BMW F30: EME fault stopping the 12V battery from charging

The problem

A BMW F30 came in with an EME fault. When the EME fails, the car can no longer charge its 12V battery.

That makes it easy to misread. The 12V battery goes flat, and the natural assumption is that the battery is to blame. But if the charging side has failed, a fresh battery just runs flat again. The real job is finding out why the battery isn't being charged.

What the EME does

EME stands for electric machine electronics. On BMW hybrids it's the power electronics unit that controls the electric motor, and it also converts high-voltage power down to the 12V that keeps the car's battery charged.

On this car the 12V battery depends on the EME for its charge. When the EME fails, nothing is keeping the battery topped up, so it slowly runs down.

The EME unit removed from the car. The label shows it is a BMW part made by Bosch.

Diagnosis and repair

The fault was diagnosed and fixed in our workshop. The unit was taken out of the car and opened up on the bench for a closer look at its internals.

The unit opened up on the bench, with its internal circuit boards visible.

The unit's internals laid out on the bench alongside the tools used.

Why this matters

If a 12V battery keeps going flat, even after it has been charged or replaced, the problem may be on the charging side rather than the battery itself. On a hybrid or EV that means high-voltage components, which need proper HV-rated equipment and training to diagnose safely.