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There was a thread on this forum that stated that one must drive an E92 about 7 miles to replace the charge taken out of the battery from cranking the engine at startup. That seems excessive.

Doesn't the alternator produce enough voltage even at an idle rpm to start recharging the engine? I would think that the appropriate metric is how long the engine is running after startup to replace the battery drawdown during one crank.

Does anyone know what the answer is?
 

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It does seem excessive.. but I was told once that because of the "bmw intelligent efficiency" or whatever garbage they called it, it would not generate any "additional power" to charge the battery unless accelerating/decelerating. This is all back in the day when my battery was about to kick the bucket and I told him I jumped the car and let it idle for like 30 minutes before I went. His response was that it did nothing idling and I had to move the car before it would even start charging the battery..
 

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It does seem excessive.. but I was told once that because of the "bmw intelligent efficiency" or whatever garbage they called it, it would not generate any "additional power" to charge the battery unless accelerating/decelerating. This is all back in the day when my battery was about to kick the bucket and I told him I jumped the car and let it idle for like 30 minutes before I went. His response was that it did nothing idling and I had to move the car before it would even start charging the battery..
That sounds bogus. I can understand if he said that the alternator voltage is too low at an idle RPM but even that sounds suspect.
 

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02/2012 X5 35d M57Y CPO 99K miles NOKIAN WR G3 12K miles
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There was a thread on this forum that stated that one must drive an E92 about 7 miles to replace the charge taken out of the battery from cranking the engine at startup. That seems excessive.

Doesn't the alternator produce enough voltage even at an idle rpm to start recharging the engine? I would think that the appropriate metric is how long the engine is running after startup to replace the battery drawdown during one crank.

Does anyone know what the answer is?
The ‘Intelligent Generator System’, Intelligent Battery Sensor IBS and the ECU are very gentle with the AGM battery, not least due to Eco Pro requirements.

It is quite likely that no charge takes place during the initial seven miles of a drive. Eco Pro advertises that it charges only on overrun. It is quite possible to completely deplete the battery due to adverse driving habits and BMW explicitly refuses warranty for batteries damaged by adverse driving habits.

An AGM battery is fragile to overcharge.
 

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Discussion Starter #6
The ‘Intelligent Generator System’, Intelligent Battery Sensor IBS and the ECU are very gentle with the AGM battery, not least due to Eco Pro requirements.

It is quite likely that no charge takes place during the initial seven miles of a drive. Eco Pro advertises that it charges only on overrun. It is quite possible to completely deplete the battery due to adverse driving habits and BMW explicitly refuses warranty for batteries damaged by adverse driving habits.

An AGM battery is fragile to overcharge.
I don't think my car came with an AGM. The dealer replaced the OEM battery under warrantee and installed a non-AGM. The dealer recently replaced it with a similar non-AGM
 

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Discussion Starter #7
alternator charging the battery

Using the onboard computer I monitored the system voltage on my 2011 335is.

Before starting the engine the voltage was 12.0 volts. Once the engine started the voltage increased to 14.5 volts and varied from about 13.9 to about 14.9 volts. So even at idle RPM the alternator was capable of charging the battery.

After driving the car about 20 miles the battery voltage rose to 12.5 volts (measured just after the engine was shut down). The next morning the battery voltage was still about 12.3 volts.

What this tells me is that it is not the distance one drives but the time the engine runs following a startup that determines the amount of energy returned to the battery.

What I don't yet know is how long must the engine run to fully restore the energy drained from the battery by one startup. Anyone have an answer?
 

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14 minutes
 

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Where did you get that number? If based on a calculation can you provide it?
So I switched out of electrical engineering my freshman year but as I dimly recall it has to do with Coulombs in = Coulombs out. It's the electrons flowing through the circuit that participate in the chemical reaction inside the battery (but not at 100% efficiency).

For they energy/power calculation make the ampere-seconds for charging equal to the ampere-seconds for discharging, and then multiply by an adjustment factor to account for the inefficiencies.

500A × 3s =1500 A-s = 2A × 750s = 10A × 150s

750s = 12.5 minutes

Figure about 90% efficiency, so the 12.5 minutes / 0.90 = about 14 minutes.

Perhaps Doug has a more detailed calculation but more input factors such as other loads (HVAC blower, lights) on the charging system would need to be included.
 

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Buy a battery tender if you drive infrequently. It keeps my battery very happy and ready to roll!


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