Engine RPM and battery charging.

[sutats]

Hi folks,

Does the engine RPM affect the speed in which the battery charges? (e.g. the battery will replenish its charge more quickly when the car is driven at a higher RPM than lower, ceteris paribus)

Thanks.

[Pumaracing]

Basically no if everything is in good condition. Even at fast idle the alternator will be producing way more current than the battery can take provided no electrical ancilliaries like headlights are on. By 2500 rpm plus or minus a bit depending on the vehicle the alternator should be capped at its maximum output of 100 amps or more so even with full ancilliary load there will be more than enough left over to charge the battery at its full rate of maybe 10 amps.

If alternators didn't produce enough power at low rpm to drive every ancilliary the car is fitted with then batteries would go flat all the time.

[SuperchargedVR6]

I've noticed some modern high power Alternators (140A or more) free wheel in one direction. Is this to stop the drive belt being damaged by the alternator's inertia at high rpm throttle closures? These beefy alternators seem to have a lot rotating mass! Probably the weight of all the copper windings?

[stevieturbo]

SuperchargedVR6 said:

I've noticed some modern high power Alternators (140A or more) free wheel in one direction. Is this to stop the drive belt being damaged by the alternator's inertia at high rpm throttle closures? These beefy alternators seem to have a lot rotating mass! Probably the weight of all the copper windings?

I think it's to increase belt life as much as reduce NVH too. As you say, there is a lot of mass attached to the wheel

[anonymous-user]

It's a belt saver yes, especially on DSG cars which now upshift very fast indeed (hence engine rpm drops v fast)

[sutats]

Thanks for your informative replies.

I average about 1500 RPM and hardly ever go over 2000 RPM (there’s over 9000 RPM available. lol). It’s a daily smooth coasting drive in slow flowing city traffic and thank goodness don’t have any electricals to run. Lol.

[cmsapms]

Max_Torque said:

It's a belt saver yes, especially on DSG cars which now upshift very fast indeed (hence engine rpm drops v fast)

How is that quick rpm drop arranged? Do DSG equipped cars have very light (rotationally speaking) flywheels?

[SuperchargedVR6]

VAG's DSG uses a slightly different Dual Mass Flywheel to the manual. It's the same size and shape but not sure what the difference in weight is, if any.

[Mr2Mike]

cmsapms said:

How is that quick rpm drop arranged? Do DSG equipped cars have very light (rotationally speaking) flywheels?

The RPM will naturally drop quickly to match the road speed when the clutch for the next gear engages - same as a manual, but it all happens a lot faster.

[Nick1point9]

There is still a torque reaction associated with decelerating the engine/flywheel, which is equal and opposite to accelerating the engine at the same rate (like when downshifting), it's just that engine friction/pumping losses/drawing against a vacuum (do DSG cars have flat shift?) is helping decelerate in the case of the upshift and hindering acceleration in the case of the downshift.

[Mr2Mike]

Nick1point9 said:

There is still a torque reaction associated with decelerating the engine/flywheel, which is equal and opposite to accelerating the engine at the same rate (like when downshifting), it's just that engine friction/pumping losses/drawing against a vacuum (do DSG cars have flat shift?) is helping decelerate in the case of the upshift and hindering acceleration in the case of the downshift.

Those losses are irrelevant in terms of slowing the engine down when a DSG upshifts - it only takes about 8ms.

[Yuxi]

I worked on an engine upgrade project earlier this year on a vehicle sold in many places around the world. At idle with aircon on full, headlights, wipers and rear screen turned on the battery is discharging. We could not decrease the diameter of the alternator pulley because it would overspeed at maximum engine revs and the client would not change the alternator spec.
It was accepted that the battery would flatten in these conditions, I am sure this is not the only car on sale new today where this is the case.

[chuntington101]

Yuxi said:

I worked on an engine upgrade project earlier this year on a vehicle sold in many places around the world. At idle with aircon on full, headlights, wipers and rear screen turned on the battery is discharging. We could not decrease the diameter of the alternator pulley because it would overspeed at maximum engine revs and the client would not change the alternator spec.
It was accepted that the battery would flatten in these conditions, I am sure this is not the only car on sale new today where this is the case.

When is someone going to sit there with the headlights on full, rear screen on demist, wipers on, radio on full blast with the cliemt control on full? Its a pretty strange senario to be in and not one i could see most people in for very long. Would it not be easier to build in a High idel sertting once the ECU detected a drop in voltage weh nsat at idle? bring the idle up to say 1000 or 1500rpm to cover the current draw?

[Yuxi]

chuntington101 said:

Yuxi said:

I worked on an engine upgrade project earlier this year on a vehicle sold in many places around the world. At idle with aircon on full, headlights, wipers and rear screen turned on the battery is discharging. We could not decrease the diameter of the alternator pulley because it would overspeed at maximum engine revs and the client would not change the alternator spec.
It was accepted that the battery would flatten in these conditions, I am sure this is not the only car on sale new today where this is the case.

When is someone going to sit there with the headlights on full, rear screen on demist, wipers on, radio on full blast with the cliemt control on full? Its a pretty strange senario to be in and not one i could see most people in for very long. Would it not be easier to build in a High idel sertting once the ECU detected a drop in voltage weh nsat at idle? bring the idle up to say 1000 or 1500rpm to cover the current draw?

You haven't sat in a traffic jam in a tropical storm in an ASEAN country have you?

[Mr2Mike]

chuntington101 said:

When is someone going to sit there with the headlights on full, rear screen on demist, wipers on, radio on full blast with the cliemt control on full? Its a pretty strange senario to be in and not one i could see most people in for very long. Would it not be easier to build in a High idel sertting once the ECU detected a drop in voltage weh nsat at idle? bring the idle up to say 1000 or 1500rpm to cover the current draw?

Most cars with aircon have an "Idle up" function anyway. Modern alternators are also ECU controlled, so in conjunction with idle speed control it certainly seems this should be a non-issue.

[chuntington101]

Yuxi said:

You haven't sat in a traffic jam in a tropical storm in an ASEAN country have you?

No this is very true i haven't!

[sutats]

I would love so much to have a R34 GTR but they've held their prices well and have a reasonable amount of miles given their age.