No.

The battery is charged ONLY on overrun, trailing throttle or alternator spinning down on its sprag clutch. Normally the alternator is ONLY maintaining accessory loads and battery current at ZERO.

You must use a battery charger. I use a CTEK 4.3 MUS weekly.

 

TLDR : It should charge it when idle.

I put a trickle charger if my car will be idle for more than a few days. Good for the battery. Helps to prolong its service life and capacity. There are also electronics that are running while the car is off, so it is good to have a power source. Otherwise, as you noticed, the battery will go flat.

Just bear in mind that a trickle charger that isn't able to detect charge level and shut off when the battery is full will damage the battery if just left on and overcharge it, which is very bad.

However, many chargers these days are smart chargers in that they will detect the battery level, and turn on or off accordingly. I just have mine always connected.

My understanding was that the alternator is rated for more amperage than the car's needs. This is to accommodate charing the battery, and also if other things are plugged into the car, such as the 12V socket, or if you have dash cams, more powerful Hifi system, playing music loud, heated seats etc.

Overall, if your electronic usage is not maxed out (blower fans on max, heated seats on max, blasting the stereo on max), it will charge the battery when idle. If you are running a bunch of things, it might not charge it. But even then, alternators certainly are rated for max load and beyond so it should be able to meet all the cars needs including the battery.

 

TLDR

Not everything can be fit to your limited attention span and comprehension. That is why there are engineers.

Intelligent Alternator Control System A Path to Efficient Dynamics
[ … ]
7 Integration of the Intelligent Alternator Control in the Vehicle Electrical System Architecture
7.1 Alternator with Communication Interface
Today's alternators are generally rated for manufacturer-specific, simulated and empirically determined charge balance cycles, and operate at electrical currents from 90 A to 210 A. Electrical power demand is rising steadily due to the increasing electrification of functions in modern motor vehicles. Consequently, peak torques of up to 60 Nm are produced at the crankshaft specifically to meet the power demand of the alternator when the engine is idling. To prevent such loads from arising suddenly and in an un- controlled manner, and to stabilize engine idling and improve fault diagnostics, the BMW Group has been using alternator con- trol units with a communication interface (BSD-Bus) since the year 2000.

This interface is used for communication between Digital Engine Electronics or Digi- tal Diesel electronics (DEE or DDE) and alternator control units and allows the engine management system to counteract sudden engine-incompatible loads. This is accomplished by the interchange of actual state variables and set-point inputs, such as alternator voltage or alternator temperature, and by incorporating these into appropriate torque-based models.

Using the torque balance calculated in these models, the engine management system is able to stabilize engine idling by, among other things, adapting the alternator control dynamics (Load Response). It is also possible to control the actual battery voltage via this interface in such a way that drops in voltage along the supply leads can be compensated for. As a result of the gradual introduction of this control technology with each new engine generation, all new vehicles of the BMW Group now have this facility for communication between the engine management system and the alternator.

To exploit the available braking energy recovery potential, it was necessary to couple the driving situation to the state of volt-age of the vehicle electrical system. In vehicle acceleration phases, the alternator termi-nal voltage is set by means of the BSD bus to or slightly below battery voltage level. As a consequence of this, the power demand of the vehicle electrical system is covered by the battery (the SoC of battery decreases) and the alternator does not place any load on the powertrain.

During overrun and braking phases, the alternator terminal voltage is transmitted to the alternator control unit with a default value higher than the actual battery voltage. As a result, the alternator not only covers the complete power demand of the vehicle electrical system, but also charges the battery (the SoC increases). The alternator now places load on the powertrain and effectively brakes the vehicle. With the introduction of the second generation of interface control units for motor vehicle alternators, it is now possible to control the power and torque characteristics of the alternator in a continuously variable manner. This is a prerequisite for engine and vehicle-compatible generation of additional electrical power in over- run/braking phases. The alternator is gener-ally controlled in a much more dynamic manner because it makes allowance for the driving situation, as well as engine and electrical power supply requirements.

7.2 Intelligent Battery Sensor (IBS)
[ … ]

https://zero.sci-hub.se/3841/bb5d73026c7ac15724fe8d28215463d4/liebl2006.pdf?download=true. 5 pp

 

BTW, when your car detects a low voltage on the battery, it will turn off many accessories. That's why your 12V DC Pump is not working. I'm sure this is also to focus on charging the battery and supplying other essential power.

