[AT] Tractor wiring

Dudley Rupert drupert at premier1.net
Thu Nov 18 02:30:13 PST 2004


Charlie:

I think you're right that the work (or energy) that the starter must put out
is the same whether it is 6 or 12 volt and that this work is equal to the
voltage at the starter times the current flow through the starter.

However, as Dean has stated below, the battery/starter wire does in fact
have resistance (i.e., it is not a perfect conductor) so there will be an IR
drop (which is sometimes called voltage drop) across the wire length.  This
means that the voltage at the starter is not actually the battery voltage
but it is equal to the battery voltage minus this voltage drop.

For a 6-volt starter to get the same electrical energy as a 12-volt starter
in theory requires it to get twice as much current.  So, if the
battery/starter wire is the same size in both a 6 and 12 volt system the IR
drop across the wire to the 6 volt starter is twice that of the 12 volt
system and thus, in reality, less than 6 volts is available at the starter.
So, the 6-volt starter actually needs more than twice as much current in
order for it to get the required electrical energy input.

Another important factor to keep in mind here is the heat loss that occurs
across the battery/starter wire.  Heat loss is equal to the square of the
current times the resistance.  As discussed above, the 6-volt starter system
requires more than twice as much current as does the 12-volt system and thus
the battery/starter wire in the 6-volt system is required to dissipate more
than 4 times as much heat.  Since the DC resistance of a copper wire is not
only inversely proportional to its' cross sectional area and directly
proportional to its' length, it is also directly proportional to the ambient
temperature of the wire (at least over quite a wide temperature range).  So,
as the resistance of the battery/starter wire goes up (due to heat) so does
the IR (voltage) drop across it resulting in even less voltage at the
starter thus requiring more current at the starter in order for it to get
the required input energy and ...  You get the point.  In summary I would
guess that for batter/starter wire sizes in the #1, #2, #4 & #6 gauge range
one would have to go up at least one and maybe two wire sizes for a 6 volt
starter system in order to keep the heat loss and voltage drop to values no
greater than that of a 12 volt starting system.

A long ramble ... hope it adds to the discussion.

Dudley -
Snohomish, Washington


-----Original Message-----
From: at-bounces at lists.antique-tractor.com
[mailto:at-bounces at lists.antique-tractor.com]On Behalf Of Dean VP
Sent: Wednesday, November 17, 2004 8:30 PM
To: 'Antique tractor email discussion group'
Subject: RE: [AT] Tractor wiring

Charlie:

It's not wattage that is the primary concern. It's IR drop. When starting
with only 6V to begin with you want as little voltage drop as possible due
to the high current.

Dean A. Van Peursem
Snohomish, WA 98290

I'm a walking storeroom of facts..... I've just lost the key to the
storeroom door


www.deerelegacy.com

http://members.cox.net/classicweb/email.htm



-----Original Message-----
From: at-bounces at lists.antique-tractor.com
[mailto:at-bounces at lists.antique-tractor.com] On Behalf Of charlie hill
Sent: Wednesday, November 17, 2004 6:52 PM
To: Antique tractor email discussion group
Subject: Re: [AT] Tractor wiring

Here is my question on this subject.

Why are battery cables for 6 volt systems heavier than those for 12 volt
applications?   Assuming both are powering a starter to turn a similar size
engine (similar load), why would the 6 volt cable need to be bigger.  It
flows more amps but less voltage and should flow roughly the same wattage in

both applications.

Charlie


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