The brief
Mr Tan brought his Tiguan in for a battery replacement, wanting it done properly by a workshop rather than guessing at it. That's the right call, a battery fitted by trained hands with the right tools, registered to the car, and backed by a warranty on both the part and the workmanship is worth more than a cheap drop-in. The battery starts the car and steadies the voltage for everything electronic while it runs. They wear out, the capacity drops, and once it can't hold a proper charge you get slow cranking and odd electrical behaviour. On a Tiguan the new one also has to be registered to the car's energy management so the charging system knows it's fresh and charges it correctly. A worn battery doesn't recover, so it needs replacing and coding in.
The diagnosis
A battery and charging test confirmed it, the battery was down, failing the load test and unable to hold voltage. The alternator was charging fine, it was just the battery at the end of its life. So it was a battery replacement, the correct type and rating for the car, fitted and then registered to the energy management so the charging would look after it properly.
The work
The old battery was removed and a new battery of the correct type and rating fitted, the terminals cleaned and the clamp torqued properly. The new battery was then registered to the car's energy management so the charging system recognised it as fresh, and any stored low-voltage faults were cleared, with the battery and the work covered by warranty. A quick run confirmed a strong crank, steady voltage, and the electrics behaving normally.
The outcome
Strong starts, steady voltage, the electronics back to normal, the new battery registered so the charging system manages it, and a warranty behind it. The Tiguan went home sorted. A worn battery only fails harder, so changing it, coding it in and backing it with a warranty gave Mr Tan a fix he doesn't have to think about.