[Gecko123]

Does idling charge the battery? I know older cars need to rev a certain RPM

I know some of you will say why not drive it to charge the battery. Without going into details I want to know if idling will change the battery.

Currently the battery is in good condition so this is just battery maintenance

[ricx]

Essentially no, your alt runs the power draws on your car and maintains, even running 100 mph on interstate what charge that gets back to your battery is negligible. If your batt is low, use a charger.

[gbalthrop]

Originally Posted by Gecko123

Does idling charge the battery? I know older cars need to rev a certain RPM...

How OLD is "older". Do you mean 1950's cars with generators instead of Alternators?

Ever seen an Ammeter or Amp Meter, that measures "NET" current/Amps In vs. Out of the battery? If the Amps or Current Flow developed by the Alternator is Greater than the Draw or Load of electrical consumers (Radio, Blower, Lights, Seat Heaters, etc.) then the battery charge is being Increased. Unless your alternator is defective, there is ALWAYS some Amps or charge being developed by the Alternator at ANY engine speed.

The Voltage Regulator on the rear of the Alternator is designed to change Amperage output of the Alternator depending upon battery voltage prior to startup, and Load placed upon the electrical system. The "Excitation Current" applied to the Alternator determines its output, and immediately after engine start, it is working hardest (and the hardest to turn) to replace the current draw from Starter Motor Activation. The primary function of the Alternator is to replenish Starter Draw, and then to maintain slightly more current IN to the battery than consumer draw OUT while the engine is running. Like keeping a jug with a drain valve filled. When the engine is started, the drain valve was wide open, and the valve remains open to SOME degree while the engine is operated.

So the short answer to your question is: "It Depends". If the Alternator and its Voltage Regulator are operating properly, the Alternator is providing Amps or Current Flow to the battery or electrical system at idle. What that Amp Value is depends upon a number of things, but particularly time since Starter Activation, and other Loads or electrical consumers being operated.

Even with ALL "Optional" consumers switched OFF, such as Lights, Climate Control, Radio, Heating elements, etc., it still requires SOME Amp Draw to run the engine, electric Fuel Pump, Coolant Pump, Radiator Fan, coils, injectors, Sensors, Electronic Modules, etc. If your battery were fully charged when you started the engine, but the Alternator is NOT producing ANY Voltage or Amps, most cars would drain the battery & stall in 30 minutes or less in daytime, with NO lights on.

If you REALLY want to see the actual numbers as far as Voltage produced, Amps being provide, Exciter Current being applied, Battery Load %, etc., INPA "Generator Messwerte" or Alternator Measured Values/ Parameters will give you Live Data so you can see the things described above. A simple Voltage measurement using a multimeter at the Jumpstart Terminals, or lighter socket with motor running, or using "Hidden Menu 9.00" to view System Voltage on the Instrument Cluster, will also provide some indication of what is going on.

George

[Efthreeoh]

Originally Posted by ricx

Essentially no, your alt runs the power draws on your car and maintains, even running 100 mph on interstate what charge that gets back to your battery is negligible. If your batt is low, use a charger.

Drop the crack pipe son.

Jeezus is this incorrect.

[Welcome to NBA Jam]

In short: yes.

Your voltage regulator and DME will determine how much energy is sent to to charge the battery. The E90 tends to maintain its state of charge around 80%.

The DME automatically increases the charging voltage and increases idle speed if it determines that energy demand is not being sufficiently met, or if the battery is in a critically low state of charge.

[Tambohamilton]

Originally Posted by ricx

Essentially no, your alt runs the power draws on your car and maintains, even running 100 mph on interstate what charge that gets back to your battery is negligible. If your batt is low, use a charger.

Eh???

The alternator is a 120A charger, if the DME/DDE wants it to be! It can hammer charge into your battery, though for sure higher rpm will allow it to generate more power.

Your battery will charge, if needed, on idle. But you'd need to let it sit idling for an hour or 2 to significantly recharge a depleted battery.