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Charging a low battery

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pwmac(at)sisna.com
Guest

Posted: Mon Aug 04, 2008 11:04 am Post subject: Charging a low battery

I  think this has been discussed before, but I vaguely remember. Why 

doesn't my truck blow the 30a fuse when I use it to charge my nearly 

dead trailer batteries (less than 9v)? I have had to do this several 

times and I just connect the truck to the trailer up start the engine 

and rev it up. No ill effects to report and the fuse did not fail.

The truck has a 110a alternator and 2 size 65 batteries (diesel). 

During this charging I see 13.8v or so on the cigar lighter plug.

Thanks, Paul

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mprather(at)spro.net
Guest

Posted: Mon Aug 04, 2008 3:07 pm Post subject: Charging a low battery

I think what's going on here is that it probably takes very little current

to raise the voltage on a dead battery from 9V to 14V - as long as no

cells are shorted.  Once the cell voltage comes up, the current drawn

drops even further - small voltage difference between the alternator

output and the stacked cell voltage.



How much current the battery can take will also depend on the internal

resistance of its cells.  It's possible that if you had batteries with

low(er) internal resistance (an RG type, maybe), you'd start blowing

fuses.  Bob posted a note a while back about certain aircraft that had

charging system problems when running low internal resistance RG

batteries.



Your hi power alternator can be convinced to provide its full output if

you connect a load which will continue to drop the bus voltage.

Regards,



Matt-

 	  Quote: 	
		   



 I  think this has been discussed before, but I vaguely remember. Why

 doesn't my truck blow the 30a fuse when I use it to charge my nearly

 dead trailer batteries (less than 9v)? I have had to do this several

 times and I just connect the truck to the trailer up start the engine

 and rev it up. No ill effects to report and the fuse did not fail.

 The truck has a 110a alternator and 2 size 65 batteries (diesel).

 During this charging I see 13.8v or so on the cigar lighter plug.

 Thanks, Paul

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echristley(at)nc.rr.com
Guest

Posted: Mon Aug 04, 2008 7:27 pm Post subject: Charging a low battery

paul wilson wrote:

 	  Quote: 	
		   



 I  think this has been discussed before, but I vaguely remember. Why 

 doesn't my truck blow the 30a fuse when I use it to charge my nearly 

 dead trailer batteries (less than 9v)? I have had to do this several 

 times and I just connect the truck to the trailer up start the engine 

 and rev it up. No ill effects to report and the fuse did not fail.

 The truck has a 110a alternator and 2 size 65 batteries (diesel). 

 During this charging I see 13.8v or so on the cigar lighter plug.

 Thanks, Paul

What is the path resistance between from the alternator, to the trailer 
	


battery and back again?



The trailer batter is at 9V and the alternator is putting out 13.8V.  

That gives you 4.8V to drive current with.  0.16 Ohms will limit 4.8V to 

30A.  In that circuit, you've got at least 30ft of wire (of unknown 

gauge), two pin connectors (probably half corroded if they look like the 

ones on the back of my truck), and two battery connections of unknown 

condition.  Half an ohm would be no problem to mix in there somewhere.



-- 



http://www.ronpaultimeline.com

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