Audi A3 TFSI e charging works one day, fails the next: how to track it down

One of the more frustrating issues A3 TFSI e owners report is inconsistent charging. Same socket, same cable, same routine, and yet one evening the car charges through to the morning and the next it stops part-way, refuses to start, or throws a vague yellow warning triangle.

The car itself is rarely broken. In most cases the culprit is one of three things: the small 12 V auxiliary battery, the charge timer/scheduler, or a setup quirk somewhere in the chain. Here is how to isolate it without a dealer visit.

The plain-English version

The A3 TFSI e has two batteries: the big high-voltage traction battery that drives the car, and a normal 12 V battery that powers the electronics, including the charging control unit. If the 12 V battery is weak, the charging controller can drop out mid-session even though the main battery is fine. That alone explains a large share of "it worked yesterday" stories.

On top of that, the MMI charge timer is the second usual suspect. If a departure time or preferred charging window is set, the car will happily plug in and not start charging until that window opens. Many owners forget the setting exists.

Step-by-step diagnosis

1. Check the 12 V battery. With the car off, a healthy 12 V battery should read around 12.6 V. Below 12.4 V and you have a likely cause. An Audi service centre can load-test it properly.
2. Disable all charging timers. In MMI, turn off scheduled/preferred charging and "reduced AC current". Try a plain plug-in session and see if it completes.
3. Try a different socket and a different cable. If the issue only appears on one socket, the socket is suspect (loose neutral, weak RCD). If it follows the cable, swap to a known-good Type 2 cable. The right spec for the A3 TFSI e is a single-phase 16 A 3.7 kW Type 2 cable from Voldt; the full Audi A3 TFSI e range also has matching accessories.
4. Update the car software and the myAudi app. A handful of inconsistent-charging reports trace back to a charge-management software bug that a later update resolved.

A bit of technical context

The A3 TFSI e charges single-phase at roughly 3.6 kW (16 A) on AC. There is no DC fast charging, the inlet is Type 2 only. That means there is not much performance variability to play with: either the session runs at its single-phase ceiling or it falls over. Inconsistency almost always means something is interrupting the session, not throttling it.

Bottom line

Test the 12 V battery, kill the timers, swap the cable, then book a service if it still misbehaves. In that order, you will catch the cause without paying for a diagnosis you did not need.