On Fri, 21 Jun 2019 21:54:38 +0100, Rod Speed wrote:

Commander Kinsey wrote

How does a lead acid battery charger (or car alternator) know when to
switch to trickle charge?

From the current the battery takes.

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?

You just look at the current going to the battery. The variably
loads like with lights isnt supplied by the battery when the
engine is running, its supplied by the alternator.

But how can the regulator on the alternator possibly know the current it's passing to the battery is going into the battery and not going straight across to the lights? If you look at the battery in your car, there are two or three thick wires coming off each terminal. One will go to the alternator, another to the fusebox for all the lights etc. Unless there's some clever circuitry monitoring each battery wire individually and subtracting the currents, the alternator can't tell the difference between a battery taking 12 amps, and a battery taking 2 amps plus lights taking 10 amps. The second one requires switching to trickle charge, the first doesn't.