[Jachu Z4]

Reviving an old thread...
You probably saved my night tonight, and I'll sleep soundly.
I had a new, low-capacity lead-acid battery (61Ah), but it was incorrectly coded, and the alternator wouldn't charge it. When I checked it this morning, it showed 11.8V. That's very low for a three-day-old battery. I checked the charge with a multimeter, and it showed 15.4V. As it turns out, the coded battery was a 70Ah AGM. Today I bought a 70Ah AGM, registered it as new, and INPA showed a charge of 11.8V. I was scared and angry. But I found this thread. There's an attachment in post #35 that explains everything.
1. When the battery is charged (mine was 13.4V when I installed it), the alternator doesn't charge (11.8V).
2. When the car accelerates hard, the alternator doesn't charge (unless it has to if the battery is weak).
3. When the car is idling, the alternator doesn't charge (unless it has to if the battery is weak).
4. The alternator charges at full power, just like a vehicle:
- when moving when you take your foot off the gas pedal above 10 km/h,
- when the vehicle brakes.

It charges at full power only when it doesn't need to use fuel. This is to avoid overloading the engine and avoid generating more exhaust gas.

The attachment provided by Helmsman provides a detailed description, explanation, and even a diagram of how the intelligent charging system works. It also explains why an AGM battery is so important in cars with regenerative braking.

Thank you for this thread, for your described cases, and for this attachment. Otherwise, I'd probably be replacing the alternator tomorrow.

Best regards, Jachu.