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What is the process for replacing the 12V battery?
How would I clean the contacts of the terminals?
My OEM battery in 2008 Jetta SE 2.5L appears rather clean, including the terminals, but I recall that my older second-hand car had huge deposits on the negative terminal (presumably from an aftermarket battery replacement). How do I make sure I replace my battery in such a way as to not cause any such huge deposits to appear?
After cleaning the terminals with a small amount of baking soda (approx a tbsp and enough water to make a paste ) and a old toothbrush. I recommend wearing gloves to protect your hands from the acid salt and lead from the terminal. The acid deposits are normal.
Assuming negative ground:
When installing the battery connect the positive first. The car body is the ground (negative)side. Doing the positive first eliminates the risk of short circuit.
When removing a battery disconnect the negative first for the same reason
Agree with @resident_heretic about the order of removing/installing the posts.
Something which has not been talked about, in a more generic sense (so this is for any vehicle), is to ensure you have the codes for your radio if needed. If you don't have the codes for the radio and or navigation system (thinking Hondas here), you're going to be in sorry shape for trying to use them when you put the new battery in. This might entail a trip to the dealer and asking them for the codes if you cannot find them on/in the vehicle. I don't know if the VW's suffer this fate, so this is just something to think about.
000 915 105 DE (OEM was 1J0 915 105 AD), but is the official replacement battery for the car in the region, do I still have to worry about the correct coding?
Replacing a battery is as simple as:
Cleaning the "contacts" is something you do when it is needed. Healthy batteries usually do not make deposits outisude. Batteries of lesser quality tend to corrode more easily or more quickly, but most batteries, if old enough, tend to corrode their terminals at some points. Someone else suggested baking soda, what I personally use is a steel brush. You can use a generic steel brush (I call it my elephant toothbrush), parts stores also sell special steel brushes for battery terminals,
where one end is a tiny "male" steel brush to clean the inside of the connector and the other end is a "female" steel brush for the electrode. This thing has served me well, I only ever use my elephant toothbrush when the deposits are excessive.
The major cause of terminal corrosion is moisture. Even a small amount of water getting into small gaps or spaces where the metal surfaces touch can lead to electrolysis, which will erode the metal. This in turn creates larger gaps, which allow more moisture to accumulate, causing the problem to grow worse over time.
A common method for preventing this is to coat the terminals with grease. This accomplishes two things: First, it seals off the metal surface from the air, which prevents oxidation. Second, it fills small gaps where moisture could accumulate in the future, preventing the opportunity for future electrolytic corrosion.
The grease must be resistant to heat, so it doesn't melt and flow away from the connection. Wheel bearing grease works nicely, or you can get dielectric grease or special products made for battery terminals.
It's possible to grease the terminals before connecting them. By using a tool such as the one in tlhlngan's answer, you roughen the surface of the terminals, exposing fresh, unoxidized metal surfaces. The grease prevents oxidation by the air, and if you tighten the terminals appropriately, the metal surfaces will crush together giving good metal-to-metal contact. Any small gaps remaining are filled with grease, not air or water.
Another factor is dissimilar metals contacting each other, which can cause galvanic corrosion even without the presence of an outside electrical charge. Having electricity flowing through the connection exacerbates this.
Posts and terminals themselves are typically made of lead alloys, so are not that dissimilar, but the cables are usually copper, and copper-lead connections will produce galvanic corrosion. Aftermarket terminals that are added to an existing cable to save time and money are particularly likely to do this.
The most important thing you can do to prevent corrosion is keep the area dry. Be careful when adding liquids to your radiator, washer fluid, or the battery itself. Some terminals come with little rubber or plastic covers, these help protect from splashing. A battery located near the front of the car may get more rain exposure than one farther back. Avoid driving through deep puddles.
If you do need to clean and wash the battery and terminals, allow them to dry thoroughly before reconnecting.