 

I would agree that the charging will be most effective while being driven. Cars are designed to be driven and much of the considerations that are put into the car revolve around that.

When an engine is running, the serpentine belt is being driven and so is the alternator. If minimal electronics are running, the excess power that the alternator can deliver will certainly charge the battery. It might not be charging it at maximum capability. Its a balance of power delivery and power draw. If the alternator can deliver 50A when idling, but the car electronics is only consuming 45A, then the remaining power can be used for other things like charging the battery.

Turning off enough electronics will devote more of the power draw to the battery. It is also possible to have too many electronics running, and will require the battery to deliver power along with the alternator and drain the battery. But running the car with no electronics running will obviously allow the battery to charge. This is why on low voltage, BMW turns off all accessories.

If the car can support those accessories with the alternator while idling, then turning off those accessories will allow that same power to then be diverted to the battery.

 

There is no “excess power that the alternator can deliver” without burning fuel. Your comprehension fails.

Oh! wait. You are driving a Sixties vintage GMC. This thread is about a 2012 or later BMW, very different.

 

More load will increase the armature and magnetic resistance that is needed to drive the required current in the alternator. In other words, a greater load will correlate to a greater force needed to drive the alternator This will require the engine to use more HP/Energy. So yes, it will burn more fuel.

This satisfies the First Law of Thermodynamics.

PS : Sixties Vintage GMCs are awesome.

 

I am being equally educated and amused by this thread. Thank you bitter_b and Doug!

 

Tow company’s often have a portable air tank, if you have AAA might ask them if that is an option.

After 6 months, battery might be to flat to jump. Maybe remove the battery and have it charged and checked might be easier then getting towed.

I’ve never used one, but they do have small jump packs, maybe a family member/friend has one you can borrow?

 

Tow company’s often have a portable air tank, if you have AAA might ask them if that is an option.

After 6 months, battery might be to flat to jump. Maybe remove the battery and have it charged and checked might be easier then getting towed.

I’ve never used one, but they do have small jump packs, maybe a family member/friend has one you can borrow?

+1
Getting back to actually addressing the o.p.'s problem rather than bickering.

Best to tackle the battery first. We don't know the age of that battery and after several months sitting it is flat as you said. In that situation if it was my car I would pay to have the battery swapped and registered. Then it might be possible to inflate the tire locally.

 

20 psi should be enough to drive it out of the garage and to a gas station with a larger pump...then drive at speed to charge.

Nothing high speed, but IMO 20psi will not led to carcass damage

I mean given that OP asks about 'idling', I assume he CAN start the car...

I thought the whole purpose of the exercise is to help the OP get his car out of the garage and running/on the road.....

 

Ooo ooo me me my turn!
Without getting into how dynamic a thermos is🤤 or who’s right and who’s wrong I figured i would cram my unsolicited opinion in where nobody asked for it 😁. Think of this as the hillbilly or 12 year old version.

I think your both correct but I kinda lean towards Doug on this one. I think everybody is trying to help so Maybe we can agree to disagree? Worked for me in the past 🤷‍♂️

From my understanding I was taught to think of the battery as a storage device for electricity. I believe that the common misconception is that the alternator charges the battery but it really just maintains it. The alternator only charges on overrun or trail throttle. BUT it will place a surface charge on the battery as a low or flat battery acts as a load until it’s brought up via a surface charge to roughly alternator voltage output. After there is a sufficient surface charge the battery no longer acts as a large load and therefore isn’t actually being charged.
Let’s say you have a good but low battery let’s say 11v. If you start the car and idle it will bring the battery voltage up to 12+ but it’s a surface charge. Your likely to be able to start and the battery acts like it’s charged but it’s not. You have not replenished the RC of said battery. At some point you need to put voltage back into the storage device occasionally via a charger. So in my mind you both are correct.
Now we have some pretty smart cars with pretty smart battery related electronics but they are mostly based on SOC measurements for calculations about the battery SOH. Then you get into the DME and registering a battery to charge more aggressively ( but again in overrun ) now with dual storage systems and regenerative braking in many models and starter/generators and 48V systems it all gets kind of…interesting and there is likely less and less of 1 answer
I think at the end of the day if someone asks if the alternator charges your battery as a general rule of thumb it should be “no…but”

hook up those trickle chargers every now and again ( model dependant LOL ) and enjoy long battery life with minimal sulfating.
Again this is all personal opinion and I’m too lazy or stupid to try and back this up with countless links and references. I have however chased battery issues for days on end so 🤷‍♂️ think whatever you prefer

 

There are many considerations that apply to a car's electrical system and it certainly isn't simple. I acknowledge Doug's discussion on the electrical system.

