Car Battery help needed.....please

Even more so if the car is used for short journeys in the winter. You're likely to need maximum load just after starting. If the alternator can't cope with that load *and* make good the starting loss from the battery, it very soon won't start at all. And this is (sadly) exactly the use my car gets at one place I work - only about 3 miles away.

The message from "Mike G" contains these words:

The heated front screen on my car's fused at 100A.

So just what is the output of an alternator on a Ford fitted with a heated front screen? My BMW doesn't have a heated front screen but has a 120 amp alternator.

Here's the breakdown for my SD1 with 75 amp one:-

Main beam 220 watts Side lights 50 Heated rear window 240 Heater fan on max 60 Engine management 120 Wipers 60 Rear fogs 42 Radio (guess) 20 === 812 watts or 67 amps.

How come the wiring doesn't melt then?

Are you sure? Rear ones sure as hell aren't.

No. I consider it as a way to make sure excessive use of accessories with the vehicle engine stopped doesn't result in the vehicle not starting. If it doesn't include this in the intelligence it's thick. ;-) Other reasons for using multiple batteries? Stock sizes are cheaper pro rata and the weight can be distributed more evenly.

Big deal. This runs all the time?

I'll bet they arrange things so it can't be used with the engine stopped. And the fuse rating doesn't give the actual running consumption - heating elements change resistance with their temperature.

Probably because it is sized to match the load? ;-)

The HFS elements are in two circuits. From memory, each circuit is wired in something like 6mm2!

When the HFS is switched on, you can hear the alternator whine as it is on maximum output.

Chris

Yes, but what you consider them as doesn't neccesarily relate to what they were designed for, does it?

I've got a lot of technical literature on CD for the Touareg; I wish I had somewhere to post it for you to see. It's quite an eye-opener.

Remember that your 75A nominal alternator won't get near it's full rating at or near idle, so in this example, the battery would not be receiving any charge at low speeds; in fact it would be helping to support some of the load.

The petrol Focus uses an 80A maximum output alternator. Assuming similar loadings to your SD1, you can add to that something like 500 watts for the HFS. Add to your list things like heated mirrors, heated washer jets, aircon clutch, rear wiper... The list for modern cars is very long!

Like I said earlier, you will find that the alternator output will not balance the full load, or even get close to it.

Chris

Yes, but what you consider them as doesn't neccesarily relate to what they were designed for, does it?

I've got a lot of technical literature on CD for the Touareg; I wish I had somewhere to post it for you to see. It's quite an eye-opener.

It still takes energy from the battery theat must be replaced.

Chris

And it's hardly beyond the wit of man to keep the service items at 12V.

They don't melt ice on the outside quickly either.

The message from Conor contains these words:

Have you seen how thick the wires are?

The message from Conor contains these words:

Front one is.

The message from "Dave Plowman (News)" contains these words:

Certainly do!

The message from "Duncan Wood" contains these words:

Not as fast as on the adverts, that's for sure! About a minute and a half to shift frost on mine.

I have an accurate voltmeter sitting across the battery in the SD1 and it does balance the load after a cold start - the EFI system on that gives a fast idle when cold. If you switch the AC on it also boosts the idle speed. Switching on the AC activates the twin supplementary cooling fans - and also disables the heated rear screen.

Heated mirrors and washer jets take next to nothing. I must admit to omitting the rear wiper on the SD1 - but it has no continuous setting so again takes very little.

I'd be most unhappy if I had to say jump start a car then find it cut out when switching on the heated screen - unless there's some over ride to prevent this. 500 watts drain over the output of the alternator sounds like *very* bad news to me. Probably why I don't have a Ford. ;-)

To paraphrase, what you actually said, was that even compared to 'peak output' "the alternator output will *not* balance the full load, or even get close to it" I think we'd all accept that at idling,and in slow moving traffic the alternator probably wouldn't keep up with the current being used.At peak output I still believe most, if not all, would. Mike.