A GTI that goes flat overnight or won't hold charge is usually a parasitic drain or a battery not registered to the car's energy management.
Parasitic drain:
- Something staying awake — a module not sleeping, a boot/glovebox light, an aftermarket accessory. Measure the sleeping current (let the car sleep ~30–40 min, then meter the draw) and pull fuses to find the circuit.
Battery registration (BEM):
- Modern VWs run battery energy management — a replacement battery must be coded/registered (capacity + serial) with VCDS/OBDeleven, or the charging system mismanages it and it dies early. Always register a new battery.
Also: a battery simply past its life, or a failing alternator (check charging voltage).
…