On 22/6/19 7:58 pm, Rod Speed wrote:


"Xeno" wrote in message
...
On 22/6/19 9:57 am, Rod Speed wrote:


"Commander Kinsey" wrote in message
news On Fri, 21 Jun 2019 23:57:52 +0100, Rod Speed
wrote:



"Commander Kinsey" wrote in message
news 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 the way batterys work, the battery voltage does change as its
charged.

That's determined by the alternator or charger.

Nope.

Yip.

Nope.

I can put any voltage I like across a battery's terminals.

Nope.

The battery then chooses how much current is drawn.

And that current changes depending on the how charged the battery is.

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.

Its more complicated than that with the current going to the
battery and the
battery is charged.

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.

No it doesn't even with a very crude battery charger.

For example, I'm currently keeping my car's battery topped up with a
bench supply overnight.* It's set to 13.8V, with a current limiter
only to prevent overloading the supply.

It actually specify the current being supplied.

The voltage stays at 13.8V all the time, sometimes 100mA is drawn,
sometimes up to 4A.* The only way I or the supply can tell the
battery is full, is by the current dropping to 100mA.* But it's
actually always full, as when 4A is drawn, that's going to a load.

What load ?* There no load with a battery being charged with a bench
supply.

Correction

Nope.

the *battery* is the *load*.

Not when the battery is fully charges and is being charged
with a bench supply that is delivering 4A to the battery.

Take the case of an alternator charging a battery at ~4 amps. The
battery is the load and it also provides, as part of that function, the
reference *voltage* that the alternator *must have* in order to control
the output.

In the process of being charged it is using electric current. That
makes it the load.

See above.

What happens to the charger when you disconnect the power with the
battery connected? It should, if designed correctly, shut down since it
no longer sees a load. Otherwise it may destroy itself.

Even when it is fully charged it will still take a trickle charge

4A isnt a trickle charge.

That depends entirely on the amp hour rating of the battery.
Also, my bench charger will start off at 4 amps, its maximum capacity.
As the battery becomes charged, that current will drop down to *1 amp*
and, from that point, it will maintain a *trickle charge*.
From Wikipedia;
For lead-acid batteries under no load float charging (such as
in SLI batteries), trickle charging happens naturally at the
end-of-charge, when the lead-acid battery internal resistance
to the charging current increases enough to reduce additional
charging current to a trickle, hence the name. In such cases,
the trickle charging equals the energy expended by the
lead-acid battery splitting the water in the electrolyte into
hydrogen and oxygen gases

The car alternator regulator is no different. It sees the battery as a
load, determines the voltage reference and pumps up its output. When the
regulator sees the battery voltage at the peak setpoint, it too will
drop the current to a trickle. If you add a load, say by turning
headlights on, that is in *parallel* to the battery and it will drop the
system voltage down a tad. The regulator will see that and pump up the
output current appropriately. The current will apportion itself to the
*two* loads as appropriate to their individual internal resistances.

Here, educate yourself;
https://www.swtc.edu/ag_power/electr...l_circuits.htm

so it is still a load even when fully charged.

Not when its still taking 4A,

If the battery is *taking* 4 amps, then it *is definitely the load*.

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?

By checking the current actually being delivered to the battery.

I guess that may be true, if the car's computer has two ammeters

It has more than one wire to the positive terminal of the battery.
So it can see what current is going to the rest of the car.

and subtracts one from the other.** But AFAIK, the alternator
regulator only works by it's own current sensor.* And that current
could be going into the battery, or past it to the loads.

Not when there is more than one wire going to the
positive terminal of the battery, and there always is.




--

Xeno


Nothing astonishes Noddy so much as common sense and plain dealing.
(with apologies to Ralph Waldo Emerson)