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On my SL43 AMG, the brake hold function sometimes works and sometimes doesn't. Could the problem be a faulty battery?
When I tested the voltage of the battery while the engine was running, it was around 14.5 volts; however, after using the stop/start button, the voltage dropped to approximately 12.5 volts and has continued to gradually decrease since then. When the automobile has been resumed after a lengthy rest or overnight when the engine is cold and the stop/start function is critical owing to low engine temperature, the hold feature performs more dependably. This is the case when the engine is cold. When the automobile is brought to a halt, it is very difficult to get the hold function to operate after the stop/start mechanism has been activated.
Is it possible that the hold feature was turned off because the battery voltage was too low (about 12.5 volts)?

#inoperative

From what I've seen and experienced, HOLD will always function while Start/Stop is active (whether the light is green or amber). HOLD continues to operate even if the driver deactivates the Stop/Start feature on the vehicle.

The only occasion that my car's HOLD function did not operate was when the Stop/Start feature was not operational owing to a low battery (for example, while I was participating in Covid).

Therefore, if HOLD isn't functioning when Stop/Start is active, I would suggest that there must be a fault somewhere, and the problem is not with the battery voltage. This is what I would answer if you asked me.

BTW 12.5v is certainly low, the battery is plainly beyond its peak, but once again, if the Stop/Start mechanism is operational, then the voltage is also adequate for the HOLD function. This is despite the fact that the battery has clearly passed its peak. I've never seen a situation in which the HOLD button was inaccessible when the Stop/Start feature was active.