2007 BMW 530 How long to recharge after a crank on a start

shopkins94 · Posted at Wed Jul 02, 2008 5:37 am

Electrical problem
2007 BMW 530 6 cyl All Wheel Drive Automatic 14500 miles
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Hello,

Do you know how long it takes for a car to recharge the battery for what was consumed during a normal start? I found the following figures on the Net and they seem about right to me for worse case.

Discharge: 200 amps for 5 seconds = 0.3 AH
Charge: 0.3 AH , 20 amps = 1 minute

So basically, once the car is cranked and started, 1-2 minutes later the battery should be recharged. Is that correct?

Thanks.

Dr. Hagerty · Posted at Wed Jul 02, 2008 7:12 am

I have had conversations with the instructors at BMW school about this issue and at first it does seem that the process should be satisfied as you have pointed out. However, the physics of the AGM batteries is such that from an engineering standpoint, the actual time to bring the charge level back to full charge is more than just a few moments. Also please consider that these cars do not stop consuming power just because you have shut the car off and locked the doors. BMW's can take up to 16 minutes to go to "sleep" and even after they do, there is still a key off draw of about 50 ma. to continue communications in the CAN BUS system. So at the end of the day the time to recharge the battery to full charge might be 20 minutes. Hope this clears this issue up for you.

shopkins94 · Posted at Wed Jul 02, 2008 10:42 am

Thanks for the quick response. Is this to say that a BMW must be driven 20 minutes every day?

Dr. Hagerty · Posted at Wed Jul 02, 2008 11:58 am

Is there a problem with that? Smile

shopkins94 · Posted at Thu Jul 03, 2008 5:48 am

Not everyone commutes an hour to work each day... What about shopping with repeated restarts or if you left the car for 3-4 days? Would that mean that you would have to recharge for 1-2 hours?

Dr. Hagerty · Posted at Thu Jul 03, 2008 7:19 am

Typically what it means is that after several weeks or months with this "pattern" of operation the battery can still start the car but it does so with diminished capacity and can shorten the life of the battery. Many times I have gotten a (brand new or just delivered) towed in vehicle that had insufficient charge on the battery to start the car. It also has a tendency to set erroneous faults in the car due to the HUGE dependency the new BMW cars have on a strong battery.