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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?
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.)
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.
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.
Dielectric grease does not conduct electricity. Proper application is important! Do NOT get grease between the terminals! This will inhibit good connection and shorten battery life. Vaseline was used for many decades, but with its own downsides. Vaseline is petroleum based, and when it gets hot it likes to run. This can cause the grease to penetrate between connections and cause problems.
Silicon based dielectric compound works extremely well because silicon won't run when it gets hot. After the silicon sees high temperatures, it will start to vulcanize. That's a fancy term for saying it will turn more solid, rather than liquid. This attribute makes it better suited for this role than petroleum jelly, and it is safe for rubber too.
If you get the grease between the connection, expect to have issues down the road. It might be 2 months, or 2 years. But eventually that grease will turn solid, and you will cause a voltage drop across the terminals. I don't need to go into battery chemistry here, but having too much voltage drop across the battery terminals will shorten the battery's lifespan.
Dielectric grease works well to prevent corrosion, but anti-corrosive spray works better in my opinion. It gets in places your fingers can't get the grease to. Dielectric grease can be used in between electrical connections, but only those utilizing low-current. Battery terminals are high current.
Not sure about dielectric grease, but I know you can use Vaseline like they did back in the old days. It works just as well, stopping moisture being able to form on your battery terminals.
