

Also, if you have one of the clamp style multimeters, you could measure directly the current through the alternator output cable (I've just got one of those so will be trying this out myself)

 

Simon.

Edited March 17, 2015 by Simon B

[Guest mcramsay]

The problem is that I have a tiny race battery, that has very little reserve, I would like to know that if sat in traffic and the rad fan comes on that I won't have to sit with my foot on the throttle to keep things ticking along. Looking at a few websites on alternators they say the alt should be spinning around 2000 rpm at idle, after doing a few sums mine is spinning around 1600. So a slightly smaller pully should achieve this.

[fry61]

We have an Odessy 680 battery ( about the size of a car radio ) & it has proved to be O.K with all loads thrown at it -- heater fan/rad fan/head & spotlights. Had thought of doubling up on the batteries but not proved necessary -- yet. The alternator doesn't give an output at tick-over, same as yours.

[Guest mcramsay]

Same battery as I am using, after idling for around 30 mins and the rad fan coming on the battery voltage was dropping to around 10.5 volts and the engine was starting to struggle. That is with minimal Load on the battery, just a gentle press on the throttle started charging again

[Guest mcramsay]

I know the chances of being sat at idle for that long are slim! But you never know when you will get stuck in slow moving traffic!