wrote:

On Sunday, May 3, 2020 at 9:12:35 AM UTC+10, 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.



--

Roger Hayter

Quite agree with the charging rate limitation. If one observes the meter
on a battery charger, the starting currents through the charger would be
around 4 amps in the worst case, coming down slowly to 0.5 amps in about
12/16 hours; if this is any guidance the alternator charging load would
be about the same, not of the order multiples of ten amps!!

Unfortunately, it isn't any guidance at all. Battery chargers are
limited in current by price, and thus the size of critical components.
They take a day to significantly charge a flat battery, while the
alternator has to recharge, say, a quarter of the capacity in a matter
of tens of minutes. I looked up maximum charge rate for lead acid wet
batteries and apparently 0.3C (a very few tens of amps for a car
battery) is recommended routinely but up to 1.5C (over 50A) can be used
for short periods with a partially discharged battery. (C of course is
a current in amps equal to the amp hour capacity of the battery to a
first approximation.)

--

Roger Hayter