BMW X5 E53 Battery Drain and Parasitic Current
Why does the X5 E53 battery drain overnight?
The cause is usually parasitic current: some system draws power even though the car is switched off. Causes have included a fan that kept running after shutdown or a door wiring harness connector oxidised by moisture. An old battery can also simply be worn out.
How much should the resting current be?
In sleep mode the current should be below 40 mA. For example 60 mA drains the battery quickly. Measure the current once the car has gone to sleep, and find the consumer by removing fuses one at a time. A single fuse, such as the interior lights, does not always solve it.
The car won't start, the battery is flat and the charging light is on - what is the fault?
In that case the alternator is not charging. The alternator diode may have burned out, and the alternator must be replaced. The 4.4i alternator is liquid-cooled, so it is worth replacing the alternator seal at the same time.
What should be noted after replacing the battery?
The original battery is 90 Ah, and a smaller one is not recommended: it labours on starting and strains the alternator. Warning lights, such as 4x4, usually go out after a short drive. If they do not go out, check the faults with a diagnostic tool.