On Fri, 21 Jun 2019 22:38:51 +0100, trader_4 wrote:

On Friday, June 21, 2019 at 5:14:50 PM UTC-4, Commander Kinsey wrote:
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.

See my other post. I agree with you, that's how every car I've worked on
has been wired. AFAIK, the voltage regulator just keeps ~13.5 to 14v
on the system, which has the alternator, battery and car loads all tied
together.

That doesn't explain why I observed my own car switching from 14.4 to 13.8, after someone told me they do that.

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.

There is no trickle charge. Apparently car batteries are fine at ~14V
while the car is running, they don't overcharge and nothing bad happens.

I would agree. I probably broke my battery by having it at about 14.5V 24/7 for months. I had incorrectly assumed all car battery chargers (although this one was very old) would not overcharge. AFAIK that charger was just a transformer and diodes, made to give out about the right voltage, probably made just for a single overnight charge. It's strange as the battery was only taking about half an amp when full, although I guess that's 7 watts of heat being constantly dissipated inside it.