On Sun, 23 Jun 2019 07:33:12 +0100, Xeno wrote:

On 23/6/19 7:22 am, Commander Kinsey wrote:
On Sat, 22 Jun 2019 16:45:08 +0100, Rod Speed
wrote:



"trader_4" wrote in message
...
On Friday, June 21, 2019 at 6:48:31 PM UTC-4, wrote:
On Fri, 21 Jun 2019 23:15:21 +0100, "Commander Kinsey"
wrote:

On Fri, 21 Jun 2019 22:57:44 +0100, Max Demian

wrote:

On 21/06/2019 21:19, Commander Kinsey wrote:
How does a lead acid battery charger (or car alternator) know
when to
switch to trickle charge? I can understand it noticing a drop in
charging current if the battery is on its own, but what if a random
changing load is connected, as there is in a running car?

The voltage perhaps.

Why would the voltage change? That's determined by the alternator or
charger. Let's say the charger/alternator gives out 14.4V
initially, to
charge the battery quickly. It'll just sit at 14.4V forever,
providing
the charger can give out enough current to charge the slightly flat
battery and power any connected loads. If the battery had no loads
connected, it would take a lot less current when it became full,
but the
voltage would stay the same. If the charger monitored the current it
was providing, how does it know if the battery is still charging at 10
amps, or if the battery is full and there's a 10 amp load?

Not really true with anything but the most primative regulator like
you might see on an old outboard. Voltage is regulated somewhere
between 13.x and 14.x, not just reflecting what the alternator can do
against the load.
Rod is right, they look at current from the alternator

Every car I've seen, the the alternator, the battery
and the rest of the car are tied to one point

But there is normally more than the one wire
at the the positive terminal of the battery.

and there is no monitor for what current
is going to the battery vs to the car load.

Wrong when there is normally more than the one
wire at the the positive terminal of the battery.

And Rod is talking computers,

Because thats what his car has.

so how did cars work prior to the 80s?

The regulator uses the voltage it sees which varys
with the load and the charge of the battery.

That was my original question - on an old car, there is no way for the
regulator to tell the difference between the battery still charging at
10 amps and the headlamps being switched on.

The regulator doesn't care. It's the voltage that matters. Whatever
*total* load there is, the voltage will change accordingly and the
current output will adjust. Ohms law will ensure an appropriate current
flow through the battery and the lights.

Fine if you want to always charge the battery at a certain voltage. But a decent charger will charge at 14.4V then drop to 13.8V float. This requires it to know the battery charging current.

They didn't have a computer didn't monitor anything
other than the voltage regulator maintained a constant
voltage of ~14v while the car was running.

Its more complicated than that with the voltage.

It was that way from the early days and auto batteries charged fine..

Generators do it differently to alternators and we arent discussing
whether they charged fine or not, we are discussing what the
regulator does when the battery is fully charged so that it doesnt
boil off the water in the battery. Thats particularly important
now that most car batterys arent refillable with water anymore.

Modern batteries are more resilient actually.

Actually less unless you want to pay for new technology like EFB or AGM.

I remember old batteries that would die if you ran them totally flat twice. Nowadays that's more like 50 times.