Ping Mrcheerful

Under those conditions the alternator will still supply enough current to work the accessories - just not charge the battery. Only experienced it once - I left my car for three weeks at Gatwick Airport and came back to a totally flat battery. Got a jump start and drove home to London. Parked up and unloaded then needed some shopping. Battery still totally flat. Fitted a spare and charged the other one for days - it did recover somewhat, but was never the same again.

Yup, but that's not to protect the alternator, that's just a sulphated battery.

In one sense, they don't! They certainly make the alternator howl.

The Ford system inhibits the HFS unless the engine is actually running, and will also turn it off if system voltage drops low enough to compromise continued running.

To some degree, the hit the battery takes when the HFS is used is compensated for by Ford Smart Charge; this boosts alternator output when the system senses a cold start has occurred (amongst other things).

As an aside, caution needs to be exercised when jump-starting a vehicle with Smart Charge. If the battery is totally flat, and the engine cold, the system can get confused. This can result in alternator output voltage rising to above 18 volts! It is also essential to use the proper calcium battery in these vehicles.

Chris

HOwever the alternator is perfectly capable of coping with that sort of current, which is an order of magnitude more than it'll put into a flat battery.

If by "coping with", you mean that the alternator will be able to balance the load, then I beg to differ.

The alternator on my car is rated at 80A maximum output; the HFS alone takes 60A!

When using both front and rear screens, headlights, heater, and all the loads associated with running the engine, you are running partially on the battery. Under those conditions, you can tell just by how much slower the heater fan is running just by the difference in sound.

Chris

In message , Chris Whelan writes

True! Fortunately the HFS acts very quickly in my Mondeo, but I prefer to use the heater "blast" button, which is even quicker. I don't there is a time-out for HFS and HRW on the Mondeo, unless anyone has noticed different?

Blah formulated on Tuesday :

Yes, I have checked all of those and they are shut down anyway by a battery saver relay which cuts their supply if the car is left undisturbed for 20minutes.

Something has though just occurred to me - If one of those switches is a little dodgy, as in on the point of opening closing, it might trigger the battery saver relay as if the car has been disturbed.

The glove box switch has a known design fault in being known to not quite turn the light off when the lid is shut. If the lid switch happened to be on its point of trigger, then it is quite feasible that it is constantly retriggering the battery saver relay and thus discharging the battery.

I would be amazed it there wasn't, certainly for the HFS.

Chris

I'll check it out when the "steamy windows" season comes back, but it rarely needs to be on for more than a minute or two, then I switch it off.

HFS timeout for the Focus is four minutes.

Chris

Thanks. I expect the Mondeo is probably the same, but I haven't noticed it. ;-)