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I recently had a new battery installed in my 2006 Subaru Forester. The shop did not use any dielectric grease, just connected the leads to the terminal and that's it. What is the preferred method of application?

  1. remove leads, apply to terminals, connect leads
  2. do not remove leads, apply to terminals and leads
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3 Answers

up vote 3 down vote accepted

Evaporated / sprayed electrolyte is corrosive. If you see corrosion on the battery posts, the leads, or in the vicinity, that's why. A layer of silicone dielectric grease over the exposed metal parts will protect them. I especially like to get some on the threads of screws in that area, to make sure they come apart easily in the future.

You want as much metal-metal surface area as possible between the battery posts and the terminals. Remember V=IR, which says that as current and resistance increase, so does voltage drop. Resistance is proportional to the cross sectional area of the conductor. So, if you draw a high current through battery leads that don't have much contact area with the posts, you will lose a lot of voltage through that connection (as heat).

So, if you want a really good connection between these metal surfaces, why would you put dielectric (non-conductive) grease in between them? Because these surfaces are not perfectly smooth. At the high points they touch, and at the low points there are gaps. Dielectric grease will get smooshed in to the gaps, keeping out moisture and electrolyte. You only need a very thin layer here, though.

As metal corrodes it expands. This will push apart the metal-metal contact at the battery posts. So, good to have the dielectric grease to stop that corrosion, and good to apply proper torque to the battery terminal.

The only times cars usually see heavy loads is during starting and immediately after as the battery is recharged. You probably won't see enough heat buildup in that time for it to matter much, until you the battery is old and you try to start on a cold morning. Then the voltage drop across the battery connection will be enough to make it difficult to start.

(It's a much bigger deal with off-grid homes and RVs, where the loads are continuous and the power source is scarce.)

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good answer, but i think it's confusing about putting the dielectric between the clamp and the post. i suggest clarifying that or doing a tl;dr as your last sentence. –  longneck Oct 28 '11 at 18:39
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I have never had problems with corrosion so I have not used dielectric grease on any of my car's battery terminals.

If the battery already has corrosion on the terminals, then I would recommend disconnecting the terminals using a wire brush to clean off the buildup and then reconnecting the leads.

If there is no corrosion then I would recommend just putting the grease over the leads and terminals. There should be no problem with either approach though.

SAFETY NOTE

But as always, when dealing with electricity be very careful when dealing with the battery and make sure not to create a circuit between the positive(red) terminal and the any metal on the car.

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MORE SPECIFIC SAFETY NOTES: - remove metal rings from your fingers. 12V won't hurt much if you get shocked, but a start battery can dump enough current (amps) through your ring to melt it. - remove negative first. Everything in your car is already connected to negative, so as soon as you remove this everything is dead. - battery acid fluid can eat holes in your clothes and skin, and blind you. Baking soda, nitrile gloves, and face masks are useful. Someone should ask a question about battery safety! –  Jay Bazuzi Mar 9 '11 at 3:39
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The wire brush to use here is specially made for batteries, and pretty cheap. It is designed to scuff both the post and the terminal. Use it before applying the grease. –  Jay Bazuzi Mar 9 '11 at 3:44
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Not sure about dielectric grease, but I know you can use Vaseline like they did back in the old days and it works just as well, by stopping moisture being able to form on your battery terminals.

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