Op asked about whether the battery would be charged while idling. The simple answer is yes, especially if Op makes sure the electronics are off. You can also think of the alternator as a power source with a power budget. Loads can be added onto the power budget until it is exhausted. The battery can add on to the power budget to deliver power and allow other electronics to be powered beyond the alternator's ability. The battery can also be a Load in which case it is being charged and not delivering power.

Unless the battery is fully charged, it will act as a load as long as the Alternator's Power Budget is satisfied. If the battery is not discharged and additional loading is added beyond the Alternator's ability, then it will be able to supply that load until it is exhausted.

As you can see in the attached power curve, the alternator will be able to deliver different power under different operating conditions. The higher the RPM, the more power and current it will be able to deliver. Depending on the electronics that are being run, the current that can be delivered at idle RPM might be enough to charge the battery and keep the electronics running. If it is not enough, then the battery would be expected to deliver the extra power. Typically, manufacturers would want the typical operating conditions to be met even with idle RPM. Otherwise, the car could malfunction if it is relying on a battery that could be depleted. That's why BMW shuts off the accessories when it is low voltage and close to depleted.

Generally, the battery and alternator share the hit. Sometimes, the battery needs to perform and help the alternator. Mostly, the alternator can charge the battery. Overall, the alternator is able to supply all of the car's needs and charge the battery, hence why our batteries are full and never depleted under normal operating conditions. The battery is there for the blips in power requirements and when the alternator is not running.

The car and it's electrical system has to be self-sustaining. This involves the Alternator always having a Positive Power Budget, even under maximum operating conditions (Or otherwise shut things off). BMW and its engineers must make sure that there aren't any loopholes that could cause the car to be stranded or operating poorly, under normal operating conditions. This includes circumstances where the battery is depleted (shut-off everything and charge it).

How many times have you been driving down the road and then the car and its electronics shut off? Never or at least very rare. The alternator can supply all the electronics that BMW has put into it, including the battery, and will otherwise behave to make sure that happens (revving higher at idle, shutting things off etc).

PS : Doug is right. This isn't a 60's Vintage GMC (no matter how awesome they are). This is a modern, intelligent, computer controlled BMW.

 

Trying to help the OP, and ignoring the whole charging on idle question, which I honestly think at 6 months in the garage is probably irrelevant...

It's possible you can jump it with a portable jumper, but if it's truly dead, then it's likely too dead to jump. AAA and most road service companies usually have commercial versions of those units, but they're probably not going to be happy to hump them from the highest floor they can get to to the third floor where they're needed. They also have air pumps or compressed air tanks. Unless the OP has friends with both of those, buying them for a one time use would be kind of expensive. And I'm assuming there are no available outlets near where the car is parked. If there are, get a CTEK conditioner and a cheap air pump, leave the battery on the CTEK overnight (it'll give you error message in an hour or two if the battery isn't going to charge), pump up the tire, and you're golden. And if you're going to leave the care unused for any period of time, you should probably have both. If there are no outlets for a charger/conditioner, then honestly you have the wrong garage for long term storage.

Otherwise, can you get the battery out of the car? With help if needed? If it's been sitting for 6 months, depending on the age of the battery when this started, it wouldn't surprise me if the battery is toast anyway. Might not be. Maybe not the simplest thing to do, but if you can't get service to the battery, get the battery to service. Have the battery checked out, and if it's still good, have it charged over night on a battery charger, put it back in the car, and have a go at it. If the battery doesn't check out, replace the battery, and go back to have it registered. A new battery will get you out of the garage. And as stated, 20 psi should let you drive slowly to air the tire, then get somewhere to check the tire out as well.

Whether or not the battery will charge at idle (I don't think it will to any significant degree, BTW) is a distraction. I have no idea how long it would take for that, IF you can even get the battery to jump start anyway.

BTW, unlike ard, I'm assuming from the original post that the OP can't start the car...

My $0.02 contribution.

 

I agree about using the Tow company’s portable air tank and portable jump start pack.

Some AAA trucks can also test your car's charging system and maybe replace your battery.