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Cap with Battery Cut off Switch


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stagger 
Member - Posts: 24
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Location: United States
Posted: December 17, 2006 at 7:46 PM / IP Logged  

Hi, I have a battery disconnect cutoff switch on my car that I dont drive very often.  I also have a cap that feeds to the sub.  Is it dangerous to use the cut off switch with the cap hooked up.  Say if I disconnect the battery (switch is on the Neg Cable) and I may not use the car for a month or so, when I turn engage the switch the cap will get a full surge of power.  I heard this can be bad for the Capacitor?

Thanks in Advance....

aznboi3644 
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Posted: December 17, 2006 at 11:07 PM / IP Logged  
What you heard is wrong...a cap can be stored charged...it won't see a "surge" of full power...nothing will be affected...I'd take that cap out if it were me...caps are useless
haemphyst 
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Joined: January 19, 2003
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Posted: December 18, 2006 at 8:32 AM / IP Logged  
azn, while you are correct, in this case you might just be wrong.
If the cap becomes disconnected from the battery, by virtue of the battery negative being disconnected BETWEEN the battery negative post and the chassis, all of the accessories in the car will pull their power FROM the capacitor, until the capacitor goes dead - which will actually happen fairly quickly. When reconnecting the negative cable, the cap will, indeed see a tremendous curent surge, which IS bad for them, not to mention the fact that the switch contacts will become pitted, and eventually unusable from this abuse.
Even sitting, a cap will self discharge a certain amount, based on the ESR of the capacitor, and after a month, there will still be an inrush.
I do agree with you completely, in the comment that removing the cap would be the best course of action. Remove the cap...
It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
aznboi3644 
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Posted: December 18, 2006 at 6:09 PM / IP Logged  
Ahh....never thought about when the battery ground is disconnected...thanks for the correction haem
specialaudio 
Copper - Posts: 58
Copper spacespace
Joined: April 15, 2006
Location: Canada
Posted: December 18, 2006 at 6:41 PM / IP Logged  

even though im rookie, i agree no cap

but why the hell is there a switch on the battery? i never seen that before

93' Cutlass
advanced_audio 
Copper - Posts: 170
Copper spacespace
Joined: July 07, 2006
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Posted: December 18, 2006 at 8:46 PM / IP Logged  
Most of the time its there to make it harder for thiefs to steal your car.
haemphyst 
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Platinum spaceThis member has been recognized as an authority in Electrical Theory. Click here for more info.spaceThis member has been recognized as an authority in Mobile Audio and Video. Click here for more info.spacespace
Joined: January 19, 2003
Location: Michigan, Bouvet Island
Posted: December 19, 2006 at 10:45 AM / IP Logged  
Thinking back, if you really want the cap, if your terminals on the switch are visible or accessible, simply connect a test light across them (bypassing the switch contacts) until the test light goes out. Once it does, THEN you can perfectly safely flip the switch, with no damage or issues at all...
It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
stagger 
Member - Posts: 24
Member spacespace
Joined: August 01, 2004
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Posted: December 19, 2006 at 6:33 PM / IP Logged  

Thanks for the replys.

The reason I use a disconnect switch is I dont use the car often, its my beater winter car, and when it sits too long, the battery drains.  Im not sure because of the cap or if I have a short somewhere pulling down the battery.

I didnt think the sudden on rush of current would be good for the cap, since it did come with a resistor you place on the terminals when you want to charge it, and to discharge it.

I always though caps did help for the high demands the sub amp takes.  Do u think it would be best to remove the cap?

aznboi3644 
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Posted: December 19, 2006 at 11:08 PM / IP Logged  
Yeah it'll help for a high current demand, for .001 seconds

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