Re: Car Battery help needed.....please

Dave Plowman (News) wrote:

In article <yqv7i.2984$sM1.232@xxxxxxxxxxxxxxxxxxxx>,
Chris Whelan <cawhelan@xxxxxxxxxxxxxxxxxxxxx> wrote:
Heated mirrors and washer jets take next to nothing.

I seem to recall something like 3A for the mirrors, and 2A for the washer
jets; a total of 10A. Not quite insignificant, and could make the
difference between charging your battery and discharging it.

Having recently been involved in trying to find a source of mirror heating
pad replacements, a three amp element would fry a mirror glass in short
order unless thermostatically controlled.

OK. Sad person that I am, I've just been out to the car with my Fluke.The
current for both mirrors was just over 3A, the value for both heated jets
was just under 4A. The total of 7A is not "next to nothing", and could make
the difference between the battery charging, or discharging.

However it makes little difference as a car electrics designer should
allow for likely loads when calculating alternator size.


They do. They allow for likely loads averaged over a reasonable period of
time. What they don't do is allow for all possible loads to be balanced by
the alternator at all times.

What do you think would happen to the National Grid if by some freak of
coincidence every household turned on every electrical device in their
house at the same time?

The designers use exactly the same principle - it's called diversity.

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. ;-)

Surely if you had any technical expertise, the last thing you would do
after having to jump-start a car with flat battery is to switch on heavy
non-essential loads!

But most people *don't* have technical expertise and many will call out
the emergency services for a jump start. FFS, most don't know what watts
are and how to convert to amps. I think you'd be surprised just how many
12 volt to 230 volt invertors are returned burnt out because people plug
an electric kettle etc into a 300 watt one. You'd think that impossible
since all domestic appliances are marked with their power consumption in
watts, but it happens all the time.

Indeed. But you said "*I'd* be most unhappy..."!

WRT the invertors, surely they should be designed to limit maximum load ;-)

I did explain earlier that the HFS supply is ECU controlled, and cuts out
the screen if the system voltage drops too low. Perhaps you missed that?

It makes sense if the alternator output can't cope.

The Ford HFS system is not new, and is well-used. Any difficulties would
have surfaced by now. Oh, and it's one of the reasons why I *would* have
a Ford. It's also popularly believed to be the main reason Ford
purchased Jaguar, as they held the patent on it.

I still stand by the two main points that I have stated; many modern
cars do not have sufficient alternator output to balance the total
possible load, and batteries of the sixties would not last long in a
modern car.

I'm quite willing to accept it applies to a heated front screen - and that
it is designed to prevent being used under certain circumstances. But that
this is done purely to save money. As regards everything else it is a poor
car that can't balance the load of the accessories likely to be used, and
relies in any way on the battery to do so, apart from extremely short
periods of time.

So you are now agreeing that for some periods of time, the electrical load
is going to be higher than the alternator can provide? Hurrah!

*The* important spec of a battery is the cold cranking current. And that
requirement hasn't changed since electric starters were invented.

Yes. I mentioned CCA way back in this thread.

The requirement of an individual vehicle type would be hard to quantify.
What would you compare say, a Focus to? A Ford Anglia? A Mark one Escort?

Modern vehicles generally run higher compression ratios, and their
multi-valve designs would need more power to crank them. OTOH, improvements
to oil technology would tent to mitigate this.

What has changed is a modern battery's ability to meet that CCA requirement.
For a given physical size, and Ah capacity, a modern battery will be able
to provide a much higher CCA value.

Chris

--
Remove prejudice to reply.
.

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