I have an E39 525iA 2001 saloon.
I had a discharged battery problem in April this year. I checked for a drain and after the system entered the sleep mode, I measured 20mA. The initial reading of the alternator output at the battery connections was 14.15v at idle. I therefore concluded the battery was at fault as it was 8 years old and after recharging on a number of occasions was again empty after a short time.
My local UK BMW dealer provided me with an OEM replacement (Varta) and I fitted it in April.
I was surprised to see the battery indicator being black after a daylight drive, from the UK to Germany, with only the air conditioning being used for over a 350 mile journey.
I have not had any starting or electrical problems whatsoever but visited my local BMW dealer in Germany regarding another problem. They informed me that the alternator was fine. They used a Bosch tester on the battery and confusingly stated that the battery had a 102% starting capability but that the battery required urgent recharging. The technician I have always found to be very competent told me not to worry about it as the starting capacity was the crucial specification to consider.
I further checked the alternator and again at idle produced 14.1v but fell to 13.5 when I activated the headlights and heater blower. I was under the impression that the constant voltage regulator fitted to the E39 was to maintian a constant 14v charge in all conditions or would it to be expected that this charge rate would fall at idle to 13.5v?
I'm curious as to the explanation and any advice or comments would be appreciated as if there is a fault with the battery I will obtain a replacement from the UK outlet?