On Sat, 22 Jun 2019 17:03:08 +0100, TMS320 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?

It doesn't.

Mine does, if I start my car when the battery is say 80% full, the voltage will be 14.4V. After a while, something causes that voltage to drop to 13.8, because something knows the battery is full and should no longer be charged at a high rate.

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?

Ohm's law.

Explain how an alternator or charger can use ohm's law to distinguish between:
1) A car battery which is full, with a load of 10 amps connected to it, like two headlights.
2) A car battery with no load, which is not full yet and draws 10 amps for the charge.