On Sat, 22 Jun 2019 18:02:30 +0100, Rod Speed wrote:



"trader_4" wrote in message
...
On Saturday, June 22, 2019 at 11:45:19 AM UTC-4, 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.

Not on any car I've owned or worked on. And more to the point, it's
obviously not needed because cars have worked with the same lead-acid
batteries

They arent the same lead acid batterys, most of the current
ones cant be topped up with extra water when needed.

and alternators with basic voltage regulators for most of the last
century.

We havent used alternators for most of the last century.

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.

Even if there is an additional wire, explain to us how you
monitor the current in the charging cable with that wire.

With more than one wire, you can see
what the load from the lights etc is.

And you dont even have to do it that way
with modern computer controlled lights now.

I doubt it knows about everything, like how much power you're using from the cigarette lighter socket etc. Easier to just measure the total load in one go, than trying to add up precise currents it thinks the wipers might be using etc.

And Rod is talking computers,

Because thats what his car has.

But the essence of his question is how cars switch to
trickle charge and AFAIK, the answer is they don't.

The current to the battery does in fact drop
dramatically once the battery is fully charged.

The chemistry of the battery sees to that itself, but it's better for the battery to lower the voltage from 14.4 to 13.8 when it's full. It's not as easy as a lithium cell where you just give it 4.2V.