On Fri, 21 Jun 2019 22:18:56 +0100, trader_4 wrote:

On Friday, June 21, 2019 at 4:54:51 PM UTC-4, 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.

Except that the alternator doesn't know how much current is going
into the battery and how much is being used to power the car.
At least not in any car I've had. The alternator is tied directly
to the battery and that common point supplies the car. The alternator
can't switch to trickle charge either. Every one I've seen, with
the car running normally, the voltage at the alternator/battery is
about 13.5 - 14V

I used to think the same, until someone in one of these newsgroups (on another topic about 6 months ago) said it drops the voltage or it would wear out the battery on long journeys. I tested my own car, by leaving the lights on for a bit, then starting it. The voltage was about 14.4. It dropped to 13.8 after the battery was filled up. The regulator must have detected the battery was full somehow and lowered the charging voltage.

Mind you after some googling, apparently a lead acid is happy being charged from 13.8 to 14.5 continuously. Although when I used to leave my car on a charger (an old Bradex car battery charger) at 14.5, it ****ed the battery after a few months. Maybe 14.5 is only ok in a car alternator circuit, which isn't usually running 24 hours a day. I think I'll always make sure it's 13.8 if it's on charge all the time - my car tends to randomly lose battery power overnight (to the alarm I believe). I was just wondering if I bought an intelligent car battery charger, whether it would ever switch down to trickle if the alarm was sucking juice, as such a charger may not expect any load. I currently have it connected to a bench supply at 13.8.