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Question about battery

Old 01-21-2013, 09:57 AM
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Default Question about battery

alright guys. dont really know where to put this thread ut ill try here and see what happens. So a few months ago i got an ipod/iphone FM TRANSMITTER WHICH WORKS GREAT. when i unplug it from the cig lighter i have to go thorugh the radio stations on the transmitter to find a good one again becuase it resets. I was wondering if i just left it plugged in would it drain my battery when the car is off? because it would be a huge luxury to not have to find my perfect FM station every time i get in the car. thanks guys!
And Mods please move this if it is in the wrong section. thanks!
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Old 01-21-2013, 11:14 AM
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It is very unlikely that your FM transmitter draws enough power to significantly drain the battery, even over a period of several days. Such devices typically draw only 50 to 100 mA, which added to the car's dark current (less than 50 mA per Ford's specs) would be a total draw of 100 to 150 mA.

So let's assume the car's dark current and the FM transmitter draw is 100 mA, the battery has a reserve capacity (RC) rating of 75 minutes (typical), and that the engine (worst case) is off for 24 hours a day. RC is the number of minutes the battery can support a 25 A draw, before the voltage drops to 10.5 V--so:

25 A * 75 minutes = 1875 Am (Amp minutes) = 31.25 Ah (Amp hours)

24 hours * .100 A per hour = 2.4 Ah per day
31.25 Ah / 2.4 Ah per day = 13 days before the battery would be drained to 10.5 V

However 100 mA is such a small load that the reserve capacity at that current would be much more than the rated 75 minutes at 25 A. It would probably take 3 to 4 weeks before the battery would no longer start the engine...
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Old 01-21-2013, 12:02 PM
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dosnt the radio have pre-sets?
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Old 01-21-2013, 03:12 PM
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Originally Posted by FrostByte View Post
dosnt the radio have pre-sets?
yes but my transmitter resets every time and i have to go through the fm channels on it .1 by .1
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Old 01-21-2013, 07:00 PM
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Doesn't the transmitter have preset options? all of them I have owned do. Actually I think the one I own stays on the last station even if I unplug it
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