On 03/05/2020 00:12, Roger Hayter wrote:
The Natural Philosopher wrote:

On 02/05/2020 21:54, williamwright wrote:
On 02/05/2020 19:05, The Natural Philosopher wrote:
On 02/05/2020 17:31, Harry Bloomfield wrote:
Dave Plowman (News) formulated on Saturday :
So you are guessing it charges a low battery at 140 amps? ;-)

No, there would need to be a rather large difference in voltage
between the alternator and battery, to achieve that and as the
battery attains a charge, its voltage rises quite rapidly.

The alternator will deliver whatever voltage it meeds to but of course
there would not need to be a particularly large voltage to charge a
battery that is capable of delivering over 700 A to starter motors
without crapping out

I would suggest nearer an initial 20amps, which would rapidly fall to
maybe 3 to 6 amps. A flat to full charge time when being driven, can
require 6 to 10 hours of being driven. Which is why it is always best
to put such a battery on a mains charger for 24 hours, if at all
possible.


Total crap. There is no pint in having a 150A capable alternator to do
THAT

You are forgetting the internal resistance of the battery.

I am the one with the degree in that subject, I am not forgetting it.
You simply have no idea of its value.

The fact is
that the most charge a car will give a flat battery is around 30A, it it
drops off very rapidly. The high alternator output is to support
accessories.

Like I said the battery has sufficiently low internal resistance to be
able to deliver 700A to a starter motor, it can sure handle 50A with
only a half a volt or so increase in charge voltage.

Bill
The charging rate must be deliberately limited to avoid damage to the
battery. The acceptable charge rate is much higher when fairly flat
than when nearly fully charged. Probably improved electronics is why
we can have much higher output alternators nowadays without risking
battery damage. The old system of just regulating the voltage could not
reliably avoid overcharging unless the maximum charge rate was
artificially limited.



BINGO.

Imagine what happens when you jump start - seen the size of the spark?

Long term charge rates are obviously low, but just after a hard cold
start the battery will soak up a lot of amps very quickly.

Because if it doesn't and its doing stop-start, the battery will be flat
PDQ .


--
"The most difficult subjects can be explained to the most slow witted
man if he has not formed any idea of them already; but the simplest
thing cannot be made clear to the most intelligent man if he is firmly
persuaded that he knows already, without a shadow of doubt, what is laid
before him."

- Leo Tolstoy