Re: Which battery lead to disconnect first

["Robert S. Wilkinson" <wilk@domain.elided>]

Alfisti,

I ordinarily wouldn't contribute to this thread (see below) but a recent
thread on another car list discussed the observation that 80% of
information on that list was wrong.  I wouldn't want that allegation to
surface here, but it's looking close.  Here's the "truth."  

1)  Disconnecting the frame-connected lead (now usually negative) precludes
the possibility of accidentally short-circuiting the battery by touching
the battery's post and a metal part of the car at the same time while
disconnecting.  Such shorting causes a huge current (1000+ amperes) that
can vaporize tools/metal and burn the mechanic, including eyes if not
protected.

2)  Other than that, it doesn't make any difference.  A 12 volt circuit
cannot shock you, unless you contact an inductor (e.g., relay coil) when it
is discharging.  Even if it could, there is still 12 volts between the
negative battery terminal and the car's frame if anything at all is powered
(radio memory, etc), so this is not a factor in choosing.  There is no
difference intrinsically between pos and neg terminals that is measurable
at this kind of voltage.

3).  Letting the positive (if disconnected) battery terminal contact the
car's metal frame short-circuits any capacitor that might remain charged
(e.g., some airbag systems, power lock, seat, radio station, mirror
memories).  This would probably be of no consequence, but if the
disconnection is for a short time, not letting this happen would allow the
memories to remain; otherwise, reprogramming would be necessary.  Doesn't
happen, of course, if the neg terminal is disconnected.

Bob Wilkinson
72 Spider


Date: Mon, 13 Jan 2003 20:11:44 +0200
>From: "John Fielding" <johnf@domain.elided>
>Subject: RE: Which battery lead to disconnect first
>
>Hi to all the Digestables,
>
>I had a good amount of off-line comments which all seemed to support the
"negative lead
>first" syndrome.
>
>I went searching on the Internet, just love this modern technology!  I
found two sites
>which happened to be USA Fire Departments and each had a catalogue of how
to tackle a
>vehicle after a collision.  Both stated that the negative lead should be
disconnected
>first.  One even went so far as to say "disconnect the negative lead first
as this will
>not produce a spark if current is flowing, whereas the positive lead can
produce a spark
>which can ignite the battery gassing products".
>
>When I was at college, many years ago, we learnt about Kirchoff's law
which simply stated
>that the current flowing in a circuit was the same at the positive pole as
the negative
>pole, so why disconnecting the negative produces no spark?? , but the
positive can,  when
>the same current is flowing in both baffles me.
>
>I also have a very faded Morris Motors workshop manual from the 1930 which
deals with
>disconnecting the battery before working on the electrical system.  In
this they state
>"always disconnect the negative lead first".  However, in those days cars
where fitted
>with positive ground electrical systems, so the negative battery lead was
the live one,
>not the ground lead!
>
>Methinks along the ways somewhere the changeover to negative earth
vehicles got muddled in
>the handbooks!
>
>I have an aquaintance who runs an auto electrical repair shop and I asked
him the same
>question, which lead to disconnect first.  He said it didn't matter as
long as the battery
>was effectively isolated from the vehicle wiring, but his preference was
the positive (on
>a negative earth vehicle).
>
>So I am still puzzled, no one can give me a definitive answer, not even
the local Bosch
>agents.
>
>The FIA in its many pages of regulations for the various motor racing
formulae deals with
>battery isolators.  They state the battery must have an effective isolator
"which can be
>fitted in either battery lead".  Battery isolators are used extensively on
commercial
>vehicles, all the catalogs I have so far managed to clap eyes on have the
main switch in
>the positive lead!  So what gives???
>
>
>John
>Durban
>South Africa
>Alfetta 1.8L turbo
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