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GM7HUD > TECHNI 14.08.04 16:48l 110 Lines 6235 Bytes #999 (0) @ WW
BID : 821527GM7HUD
Read: GUEST
Subj: Re: Car Battery Confusion.
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<DK0WUE<HA3PG<IK1ZNW<VE3FJB<W4JAX<WB0TAX<
GB7ESX
Sent: 040814/1243z 74149@GB7ESX.#31.GBR.EU $:821527GM7HUD [Witham, Esx]NNA V3.1
Dick, VK3ABK wrote:-
"A car alternator is designed to run the entire car electrical system, and
to do this hours on end. The rated output, typically 40-45 Amp, is more
than adequate to run the car electrics, and the battery is only used for
cranking and running lights etc when stationary. Once the car is running,
the battery only serves as a circuit control device."
Then Jeff G4XNH wrote:
> From past experience, I must disagree with those comments. Years ago, I
> once had to drive a car from A to B and had to remove the battery. I
> started the car and kept it running whilst I removed the battery. It ran
> fine for a mile or so and then I started experimenting. First I turned the
> side lights on and it continued to run with no difference. Then I switched
> on the dipped beam and the engine stopped. It would NOT supply the current
> necessary to provide enough power to light the dimmed lights. I tried this
> several times with the same result. Run main beams and no chance of the
> engine continuing to run.
Well, number 1, you're going to break the charging system if you run it
without a battery attached. And number 2, Dick is correct. The alternator
has to be big enough to power all the items in the car or you'd not get
very far. If it wasnt able to, then where does the power come from to power
the lights when you drive? From the battery? Well yes but where does the
charge come from to replace the charge being used from the battery? If the
alterator couldn't power all the loads and charge the battery then
eventually you'd run out of charge and the car would stop. If the
alternator can't power all the loads what does?
You have to consider the electrical system of a car as a whole. The
alternator output is a function of rotor speed. Hence full generation
capability occurs at moderately high revs. At such speeds the alternator is
sized to be able to drive the full load of the car. Moreover the output of
an alternator is much greater at low revs than a similarly rated dynamo.
However, extended running of all items and low revs will require the
missing current to be supplied by the battery. In normal cicumstances, the
current used to power the lights comes straight from the alternator unless
the car is effectively stationary. In which case the battey fills in. When
the revs rise as you move off, the alternator output will rise rapidly
enabling the used charge in the battery to be replaced and to power the
lights.
The charging system has to be able to power all the items fitted. It might
not be able to do that and charge the battery unless you are hammering
along the motorway but the occasions when everything is on is remote.
There's normally a bit of capacity left to replace the missing charge. Only
if you do lots of engine starting, short trips and have heated windows,
chairs, fans, lights, wipers, on all the time will the battery never get
fully recharged after starting the car. A well maintained car will be able
to replace the energy used to start the car with normal driving in about 5
miles or so. Excessive cranking requires further to be travelled. If you do
such driving in a UK winter then you'll probably benefit from a few hours
with a battery charger.
If you remove the battery from the circuit, then you rely entirely on the
alternator. However, the output from the alternator is only fed into the
car electrical circuit when the battery terminal voltage drops below the
level set by the charge regulator circuits. With a battery, when the
voltage gets high enough, the alternator is disconnected and the battery
provides all the power. Without a battery, the voltage will rise to the
output voltage of the alternator which is higher than the charge voltage
and so the alternator is disconnected. Now there is no battery to fill and
so the voltage immediately drops below the charge cutoff point and the
alternator is reconnected. This results in the voltage rising and being
cutoff. The voltage is thus a fast chopped DC supply. This doesnt do the
regulator any good. It will see off the contacts in a mechanical regulator
quite quickly and most electronic ones expire almost immediately. With an
old Kettering ignition systems, this will be OK to run the engine but is
highly unlike to be tolerated by a modern computer controlled engine
management system with fuel injection.
If we continue with our batteryless car running on idle, then we have a
rough DC voltage available. However, we know the current available is a
function of revs so if the load is increased by switching on additional
demand such as lamps, then eventually you will demand current than
available, the voltage when the charge circuit connects the alternator will
be less than required to generate a spark and the engine will stall.
Running the engine at fixed high revs may allow you to run more loads but
if you try to drive along in such conditions, when you change gear etc, the
revs will fall below such a point where the rough voltage is insufficient
to generate a spark. The engine normally stops (or splutters) at this
point.
There are ways of increasing the output of the charge system. One is to fit
a bigger alternator. The other is to fit a bigger pulley so the alternator
turns faster at slower engine revs. Both these changes are done by today's
youth so that they can have sufficient power at crawling speeds to power
their megawatt sound systems. There again, a few drivers know that they can
only run their systems for a few minutes at crushing volumes before their
car will stop. The skill being able to annoy sufficient people before they
need to turn it down to allow the battery to recover :-)
Back to the original point of disconnecting the battery before applying a
charger. If the charge voltage cannot rise above 14.4V for nominal 12V
systems, then it is safe to attach the charger when the battery is still
connected. So I use the 1976 vintage transformer/metal rectifier charger
(producing 16V+ at 100Hz to charge batteries not connected to cars and a
13.8V 10A regulated PSU (classic 723 + BFY51 + 2*2n3055) to charge
batteries in the car.
73 de Andy GM7HUD
The operation of car charging systems is officially recognised as not being
rocket science.
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