Dave's example above shows that the alternator output only has 8A "spare" capacity. He has left off his list a number of things that many more modern cars have. In particular, a heated front screen takes inthe region of 40A!
Do you still believe a 75A alternator could balance a load of perhaps 100A? If so, would you be my accountant please?
Chris
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.
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!
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?
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.
Chris
Dave Plowman (News) ( snipped-for-privacy@davenoise.co.uk) gurgled happily, sounding much like they were saying :
Does it need to?
Probably not, but I expect cars with more electrical accessories like heated seats and windscreens, would have higher output alternators. The 120amp one fitted to Daves 528i BMW for example, which BTW does not have a heated screen would apparently cope even if it did have one. Mike.
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.
However it makes little difference as a car electrics designer should allow for likely loads when calculating alternator size.
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.It makes sense if the alternator output can't cope.
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.
*The* important spec of a battery is the cold cranking current. And that requirement hasn't changed since electric starters were invented.
When calculating any load you apply a diversity factor. Try totalling up the possible *maximum* load in your house - including all those heaters tucked away just in case - and you'll find it exceeds the rating of the main fuse several times over.
Nope. All petrol Focuses have 80A alternators for example.
The point I was trying to make to Dave was that car batteries have evolved from those used in the sixties. I stated that a battery from that era would not last long on a modern car because of the higher electrical demands. Dave stated that:
"The only real load on a battery is the starting one. And that hasn't changed. The load from the extra electrics is taken care of by the alternator."
I'm afraid that for a modern car, that statement is simply untrue, and I'm reasonably happy that I've shown that.
They all cope for nearly all of the time by virtue of the fact that cars are not usually driven for very short distances at night in the winter, with most of the electrical load on. Those that are *will* eventually run the battery down. (Ask my stepson - he's an AA patrol :-)).
The vast majority of cars spend at least some of the time running without maximum electrical load. This counters the periods when they are running with heavy loads that the alternator can't balance.
Chris
Dave Plowman (News) ( snipped-for-privacy@davenoise.co.uk) gurgled happily, sounding much like they were saying :
Just like the alternator in a car, then.
Did anybody claim otherwise?
It wasn't even true for most cars then, 35Amp Alternators where still standard fit in the late 70s and that's not enough to run the rear demist, all the lights and the interior fan. Fortunately the radio was an option.
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.
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.
Indeed. But you said "*I'd* be most unhappy..."!
WRT the invertors, surely they should be designed to limit maximum load ;-)
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!
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
The message from Chris Whelan contains these words:
And that's what the battery is for! To tide the system over during periods of excess demand.
Indeed. I've replaced many a broken diode pack in a Lucas 35ACR...
Even worse, dynamo equipped cars had a theoretical maximum output of 17A! Even with the mechanical regulator carefully tweaked, I was never able to get the maximum output.
Chris
My first car 40 years ago had a 70 amp/hr battery. My current car has a 70 amp/hour battery. Please explain how they are different as regards this discussion?
Both my cars are perfectly capable of doing this. I doubt an AA man investigates the condition of the battery - they can and do fail which can prevent a car starting regardless of how it has been used. Of course it's possible Ford penny pinch of the alternator size. I've never owned a modern one.
Indeed. But if that 'excess' demand is 500 watts over more than a very short time you'll end up with a flat battery. And if the alternator is sized *just* to cope at full output with the 'normal' load it will also be slow to recharge a low battery - if that 'normal' load is in use.
Indeed. I had exactly that problem with a Rover P6 3500 and ended up changing the alternator for a higher output one. Strangely from an earlier model which although identical in electrical load had a larger alternator. BL penny pinching at work.
Radios in those days drew such a small a current as to be insignificant, really. Unless you go back to valve types. Before alternators.
Your doubts are ill-founded.
Patrols are not allowed to just swap out a battery without carrying out an extensive test of both battery and charging system. The vehicles carry specialist equipment for just this task.
I've owned nothing but Ford since 1983. I've had one four-year old battery fail; a jump start was needed. Other batteries have been replaced on a precautionary basis as some of the vehicles were quite old when purchased. I have never had a battery go flat. It's reasonable to assume that the alternators of all those vehicles were appropriately sized.
Chris
I already did!
"The way modern batteries are constructed, with complex shapes for the plates, more reliable intercell connections and many more factors make batteries far more durable than in the past. I stand by my statement absolutely; batteries made to sixties standards would simply not give good service in a modern car.
If you want to get more detail, the Varta site is quite informative."
Chris
I've yet to find a maker's site where they don't extol the virtues of their products.
My experience says batteries don't last any longer. If they did, they would be offered with a longer warranty. 'Lifetime' warranties for batteries were around in the '70s. When i last bought a battery I was offered the same one with two prices - depending on the warranty I chose.
Contrast that to cars - with some offering a 7 year warranty.
OK. Information on plate design, silver calcium technology et al is out there. There's no point in me finding the links for you because you simply don't want to accept that the statements you made were incorrect.
Your experience totally contradicts mine. I've just replaced the Focus battery at eight years old. It was still able to start the car. I would have expected to have used at least two on a 60's or 70's car.
Were they? I never saw one!
Car batteries in the sixties were offered with a one-year warranty if a cheap one, or two years if you paid a bit more. The minimum now is usually three years, with four or more for premium ones.
Why are they able to offer a longer warranty? Because they are better made - just like the batteries!
Chris
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