On 22/6/19 7:54 am, Commander Kinsey wrote:
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.

13.8 - 14.2 is the normal charge range. They will start out high when
charging and drop back to the lower range when the battery reaches full
charge. 14.4 is a tad on the high side and might gas the batteries up a
little. BTW, trickle charge relates to the charging *current*, not the
voltage. A 1 amp trickle charge will keep a battery topped up against
natural charge loss when unused.

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

Yes, they do that but it isn't *switching* per se. They drop to the
lower voltage as the battery becomes charged. Turning a load on, like
the headlights, will cause a more sudden increase in voltage since you
have increased the current requirements. The battery is a *load* on the
system and so is anything you turn on.

*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.

13.8 is the normal maximum charge you want to be putting into that
battery. anything over runs a risk of gassing, etc. ie. overcharging.

--

Xeno


Nothing astonishes Noddy so much as common sense and plain dealing.
(with apologies to Ralph Waldo Emerson)