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.

Lithium batteries are a whole other thing and have smarter chargers
matched to the battery. Mix them up and your pants might really be on
fire.

Lithium are actually very easy to charge. For a single cell, just provide 4.2V, with the current limited to what it could handle (or what the charger can handle). A lithium cell will take virtually zero current when full if you give it 4.2V. It in fact will stay at 4.2V when you disconnect it from the charger. I have a charger here for 18650 torch cells - I think it's just a 4.2V 1A current limited voltage regulated supply. Up to an amp, up to 4.2V.