On 06/03/2017 08:57, Martin Brown wrote:

Several miles in winter when headlights tend to get used more may not be
enough to restore the energy taken from the battery to start the car.

Assuming there are no poor connections the charge rate won't vary much
with the load from lights etc. The alternator maintains the same voltage
irrespective of load. The alternator is more than capable of powering
the lights, heater motor, etc, and also charging the battery.

I drive a vehicle with digital ammeters and I observe this fact
frequently. I turn the headlights on, the voltage and charge rate dip
down momentarily, then they resume their previous position.

The idea that you can't charge the battery if you have the lights and
the heater blower on is a hangover from the days of dynamos. They had a
max output of, typically, 22A, and SFA on tick-over. A modern alternator
will produce 20A at tick-over and over 80A when the engine is running
normally. The alternator in my 20 year old motorhome will power a 24V
45A microwave and still charge the battery at 20A, power the headlights,
etc. (Yes, we use the micro when we're moving)

Bill