Charge or jump start?

[kambites]

Krikkit said:

Is that not the standard ECU idle learning program once flat?

I hadn't thought of that, but it doesn't do the same if I disconnect the battery when working on electrical stuff.

Of course, being a Lotus, it's possible that the ECU is just broken. Whatever the cause, it's fecking annoying because it's quite hard to drive smoothly with the engine idling at 2500rpm.

My alternator is only 80amp, I think.

[ruggedscotty]

Okay lets put this one to bed.....

Engine off measure the battery voltage.

Engine on measure the battery voltage.

These will differ - the higher voltage being present when the engine is running as the alternator is providing power.

How does it do this ?

Ok basic - you have four coils - three coils are located in the rotor - these rotate (hence rotor) and generate electricity in the form of three phase voltage. This voltage is rectified and fed out to the battery. Its done this way as everything is connected to the battery so all the cars electrical requirements are connected to the battery. If you connect the alternator output to there then by default you are connecting to everything else.

With an electrical supply what ever has the highest voltage will feed the load. So this is why you have a higher voltage across the battery when the engine is running - the alternator supplies the voltage and this voltage difference between the battery and the alternator will cause a current to flow into the battery, charging it.

So how does the alternator make electricity ? I said you have four coils - three forming the rotor - there is one coil called the stator - this is stationary. This connects to the battery through a regulator. The regulator sensing the voltage being produced by the alternator. if this voltage is low then the regulator senses this and increases the current flowing through the stator - this increases the magentic field in the alterntor which increases the voltage that is generated by the rotor turning in that stator field. You need to draw mechanical energy into the alterntor through the rotor being attached to the engine crankshaft. The stronger the field produced by the stator the harder the rotor has to turn. This loads the engine and the engine electronics will pick that up and increase the fuel to the engine to try and maintain speed.

We have 2.5 MW alternators at work attached to quad turbo v16 engines that run at 1500rpm so you dont need to run them at crazy speeds to get big power. Its all in the field you spin the rotor in. The stator coil generating a magnetic field that the rotor turns in - and it is the coils cutting this magnetic field produced by the stator that generates a current flow. The rotor revolves so one half turn its generating a current flow in one direction and then the other half turn it generates current in the other direction giving you ac. This alternating current generates an alternating voltage internally thats converted to dc for output to the battery.

As soon as that engine is turning and the alternator is going is generating voltage. As that voltage is above the voltage of the battery its the alternator that supplies all the current and runs everything on the car

[normalbloke]

ruggedscotty said:

. Guff.

The poor chap only needed a maintenance charger.....

[battered]

Poor chap actually didn't need that, jump leads and taking it for a ride around town once a week would have seen it.

A maint charger just means he can forget about this if he wishes.

Good explanation scotty, FWIW in modern ECU's AIUI the idle speed is controlled by ignition advance not fuel (petrol engines). The load comes on, the engine slows down, the ECU says "hello, what's this?" and dials in a bit more advance, the engine speeds up, everything settles down. Take off the load, the reverse happens. That's a detail though.

I like the thought of a 2.5MW alternator. That'll run your radio.

[Flip Martian]

normalbloke said:

The poor chap only needed a maintenance charger.....

Well the CTEK arrived yesterday lunchtime and 24 hours later the battery was tested, desulphated, charged, tested and all done. Started like a dream of course. Wife is happy as her little Suzuki thing only gets driven short distances and gets left a bit in the winter, so she's reassured she won't be stuck with a car that won't start (although I doubt it would suffer that much). A great bit of kit - fit it, switch it on, let it work, take it off. Done. With 3 cars in the family, and possibly a 4th after xmas, no doubt it will prove a good buy.

[Mr2Mike]

kambites said:

There would be nothing miraculous about it. There is no theoretical reason engine speed should have any bearing on the output of a generator running from the engine;

Basic physics dictates that the unregulated output of an alternator will be proportional to it's rotor speed (rate of change of magnetic flux), and therefore engine speed. The regulator can't increase the output over and above the unregulated output for any given engine speed.

This chart shows alternator output current vs shaft speed for a few different alternator models:

Edited by Mr2Mike on Friday 9th October 11:39

[kambites]

Mr2Mike said:

Basic physics dictates that the unregulated output of an alternator will be proportional to it's rotor speed

Indeed, but the key word there is "unregulated". There's no theoretical reason (although there may be many practical ones) that you couldn't set the alternator up to be able to produce the maximum that a battery can charge at at idle.

I don't believe that chart; that's saying that all of those alternators put out no current at <1000rpm? So if you left a car idling it'd eventually flatten its battery and die?

[skyrover]

I find ctek chargers a bit poor for recovering flat batteries.

Your better off getting something that can put out a bit more amps to overcome any resistance, whereas ctek chargers will simply refuse to charge,

[kambites]

skyrover said:

I find ctek chargers a bit poor for recovering flat batteries.

Your better off getting something that can put out a bit more amps to overcome any resistance, whereas ctek chargers will simply refuse to charge,

Hmm, mine worked(ish) on my MGB battery which had been left flat for about a year.

[skyrover]

I bought 2 at the recommendation of a friend... neither would touch my flat land rover battery.

In the end I had to use a heavier duty charger with a 90 amp boost capacity to break the resistance and get the charge cycle started

[Mr2Mike]

kambites said:

Indeed, but the key word there is "unregulated". There's no theoretical reason (although there may be many practical ones) that you couldn't set the alternator up to be able to produce the maximum that a battery can charge at at idle.

