"Commander Kinsey" wrote in message
news
On Sat, 22 Jun 2019 01:17:29 +0100, wrote:

On Fri, 21 Jun 2019 23:55:31 +0100, "Commander Kinsey"
wrote:

On Fri, 21 Jun 2019 23:48:01 +0100, 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 There is
nothing they can do about the load so they don't give a **** about the
lights. The regulator just watches alternator current and cranks up
the voltage to keep it up. When current drops off it is either because
there is less load or because the battery is taking less of a charge.
That is when the voltage ramps down.
If current gets really low, like the battery is charged and there is
not much load they hit the bottom of the range ... essentially a
trickle charger that is also running the radio and heater fan.

Take this situation: You start your car with the battery 3/4s full.
The alternator provides a high voltage and charges it quickly. It's
dark and cold, you have demisters, lights, etc on. How does the
alternator know when the battery is full? As current will continue to
be taken from it to power all those loads.

As for a smart charger, usually the voltage is all they need to know.
When the voltage drop across the battery starts to rise. it indicates
the battery is charging and at a certain point they either turn off or
turn to trickle.
Different batteries have different "fully charged" voltage levels so
they usually have a switch for different types . (deep cycle, AGM etc)

I guess charging a car battery with a charger plugged into the house
won't work if you have a load in the car like lights (or in my case a
faulty alarm). The charger will think the battery is still drawing a
fair current and isn't full, when in fact it's the load eating it up.

Plug in chargers are voltage regulated so that load would pull the
voltage down and the charger would try to bring it up. Once the
battery was charged the charger would either see a higher voltage and
ramp down or it would stay there at. what it could drive.

No, if you charge a battery with no load, the charger will sit at 14.4V
(provided it can give enough current to do that) until the battery drew
little current, then it would back off to 13.8V.

But with a load, the charger thinks the battery is still charging, when in
fact it's the load taking that current. So it will stay at 14.4V forever
and **** the battery.

That is
probably a good reason to turn all that crap off when you are charging
a battery off line

I don't have anything deliberately loading it.

and you might want to take the negative lead to the
car loose if you have too much residual load.

Too much hassle. Anyway if I was going to do that, which I used to, I
wouldn't even need a charger as the battery would never go flat.

Maybe you could figure
out what was killing the battery in the first place then.

I know it's the alarm,

No you don't.

as it still does it with every damn fuse pulled out.

Could still be a partial short in the wiring or with a
steaming turd with wheels frog car, something else
that isnt fused. It unlikely that the alarm isnt fused
because a fault in the alarm could set fire to the car.

The alarm is hidden away and difficult
to disconnect, to stop thieves doing so.

You previously proclaimed that all alarms should be permanently disabled.