On Sat, 22 Jun 2019 11:41:15 +0100, Xeno wrote:

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.

I believe the reason Rod mentioned load as being seperate to the battery is this thread is about my car, with a fault that draws current from the battery, and it's also connected to a bench supply. In this situation, we refer to "load" as the faulty alarm system. I originally asked how an alternator could distinguish between the battery charging, and a load such as your headlamps being on.

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.

Bull****. You're telling me that a bench supply with a battery on the output and no 240V input will blow up if it doesn't shut down? Wrong. Absolutely wrong. The output end of the supply (which probably ends with smoothing capacitors) is just kept at the normal output voltage by the battery.

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*.

How ****ing big is that battery?! When I charge a 60Ah lead acid at 13.8V, it drops to about 150mA when full.

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

Trouble is you might want to do a bulk charge first, at 14.4V.

The car alternator regulator is no different. It sees the battery as a
load, determines the voltage reference and pumps up its output.

All it can do is supply a certain voltage. 13.8V is safe. If it uses 14.4V or higher, it has to know when to back off to 13.8. It cannot do that if there is an external current draw like headlamp, as it won't know if they're switched on, or if the battery is drawing that current.

When the
regulator sees the battery voltage at the peak setpoint,

You can't tell a battery is full by voltage. You can only tell by it drawing less current. The voltage is determined by the charger.

it too will
drop the current to a trickle.

Actually it drops the voltage, to 13.8. I used to have a solar battery regulator which had very detailed instructions saying how it worked. It took the voltage of the solar cells and altered it up or down a bit to suit the battery condition.

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.

But it can't tell the difference between current going to the lights and to the battery.