Charging battery while engine running

Smeeagain · Post by **Smeeagain** » Tue Nov 15, 2011 1:48 pm

Please settle a dispute

Can the battery be charged on 20047X5 E53 3.0d Sport whilst the engine is running but the car is stationary or does the car need to be driven ?

I don't mean connecting a battery charger - I mean does the alternator charge the battery in that state

Smee

pvr · Post by **pvr** » Tue Nov 15, 2011 2:26 pm

The alternator should deliver 14v, so yes, it will charge.

Probably won't charge a lot if you have all consumers on such as A/C, rear window heaters etc ...

Raj · Post by **Raj** » Tue Nov 15, 2011 4:20 pm

Yes it will. The alternator supplies the charging system with current and it will charge the battery as long as the control module believes the battery requires charging even in a stationary state.

I your battery is flat and you jump start it, you may just let it idle for sometime to provide a charge for the battery.

SimplyRed · Post by **SimplyRed** » Tue Nov 15, 2011 4:26 pm

The man from Aston Martin told me something that I cant get my head round.

He said that the battery will charge faster at idle if the headlights are on.

(something about the alternator)

Can there be any truth in that?

Raj · Post by **Raj** » Tue Nov 15, 2011 4:34 pm

I'm a little unsure at that. The alternator simply provides the same current. If anything I would have thought the opposite as more of the supplying current would be required by the headlights.

SimplyRed · Post by **SimplyRed** » Tue Nov 15, 2011 4:38 pm

[quote=""Raj""]I'm a little unsure at that. The alternator simply provides the same current. If anything I would have thought the opposite as more of the supplying current would be required by the headlights.[/quote]

Yeh thats what i thought, he said that at idle the alternator may not even be doing its job and that by siwitching the lights on the small current it uses would get the laternator running. Beats me???? He is the senior technician at Lancasters so he must know something???

Rusty · Post by **Rusty** » Tue Nov 15, 2011 6:27 pm

Could easily be something in it.

If the alternator isn't seeing a suitable current draw, it may not switch in so as to reduce emissions through unnecessary loading, thus if you apply a load (headlights etc) the alternator kicks in and so charges the battery.

This may be of a greater application if the alternator is not even driven until loaded, this is sometimes done by means of an electo-coupling.

Just my thoughts but may help.

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Raj · Post by **Raj** » Tue Nov 15, 2011 8:03 pm

Possibly, but as in my earlier example when you have a totally flat battery you jump start it and can let it idle for sometime and it will increase the charge of the battery as when you switch the car off and try and start it there is charge to start it again. Will have to try and find out for sure.

X5Sport · Post by **X5Sport** » Tue Nov 15, 2011 8:33 pm

It's all down to that magical device called the regulator which will monitor the battery voltage and hence the 'load'. Increase the load and you reduce the voltage and so the regulator gives more output. But, this does mean that the alternator needs to be spinning at an optimum rate or it simply cannot produce enough output, so although you will get a charge, it may need a few more engine rpm to do the job properly. A slightly fast idle is better in these cases.

For the really technically minded, the regulator monitors the battery voltage and stator voltage in the alternator itself. The regulator then controls the rotor field current to suit the load/demand required.

The lowest optimum rpm for most low frequency automotive alternators (they generate AC which is converted to DC via the Rectifier) is around 3,000rpm and hence they are normally driven at 2-3 times crankshaft speed, hence you need between 1,000 and 1,500 rpm on the engine to give the minimum optimum rpm on the alternator.

The Aston-Martin contact is sort of right, in that extra load means increased output from the alternator, but it also depends upon the rpm being available. Really fancy alternators no longer use Regulators as they have 'electronic control units'. What's the betting BMWs have those then?

With the old dynamo systems, output depended upon rpm to a much higher degree.

Smeeagain · Post by **Smeeagain** » Tue Nov 15, 2011 10:48 pm

So if they need 1000-1500 rpm then they're not really charging while stationary then ?

Smee

X5Sport · Post by **X5Sport** » Wed Nov 16, 2011 9:47 am

It will charge, but the output from the alternator will be lower than it might be at a fast tick-over so the charge rate will be less and it will take longer.

It's always better to use a proper charger. Better for the engine as it won't soot up, better for the fuel cost too!! A Charger always gives a constant steady rate which is what a battery really needs.

Raj · Post by **Raj** » Wed Nov 16, 2011 10:33 am

So I guess to answer the original question, yes it will be charging although not maybe at optimum performance.

X5Sport · Post by **X5Sport** » Wed Nov 16, 2011 11:12 am

[quote=""Raj""]So I guess to answer the original question, yes it will be charging although not maybe at optimum performance.[/quote]

Er........well, since you put it so succinctly .........yes..

Raj · Post by **Raj** » Wed Nov 16, 2011 12:27 pm

[quote=""X5Sport""][quote=""Raj""]So I guess to answer the original question, yes it will be charging although not maybe at optimum performance.[/quote]

Er........well, since you put it so succinctly .........yes..

[/quote]

How eloquent my dear sir.....

Between us all we're unstoppable.....lol

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