Every December, I start hearing the same questions at the workshop. "Kris, my Start/Stop has stopped working. Has something broken?" And then there's the other one: "I've changed the battery, and now the car's showing faults." Here's the thing, my friend – those two problems are connected. And VW battery coding is a big part of the story. Let me explain what's going on so you don't end up spending money you didn't need to.
I've already explained how the system itself works in my article "Start/Stop – What Is It and How Does It Work?". This time, I want to show you why winter changes everything and just how important your battery really is.
Why does Start/Stop stop working when it gets cold?
Because your car is trying to make sure you don't get stranded. Its computer constantly monitors how much charge is left in the battery and what condition it's in. If it thinks there won't be enough power to restart the engine after switching it off at the traffic lights, it simply keeps the engine running. That's not a fault. Quite the opposite – the system is looking after you.
Winter is the hardest time of year for your battery. Cold weather reduces its capacity, and that's exactly when we switch everything on – the heater, heated seats, heated windows, heated steering wheel, headlights and wipers. Now add those short journeys to work and back. The alternator simply doesn't have enough time to put back the power you've used. So Start/Stop might stay inactive for weeks. But that doesn't necessarily mean anything is broken.
When does Start/Stop not working point to a real problem?
- Start/Stop still doesn't work even after a long motorway drive.
- You get a low battery warning or a system fault message on the dashboard.
- You notice the starter motor turning more slowly than usual in the morning.
- Your battery is already more than 5–6 years old.
If you notice two of these things happening together, take my advice and get the battery checked. Don't wait until one freezing morning when the car refuses to start to find out that the battery needed replacing.
Why is VW battery coding necessary after replacement?
This is where a lot of people get it wrong. In a car with Start/Stop, the battery isn't just a box full of electricity with two terminals. There's a sensor on the negative terminal that monitors current, voltage and temperature. And the computer that controls charging knows exactly what battery you've got – its capacity, type, manufacturer and serial number. It uses that information to decide when to charge the battery and how much charge it needs.
Now imagine you replace the battery but skip the coding. Your car still thinks it's dealing with the old, worn-out battery, so it keeps charging the new one in exactly the same way. The new battery can end up undercharged or overcharged, Start/Stop still doesn't work, and the battery's life gets cut short. Then the owner comes to me and says, "Kris, batteries these days are rubbish." And I have to explain that the battery itself might not be the problem at all.
If your car has Start/Stop, always fit the battery type specified for it – AGM for AGM, EFB for EFB. A standard battery simply can't cope with that kind of use.
How do you replace and code a battery properly?
- First, we check what battery is currently fitted and what the manufacturer specifies for the car.
- We fit a new battery of the correct type and capacity.
- Using a diagnostic tool, we register the new battery's details in the car.
- We clear any fault codes and check that the charging system is working properly.
The coding itself only takes a few minutes. There's nothing complicated about it when you know what you're doing. But! Don't skip it. Replacing a battery isn't just a matter of undoing two terminals, swapping the battery and tightening them back up.
How can you look after your battery in winter?
If your car sits unused for long periods or you mostly drive short distances, I'd recommend connecting it to a good battery charger overnight once every two weeks. Modern chargers monitor the charging process themselves and prevent the battery from being overcharged. It's a little bit of care that could save you from having to call roadside assistance when it's minus ten outside.
A tip from Kris Mladenov: If Start/Stop stops working in winter, don't immediately assume something's broken. Check the battery first. And if it turns out you need a new one, don't skip the coding. I'm telling you this for your own good – five minutes of work could save you from having to buy another battery a year later.
I hope this article saves you at least one freezing morning spent wondering where you're going to find a set of jump leads. And if you've found it helpful, share it with your friends who drive cars with Start/Stop too.
The solution in our shop
Genuine Audi battery charger. Keeps your battery charged when the car is parked for long periods or you mainly use it for short journeys.
View in our shopRead more
- Electrical Problems and Control Modules
- Diagnostics
- Start/Stop – What Is It and How Does It Work?
- G12, G12++, G13 and G12evo Coolant: Can You Mix Them?
- Leaving Your Car Parked for a Long Time: What to Do Before and After
- The New Year and Your Mileage: What to Check After the Holidays
Frequently Asked Questions
Why doesn't Start/Stop work in winter?
Because your car keeps an eye on the battery's charge and won't switch off the engine unless it's confident there's enough power to start it again. In winter, the cold and all the electrical equipment you're using put a much heavier load on the battery.
Do I need to code a new battery?
Yes, if your car has Start/Stop and an energy management system. Without coding, the car carries on charging the new battery as though it were the old one. And that can seriously shorten its life.
Can I fit a standard battery instead of an AGM?
No. If your car has Start/Stop, stick to the battery type the manufacturer specifies – AGM or EFB. A standard battery simply isn't designed to cope with that kind of workload.
How long does a Start/Stop car battery last?
Usually around 5–6 years, provided it's been coded correctly and you don't leave the car sitting with a flat battery for long periods.