Looking for measurable resistance that causes large voltage drop

[motolectric]

Hi,

A few years back I came across a web page that detailed the very small increase in measurable resistance that causes quite large voltage drops especially in the 12-14 DC volt range. I thought I had it bookmarked but cannot find the URL.

Basically the text stated that the increased resistance was very small, almost unmeasurable (although it could be measured using sophisticated gear). It pointed out that trying to determine things like voltage losses using measured increase in ohms was not a good way to go about it and that showing the voltage loss during using a DMM was a superior approach.

The text did quote the exceedingly small increase in ohms that produced fairly large voltage drops.

I'm looking for this type of information.

Any tips/pointers is much appreciated.

M./

 

[mneary]

An fairly large voltage drop can be accomplished in an exceedingly small resistance by applying an exceedingly large current.

 

[motolectric]

An fairly large voltage drop can be accomplished in an exceedingly small resistance by applying an exceedingly large current.

That's exactly the situation I have.

12.65-12.79 (lead-acid to AGM) volts and a .7 kilowatt starter motor (i.e wants 53-55 amps or so). The original equipment circuit takes 7 seconds to start a motorcycle and mine (all copper from tip to tip except for the tin plating on the terminals) starts the bike in 1 second.

I have shade-tree mechanics that want to get into measurable resistance arguments and I need an authoritative resource to point them to to satisfy them that I am not making things up (i.e the increased resistance is almost unmeasurable but the result of removing that unmeasurable increase is easy to see).

So hopefully I can get a site with figures shown and or some sort of equation that will allow me to show them hard numbers.

I'm also looking for any data on the added losses as current crosses from one type of metal to another, copper to steel and copper to brass etc. And of the losses of wire to terminal transitions (of even a brand new junction).

Any pointers towards hard numbers on things like the loss of 2, 6 inch pieces of copper wire with a brass butt connector in the middle vs. a straight 12 inch piece of wire is also desirable.

Part of this is what I would call a "green initiative". My circuits will allow motorcyclists to extend the life of their batteries 2-3 times and that means a lot less toxic lead-acid batteries to be properly disposed.

Again, thanks for the quick reply.

M./

 

[bychon]

You are using the standard method of measuring very small resistances. That means: running a high current through the resistance and measuring the voltage loss in the low resistance item. You didn't mention measuring the voltage loss with a meter, but you are measuring it with a starter motor! A motor running faster and better is a sure sign of a better voltage supply. You haven't "made up" something that was previously unknown or mysterious.

I don't know about losses in metal to metal transitions, and I'm too lazy to set up a high current tester. Maybe somebody else will answer that.