The brief
The Tiguan had a battery that was getting on, with corrosion building up around the terminals, slow cranking, and the kind of electrical niggles that come with a tired battery. He brought it in. Terminal corrosion is more than untidy, it raises resistance, weakens the charge getting through, and can become a fire risk if it's left, so it wants sorting. 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 the slow starts and odd 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 corroded, worn battery doesn't recover, so it needs replacing, the terminals cleaned, 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, with the terminals corroded. 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, fitted with cleaned terminals and then registered to the energy management so the charging would look after it properly.
The work
The old corroded battery was removed, the terminals and clamps cleaned and protected, and a new genuine VW-spec battery of the correct type and rating fitted, 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. 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, clean terminals, and the new battery registered so the charging system manages it. The Tiguan went home sorted. A corroded, worn battery only fails harder and the corrosion only spreads, so changing it, cleaning the terminals and coding it in put the car right before it became a no-start or a hazard.