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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?

  • I've cleaned my terminals using baking soda mixed with water. – MooseLucifer Aug 17 '16 at 22:52
  • @MooseLucifer, so, I buy baking soda, and mix it in distilled water that I already have from topping up my old battery? How much should be mixed? – cnst Aug 17 '16 at 22:54
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    You need to make sure that on your model if you disconnect the battery it will not clear the BCM of it's security keys and force you to get a key faub reassociated with it. In my 2011 Infiniti there is a special battery replacement process so you don't have to get your BCM reprogrammed. – DucatiKiller Aug 17 '16 at 23:38
  • I do not know about WV, but Volvo, when connect battery back, ignition key must be in position II. So, read Your cars manual for procedure. – Guntis Aug 18 '16 at 17:48
  • Related question – Zaid Aug 18 '16 at 20:53
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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

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Agree with @resident_heretic about the order of removing/installing the posts.

  • Remove the negative first and reconnect last when changing out the battery. This prevents shorting the positive battery terminal to ground while swinging a wrench to tighten things.

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.

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    Just another thing to consider is that some newer cars require the battery be coded to the engine computer. If it's not the charging system will burn out the battery fairly quickly. – Ben Aug 18 '16 at 10:55
  • @Ben - Salient point! Manufacturers are making it harder and harder to do things as a consumer, eh? – Pᴀᴜʟsᴛᴇʀ2 Aug 18 '16 at 21:39
  • @Ben, do you have some source for this? If I'm buying the battery directly from the dealer, which has a different brand, and a part number of 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? – cnst Aug 19 '16 at 3:28
  • fwiiw, i've posted a new question in regards to the correct battery replacement for a Jetta: mechanics.stackexchange.com/questions/35403/… – cnst Aug 19 '16 at 3:58
  • @cnst check out the vcds tour page ross-tech.com/vcds/tour/recode_screen.html it references the battery recoding. I've also seen mention of audis as early as 07 needing recoding. note these are all agm batterys. check with your dealer to make sure. – Ben Aug 19 '16 at 11:37
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Replacing a battery is as simple as:

  1. disconnecting the 2 battery terminals
  2. removing the old battery
  3. inserting the new battery
  4. reconnecting the 2 battery terminals

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,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.

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    Whoa those brushes are neat. If I didn't own a dremel I'd totally use them. – Jason C Aug 18 '16 at 1:26
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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.

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    +1 for answering the question behind the actual question. – the-wabbit Aug 18 '16 at 7:13
  • Thanks for the tip with the wheel bearing grease. I noticed some people, including you, recommend greasing before applying the clamps. Is this really a good idea? Won't it increase resistance between the terminals and clamps? – I have no idea what I'm doing Aug 18 '16 at 7:40
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    @IhavenoideawhatI'mdoing: When you tighten the clamps they displace the grease so you have metal-to-metal contact. I've never had (or heard of) any issues with either current or voltage from greasing the terminals. Contrast that to the problems that everyone's seen from not greasing the terminals and it's pretty clear that greasing is a win. – TMN Aug 18 '16 at 15:25

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