Yes you could, but you'd have to have the alternator spinning so fast that it would overspeed and destroy itself when the engine RPM reached it's maximum.

kambites said:

I don't believe that chart; that's saying that all of those alternators put out no current at <1000rpm? So if you left a car idling it'd eventually flatten its battery and die?

Look at the chart, SHAFT sped not engine speed. Alternators run faster than the crank speed.

All automotive alternators have a similar performance profile to this. If you buy a reconditioned alternator you sometimes get a print out of it's measured performance.

Honestly, this is O-level/GCSE physics stuff.

[kambites]

Mr2Mike said:

kambites said:

Indeed, but the key word there is "unregulated". There's no theoretical reason (although there may be many practical ones) that you couldn't set the alternator up to be able to produce the maximum that a battery can charge at at idle.

Yes you could, but you have to have the alternator spinning so fast that it would overspeed and destroy itself when the engine RPM reached it's maximum.

Or you just increase the number of turns and hence the field strength.

I'm aware of how an alternator works - indeed I've built a very crude one in the past. However, it is perfectly possible to set one up to generate significant current at 800rpm and still be perfectly happy at 8000rpm. Now maybe manufacturers don't do this for cost/complexity/whatever reason but it is perfectly possible.

[kambites]

Mr2Mike said:

Look at the chart, SHAFT sped not engine speed.

Ah, oops.

I've never measured the relative size of the pulley wheels on the crank and alternator, I don't really have a feeling for how different they are. I suppose it must be at least a five to one reduction in circumference.

Edited by kambites on Friday 9th October 11:52

[R8VXF]

Talking of jump starting, I was told in Halfords that it would be dangerous to jump start my 1.2 Corsa with the VXR8. Any truth to this?

[TooMany2cvs]

skyrover said:

I bought 2 at the recommendation of a friend... neither would touch my flat land rover battery.

In the end I had to use a heavier duty charger with a 90 amp boost capacity to break the resistance and get the charge cycle started

That's going to be the case for any smart charger - they need to "see" that they're connected to a battery, right way round, in order to start outputting. If the battery's too flat, no smart charger (unless it's one with a "No, honest, trust me - you are" button) will do anything.

Some of the bigger Cteks offer a "supply" mode - where it'll put sparks out without "seeing" a battery. That'd get your flat-flat started.

[normalbloke]

TooMany2cvs said:

skyrover said:

I bought 2 at the recommendation of a friend... neither would touch my flat land rover battery.

In the end I had to use a heavier duty charger with a 90 amp boost capacity to break the resistance and get the charge cycle started

That's going to be the case for any smart charger - they need to "see" that they're connected to a battery, right way round, in order to start outputting. If the battery's too flat, no smart charger (unless it's one with a "No, honest, trust me - you are" button) will do anything.

Some of the bigger Cteks offer a "supply" mode - where it'll put sparks out without "seeing" a battery. That'd get your flat-flat started.

But then the 2 battery in parallel trick comes in handy.

[Howard-]

HustleRussell said:

Either really. It's a modern car so I'd have no qualms about just jumping it and taking it for a drive.

On the contrary, you need to be careful jump-starting modern cars. I know Ford's SmartCharge system is very sensitive to this and can cause huge voltage spikes which can (and do) fry various electronic modules. I know someone who had to replace the body control module in their 2008(ish) Fiesta as a result of jump-starting. I think if you must jump-start then it helps if you don't start the 'helper' car's engine - just hook the battery up to yours.

[Hooli]

R8VXF said:

Talking of jump starting, I was told in Halfords that it would be dangerous to jump start my 1.2 Corsa with the VXR8. Any truth to this?

It's bks.

However I wouldn't leave a car with a big battery connected to a car with a small battery for ages. The car with the larger battery will normally have much higher alternator output to charge the larger battery. This can mean the charging ampage is too great for the smaller battery on the other car.

I can't see it ever being an issue with cars, just something to bear in mind with bikes etc.

[Robert Elise]

TooMany2cvs said:

skyrover said:

I bought 2 at the recommendation of a friend... neither would touch my flat land rover battery.

In the end I had to use a heavier duty charger with a 90 amp boost capacity to break the resistance and get the charge cycle started

That's going to be the case for any smart charger - they need to "see" that they're connected to a battery, right way round, in order to start outputting. If the battery's too flat, no smart charger (unless it's one with a "No, honest, trust me - you are" button) will do anything.

Some of the bigger Cteks offer a "supply" mode - where it'll put sparks out without "seeing" a battery. That'd get your flat-flat started.

i use a smart maintainer with just .8 amp max output. perfect for maintaining a healthy battery and little chance of over heating were things to go wrong while you're away for weeks. (the one fire i'm aware of with a 5amp output was admittedly someone who shut his conditioner under a rag under the bonnet....not me!).

[battered]

normalbloke said:

But then the 2 battery in parallel trick comes in handy.

I've heard this one, never used it myself. However in the motor trade it's a favourite. Connect a spare battery to the dead one with jump leads, connect the charger to the spare, walk away for a few hours or overnight. Charger sorts them both out, spare battery gives the dead one enough oomph to wake it up, possibly by passing heavy current through the cells and dislodging the sulphate film, who knows?