Newsgroups: rec.autos

In rec.autos, david.bonds@cutting.hou.tx.us (David Bonds) writes:

      >>I've been told that petroleum jelly is great at keeping battery terminals
      >>from corroding, just put on the inside of the cables (where they contact
      >>the terminal), and you shouldn't have any problems with corrosion til you
      >>need a battery replacement anyhow!
      >
      >  This is a good way to make SURE your car won't start.  The
      >  petroleum jelly (or Vaseline) is an excellent insulator.  Even if

      [This is correct, vaseline is an excellent insulator (dielectric)
      as are most greases. Conductive greases usually contain powdered
      metal so that there are electrically conductive paths. When making
      an electrical connection, one must take care to prevent the
      formation of insulating oxides or other insulating compounds.
      Electricians will liberally smear power line connections with
      grease to seal out corrosive oxygen from the atmosphere. If the
      connection corrodes you will get at least 2 bad results:
      You will have the extra resistance in series with the load so less
      voltage will appear at the load. The voltage lost will equal the
      resistance of the connection times the load current. You can see
      that loads which require high currents will suffer the greatest
      loss. The other bad effect is that the connection will heat up due
      to the resistive heating I^2*R ( I am not going to worry about
      phases etc). The heating of the connection will further raise the
      resistance since the conductor's bulk resitance is an increasing
      function of temperature. Eventually, the circuit can go open or
      have so much resistance that it cannot supply adequate power, or
      it can cause a fire.

      So what is the effect of greasing the contact area and the
      surrounding area? In a car you want to prevent the sulfuric acid
      from forming resistive sulfates and oxides as outlined above. The
      grease is easily displaced by the clamping force of the
      connection. The total area of metel-to-metal contact will be
      approximately the same whether grease or air is present in the
      connection. If the grease is inert to acid, the connection will be
      preserved. Most greases, vaseline, cup grease etc. are resistant
      to acids and will work quite well; but be very certain that ALL
      corrosives are cleaned from the connection before applying grease
      or you will lock in the acid and still have problems. Plain water
      will work quite well ( a little baking soda will aid the
      neutralizing of the acid). Use caution when cleaning the battery
      so water and baking soda does not enter the electrolyte. It's a
      good time to top off the water in the battery. Now smear grease on
      the post and top of battery around the post. Attach the clamps and
      coat them with grease. Do not use conductive greases as the
      conducting medium will most likely corrode in acid.  I have found
      a jack knife to work the best in removing the resistive layer from
      the posts and the clamps. Most of the brush types don't work well
      unless the battery was OK anyway.
      Bob]


Newsgroups: rec.autos