30A fuse blowing (truck to camper circuit)

[Pete D]

(Note to all the normal people in the forum-- this kind of stuff gets me unreasonably excited; I know it's odd, but at least it's not immoral!!) Just to be argumentative, what kind of power loss would a 0.4 Ohm snubber resistor cause in a normal charging circuit? Seems like cheap insurance to me.

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Doesn't it seem odd to anyone else that we have the NEC for house wiring, but (evidently) nothing like that for vehicles? Not to directly knock anyone, but the egregious wiring I have witnessed, e.g. failed tapcons in the ceiling of my 2001 Hawk, make me think a little bureaucracy in RV wiring might be a good thing...

I think the RV Industry Association (RVIA) sets standards of some kind, but dunno how much force they have. One problem that I see is the conflict in color codes between commercial/residential and automotive/marine wiring.

Hmm, E=IR? (Been a looong time and I wasn't real good then...)

http://www.the12volt.com/ohm/ohmslaw.asp

Lessee, 12V / .4 Ohm = 30Amps possible through the resistor, but looking at it another way, there's a very large voltage drop across the resistor at high current, so nothing would be happening? At a 10Amp flow, the drop would be 4V; at a 2Amp flow, the drop would be 0.8V and that would be enough to terminate the charging process way too early.

I recall that a small (Ohm-wise) ballast resistor in an ignition system drops the voltage to the coil from 12VDC to 6VDC during starting and that ceramic resistor gets very hot in the process.

 

[Mark W. Ingalls]

A pretty dead battery will still have some voltage, say for argument 11 volts. Assume the charging alternator will supply 13.5 volts to the system. That lets 2.5 V drop across the resistor, supplying 6-1/4 amperes of charging current to the battery.

As the battery charges, its voltages goes up, so there is less drop across the resistor. I agree that a constant resistance current snub is not a very efficient charging method, that is why current limiters use transistors as variable snubs.

I thought winter200's idea of a light bulb was a good one.

 

[Pete D]

Prior to the electronic advances, the telephone company used specially designed light bulbs as ballast resistors on special service circuits; they were called Ballast Lamps IIRC. In this case, one good thing would be that if the voltage were to get too high, the bulb would burn out, acting like a fuse.

 

[HERR42]

all the specs from optima and interstate expect a 30 amp charge current limit....or a max battery temp.

a solid state current limiter would be cool but the wiring would need to be beefed up.

dual parallel batteries would escalate that demand.

..the existing wiring is good for 10 amps continuous i think. a 10 amp current limiter would be the ticket.
..but even then, with a load greater than 10 amps the voltage would take a dive...

might be best putting a disconnect on one so that one can be charged at a time.