The car has an emergency low-battery mode, when demanding more power than it can sustain, eg. driving uphill at speed pulling a heavy caravan and the battery approaching its absolute minimum state of charge. First it will give a "reduced power" warning on the dashboard and nothing else. If you back up on the accelerator a bit, it will disappear again, otherwise the car will reduce power to divert charge to the battery. If that is not sufficient, it will go into "turtle" mode, display a turtle icon and divert maximum charging power to the battery leaving only a minimal amount to propel the car at low speed. As soon as the unsustainable situation is resolved i.e. you are at the top of the hill, it will charge up the battery to its normal operating minimum level and resume normal operation. Obviously it can happen in any heavy-load situation, but it is a rare occurence. I only saw the "reduced power" warning once in 140.000 km, when driving up a long motorway incline at 160kph in a fully packed car with a skibox on top into a strong headwind.
It is one of the reasons that I run on "charge" at higher motorway speeds to keep the battery topped up as much as possible.
