This guide is about the 12-volt battery that starts the engine and powers the car's electronics. It's a different part from the small coin battery inside your key. If your fob is the problem, our guide on how to change the battery in your Audi key fob takes about five minutes. If the fob dies before you can swap it, here's how to start your Audi with a dead key fob.

The 12-volt battery is a bigger job. It's tied into the car's charging system, and that's where coding comes in.

Older Audis like the 2013 Q5 don't show a battery health percentage in the MMI menus. You can still get a clear picture of where your battery stands from a few sources.

Check the battery's age. Most batteries have a date code on the label. AAA puts the average battery life at three to five years. If yours is in that range and still the original, it's worth testing before the cold months.

Take a voltage reading. With the engine off, a healthy, charged battery should read about 12.6 volts on a basic voltmeter. With the engine running, a reading between 13 and 14.5 volts means the alternator is charging normally. A battery that keeps dropping below 12.6 volts after a full charge is struggling to hold one.

Watch the start-stop system. On Audis with start-stop, the system is a useful early warning. Audi's service information for models including the 2013 to 2016 A6 and Q5 says the engine won't shut off at a stop when the battery charge is too low. If start-stop has stopped working in mild weather with the engine fully warmed up, the battery may be weakening.

Notice how the car starts. A slow, labored crank, dim lights at startup, or electrical warnings on the dash are common signs. A red battery light usually points to the charging system rather than the battery. Our guide to Audi dashboard warning lights covers what that light means and how urgent it is.

Get a professional test. A dealership battery test checks the battery's condition under load and can read the data stored by the car's energy management system. It's the most complete answer, and it's part of a standard service visit at the Audi Great Neck Service Center.

Driving habits matter too. A car that mostly makes short trips, like quick errands along Northern Boulevard, may not run long enough to fully recharge the battery. Cold mornings in a Manhasset driveway add more strain.

Older cars had a simple charging setup. The alternator charged the battery, and any battery that fit the tray worked the same way.

Modern Audis use a battery management system that watches the battery's condition and adjusts charging over time. As a battery ages, the system changes how it charges that battery to keep it topped up. Midtronics, which makes the battery testing equipment many service departments use, explains that the car can't detect a new battery on its own. Until the new one is registered, the system keeps using the charging plan built for the old one.

The car stores details about the installed battery, including its part number, serial number, manufacturer, and capacity. When a new battery goes in, those details need to be updated so the system starts fresh.

Audi battery coding, sometimes called adaptation or registration, is done with a scan tool plugged into the car's diagnostic port. Ross-Tech, the company behind the VCDS diagnostic software used by many Audi specialists, says a large majority of modern Audis require it. Depending on the model, the technician enters the new battery's:

• Part number and serial number
• Manufacturer
• Capacity, measured in amp-hours
• Battery technology, such as AGM or EFB

Skipping this step is what catches most DIY owners off guard. In most cases the car starts and drives normally after the new battery goes in, so nothing seems wrong. The problems show up gradually.

According to Midtronics, a battery that isn't registered can be overcharged or undercharged. Overcharging can overheat the battery and dry it out, and undercharging lets damaging deposits build up on the battery plates. Either way, the new battery wears out sooner than it should.

Start-stop is often the first feature affected. Audi's start-stop service information lists "the battery has not been adapted with a scan tool" as one of the conditions that keeps the system from working. Even after a correct adaptation, the system needs about 10 start cycles to finish calculating the new charging curve. Some owners also see electrical messages or stored fault codes after an uncoded swap, which can send them looking for a problem that's really just missing paperwork in the car's computer.

Coding only works if the new battery is the right type and size for your Audi. Many Audis with start-stop use an AGM (absorbent glass mat) battery, which is built to handle frequent restarts. Others use an EFB (enhanced flooded battery).

VARTA, a major battery supplier for German automakers, says a start-stop battery can only be replaced with one of the same type. An EFB can be upgraded to an AGM, but putting a standard battery in place of an AGM or EFB leads to rapid wear. Ross-Tech adds that a replacement needs to match the required capacity, chemistry, and internal resistance. If the battery type changes, that setting has to be updated during coding as well.

This is why the bargain battery from a big-box store isn't always a direct substitute, even when it fits the tray and the terminals line up.

Where the battery sits depends on the model and generation. On some Audis it's under the hood beneath a plastic cover or near the base of the windshield. On others it's under the trunk floor, with jump-start posts under the hood so you don't need to reach the battery to jump the car. Your owner's manual shows the exact location for your car.

An A3 battery removal usually means taking off a cover and a hold-down bracket before you can reach the terminals. On larger cars like the A6, the job can involve removing trunk panels and working in a tight space. Either way, the physical swap is often the easy part.

Newer Audis add another layer. Many current models use a 48-volt mild hybrid system with its own lithium-ion battery alongside the 12-volt system. That's not a battery for a driveway repair, and it's a good reason to leave newer models to trained technicians.

Disconnecting the battery can also reset a few things. Depending on the model, features like one-touch windows or the clock may need to be set again, and some older models may ask for a radio security code.

A DIY Audi battery change is possible if you have three things: the correct replacement battery, a scan tool that can perform Audi battery adaptation, and the battery's part number, serial number, and capacity on hand when you code it. A basic code reader that only reads and clears fault codes won't do the job.

If any of those are missing, the dealership is the simpler route. When you have your Audi battery replaced at Audi Great Neck, the service team:

1. Tests the old battery and charging system first. This confirms the battery is the problem. A weak alternator can make a good battery look bad.
2. Installs the correct battery for your model. That means the right type, size, and capacity for your Audi's electrical system.
3. Codes the new battery to the car. This resets the charging strategy so the new battery charges correctly from day one.
4. Checks for stored fault codes. Anything left over from the low-voltage period gets cleared so it doesn't hide a real issue later.

Genuine Audi replacement batteries installed at the dealership come with their own limited warranty, and the service team can go over the current terms when you book. The service department at 795 Northern Boulevard is open Monday through Friday from 7:30 AM to 5:30 PM and Saturday from 8:00 AM to 4:00 PM. Complimentary shuttle service and loaner vehicles are available with service, which helps if you're coming from Port Washington or Roslyn and can't wait at the dealership.

An Audi battery replacement goes smoothly when you test the battery before it fails, match the replacement to what your car was built for, and finish with coding so the charging system knows what's installed. Coding is quick to do and costly to skip, since an unregistered battery can wear out early and take start-stop with it.

Before winter, it's a good time to have the battery tested along with the rest of the items in our Audi winter checklist. Schedule a service appointment online, check current service and parts specials, or contact our team with questions about your model.