On Sun, 23 Jun 2019 07:46:39 +0100, Xeno wrote:

On 23/6/19 7:26 am, Commander Kinsey wrote:
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.

What do you think is happening when you have a fully charged battery but
have the AC on, the headlights on, the driving lights on,

The what lights?

the seat warmer on, etc, etc.
The *internal resistance* of the battery will be high when it is fully
charged

Really? How does it start your car then?

so most current will flow to the *other loads*.

As far as the alternator is concerned, a lot of current is going out of the alternator. It doesn't know it's to loads, it could be trying to charge a very flat battery. The first requires 13.8V, the second 14.4V.

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.

Depends on the car. Some cars have a lot of parasitic loading, some have
minimal parasitic loading, very few current cars have no parasitic loading.

Mine is absurd, it can make the battery go from full to unable to start (and that's a new battery) overnight. There's clearly a fault, presumably with the alarm, as it will do it with all the fuses taken out.

Batteries lose charge even if nothing is connected to them. It's called
*self-discharge* and you can educate yourself here;
https://en.wikipedia.org/wiki/Self-discharge

Irrelevant, I use my car every 3 days at the least

It's why people put their batteries on a trickle or float charger when
in storage and not being used.

More education here;
https://en.wikipedia.org/wiki/Float_voltage

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

I know it's the alarm, as it still does it with every damn fuse pulled
out. The alarm is hidden away and difficult to disconnect, to stop
thieves doing so.