On Sat, 22 Jun 2019 00:57:57 +0100, 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.

I can if the charger / power supply is powerful enough.

The battery then chooses how much current is drawn.

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

Correct, it's the current it checks.

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.

No, it LIMITS the current. I set 13.8V, 2.5A. It will back off if either is exceeded.

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.

The faulty frog car with the dodgy alarm.