The alternator can charge the battery at low RPMs, but it does so very slowly. If you're running accessories like heated seats, screens, blowers, stereo, or headlights, the power from the alternator is first prioritised to keep these systems running. This means that only the leftover power, which could be minimal at low RPMs, is available to charge the battery. At idle, you might even be drawing power from the battery rather than charging it, slowly depleting it over time. Remeber, turning over a V8 requires a lot of power, and that needs to be put back!
However, this doesn’t mean you need to drive like a maniac to ensure the battery gets charged (though it’s more fun if you do!). As I’ve mentioned before, the output of an alternator isn’t linear — a modest increase in RPMs results in a significant jump in output current.
For example, an alternator output curve is shown below. It may not be identical to the one used in the F-Type, but it's similar in terms of rating and behaviour. Note that the speed is for the alternator, not the engine.
If we assume the car’s pulley ratio is approximately 2:1, this means that for every turn of the engine, the alternator spins twice. At an idle speed of around 850 RPM, the alternator would be turning at about 1700 RPM, which is less than a third of its rated power. However, as RPMs increase, the output from the alternator rises quickly. Full current may be reached by 2500 - 3000 RPM, but that's not an engine speed the car typically cruises at.
The point of all this is to know that on a drive your battery may not charge as quickly or as fully as you might think and that depending on your car's usage, a battery charger might be a good investment
