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large voltage drop when cranking car

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Forum Name: General Discussion
Forum Discription: General Mobile Electronics Questions and Answers
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=109604
Printed Date: November 29, 2021 at 9:18 AM


Topic: large voltage drop when cranking car

Posted By: benjiboy
Subject: large voltage drop when cranking car
Date Posted: December 09, 2008 at 10:05 AM

I am a bit confused on what would cause a substantial voltage drop (12v down to 8v!) when I am cranking my car.

The battery is relocated from the engine bay to the trunk so its a "custom" install. 1991 Camaro.

I have 1/0 welding cable from the battery to my battery cut-off switch.
Same cable from the switch up to the starter.
Same cable from the battery to the frame GND.
A 4ga. ground strap from the motor to the frame GND.
AND a ~4ga. cable from the battery to the engine GND.

I should be covered there, correct? Is this a starter or battery issue (I can not rule those out yet)?



Replies:

Posted By: ckeeler
Date Posted: December 09, 2008 at 10:31 AM
get the battery load tested (out of the car). it sounds like you need a new one




Posted By: i am an idiot
Date Posted: December 09, 2008 at 1:34 PM
If you are reading the voltage AT the battery and it is dropping to 8 volts there, then i too vote for a defective battery.




Posted By: DYohn
Date Posted: December 09, 2008 at 2:35 PM

Agreed.  However, the cable run from the relocated battery back to the starter can add resistance and drop battery voltage, so you should measure the cable resistance and see if maybe you have a corroded connection.  Where are you reading system voltage?

As an aside, I have seen batteries drop to 8 or 9 volts in very cold country when cranking a car that's been sitting out all night.



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Posted By: benjiboy
Date Posted: December 09, 2008 at 9:37 PM
Well, I am changing the stock starter to something that is much more capable of turning over the high compression so that may help the starter drag a little bit.

I have the 1/0 from the kill switch running to a distro block in the bay and then out of the distro block to the starter. I can shorten this long feed by maybe 1 1/2 feet but I don't think that is the issue. I am measuring the voltage at that distro block maybe 2 1/2 feet from the starter.

All the cables and lugs are pretty much new so corrosion is probably not a factor. But, I did grab a battery from a friend that has been sitting outside but supposedly is good. It read 12.53 volts when I checked it on my workbench.




Posted By: benjiboy
Date Posted: December 09, 2008 at 9:43 PM
Another note..

I am using this disconnect switch rated at 20 continuous amps and 125 surge amps. It has no alternator disconnect and that alternator charging lead is hooked up to it as well.
https://store.summitracing.com/partdetail.asp?autofilter=1&part=SUM-G1432&N=700+4294925143+4294859645+115&autoview=sku

Could this be any part of the issue?




Posted By: ckeeler
Date Posted: December 09, 2008 at 10:16 PM

ckeeler wrote:

get the battery load tested (out of the car). it sounds like you need a new one

your first step^^^^. then go from there.





Posted By: i am an idiot
Date Posted: December 09, 2008 at 10:17 PM

Checking battery voltage without a serious load on the battery will tell you nothing about the state of the battery.   If this 20 amp/125 amp surge relay is in between the battery and the starter, then yes that may have a little to do with your problem. 





Posted By: benjiboy
Date Posted: December 10, 2008 at 3:10 PM
Yes the 20/125 amp switch is between the 1/0 cable from the battery to the starter.

I am going to shorten the 1/0 main feed by at least 4 feet, run 1/0 to the starter (it was 2ga before not 1/0 as I had mistakenly thought!), solder all my lugs on the cables (they are crimped now), get a higher rated cut-off switch with the alternator disconnect provision, install the higher performance starter, and run 4ga (instead of my 6ga and 8ga combo) for the alternator charging wire.

This should ffin do it!




Posted By: js305
Date Posted: December 14, 2008 at 1:53 PM
Uh, I am assuming you have a pretty high compression engine by what you are saying. Does the engine still turn over and start OK? I think a bigger battery (higher cranking amps) is in order anyway. Everything else sounds good to me.

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Joe in Texas




Posted By: benjiboy
Date Posted: December 14, 2008 at 2:54 PM
The battery was a tired Optima Red Top starting battery at 800 CCA and maybe 940 CA.





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