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Old 10-05-2012, 02:11 PM
tampamark tampamark is offline
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Location: tampa fl
Join Date: Jan 2009
Posts: 3,311
Mein Auto: 650 Sold - '12 Jaguar XK
Originally Posted by avoiceoreason View Post
So if I understand this correctly, then it would be a greater drain on the battery to drop the top via comfort access vs. while the car is idling. Correct?

Do you (or anyone) have a trickle charger that you recommend? If I have to get the battery replaced, I might try your method and see if that helps. How long has your battery lasted since using this weekly trickle charge approach? And for that matter, how long should the battery last in the first place?
It is absolutely a bigger drain when dropping the top with the car off as opposed to idle. At least at idle the system is partially able to supply the power. I only say that the car cannot sustain the system properly at idle since it is documented that short drives kill the battery. Obviously sustained driving is required to recharge the battery properly.

It is simple math, the car has so many electronic components that it just can't keep up without having the RPM at higher levels (and therefore the alternator spins faster to generate more voltage). Too much draw for not enough voltage! I know that the compressor will shut off on some cars under extreme load, but the V8 on the 6 wouldn't even acknowledge something as small as an alternator shutting off, hell I can't tell when the compressor kicks on and off. I used to have a 4 cylinder Celica in college that I would use the A/C button like it was a turbo boost. Turn it off and it felt like I had 40 more hp.

I have never checked the BMW, but on my motorcycle when applying a volt meter to it at idle it barely puts out above 12v, at 5,000 rpm (out of a max of 12,000 rpm) it is at peak output of close to 15v. Any higher RPM and the regulator is capping it from going too high.

Others have had the alternator fail in the other direction...the regulator fails which is in charge of capping the voltage that is output so that it doesn't drive the electronics wild or provide to high of voltage to the battery and killing it that way.

Yorgi has provided several recommendations for Battery Tenders, here is a thread that goes over different models in detail along with some recommendations:

Originally Posted by roseng View Post
"non-favorable driving profile". I'm sorry but someone should get an award for this term. I would prefer to think of it as a "non-realsitic design criteria" issue.

Last edited by tampamark; 10-05-2012 at 02:12 PM.
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