Lead acid battery charger (or alternator) switching to trickle with load present?

How does a lead acid battery charger (or car alternator) know when to switch to trickle charge? 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?
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From the current the battery takes.

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

is

is

s passing to the battery is going into the battery and not going straight a cross 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 alterna tor, 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.
Unless there's some clever circuitry monitoring each battery wire individ ually and subtracting the currents, the alternator can't tell the differenc e 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.
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That doesn't explain why I observed my own car switching from 14.4 to 13.8, after someone told me they do that.

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

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.

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.

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.
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wrote

The computer knows whats going to the battery and you can see that with an ODB2 dongle.

And it's the voltage across the one going from the alternator to the battery that allows the computer to know how much current is going to the battery.

Yes there is, its called the computer.

But the computer can. And knows if the lights are on too.

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My dongle only lists faults.

Bullshit. How could it possibly know if the current flows into the battery or goes to the other wire leading to the fusebox?

So what happened with older cars before they did that?

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wrote

Because it's a steaming turd with wheels frog car.
Most show all sorts of things.

We'll see...

By measuring the voltage drop across those cables, stupid.

The voltage across the battery changes as the battery is charged.

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Basically all cars before say 2010, the alternator VOLTAGE is regulated to 13.4 to 14.5 volts and the battery draws as much current as it need to, to charge. When the battery is fully charged it draws only a small current. The exact voltage the alternator is regulated to actually changes a little with temperature, up a little in cold down in hot. That battery university web site has a lot of good detail.
BUT,
I did hear that some newer cars have some weird mode where the computer sets the alternator to a lower voltage as a fuel saving measure. That may be what the poster was seeing.
What year was the car you saw this behavior?
m
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The OP's 2002 Renault clearly isnt.

2002
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On Monday, June 24, 2019 at 2:06:04 PM UTC-4, snipped-for-privacy@yahoo.com wrote:

I agree, both with how traditional VRs worked and that newer cars may have something more advanced. What I disagree with is people making up pure BS, speculation, and then asserting it as fact. That fool Kinsey even dismisses the idea of supplying some cites or references. And also it's obviously not essential to do a new method, because the traditional dumb VRs worked for more than a half century with the same lead acid batteries. There is no need to back off to trickle charging, etc.
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wrote:

Different lead acid batterys in fact.

But if the system deliberately charges more aggressively initially so that the battery is more fully charged with start stop driving, particularly with engines that now are stopped automatically when the car is stopped at the lights etc, it may well be very desirable to back off that charging when the battery is charged.
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On 25/6/19 4:06 am, snipped-for-privacy@yahoo.com wrote:

It is a fuel saving measure. Also, some systems have a 3 stage charging system - charging, trickle and float. This is very beneficial when you have two batteries with only one being a starter battery. A lot of 4 wheel drives for instance.

--

Xeno


Nothing astonishes Noddy so much as common sense and plain dealing.
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On Friday, June 21, 2019 at 4:54:51 PM UTC-4, Rod Speed wrote:

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
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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 fucked 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.
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wrote

Have fun explaining how you can have an ammeter which shows the current that is going to the battery.
Ditto with an OBD2 dongle.

Yes.

Nope. And there isnt normally just the one wire at the positive battery terminal.
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Er that's what he just said, it can't. The car must have more than one meter, on every wire coming from the battery, so it knows what's being consumed and what's used for charging.

I believe mine has three. Alternator, starter, everything else. The alternator itself can't know where it's current goes once it reaches the battery terminal. Does it go to the lights or into the battery itself?
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On Friday, June 21, 2019 at 6:44:36 PM UTC-4, Rod Speed wrote:

Because then there is an ammeter, dummy. I have yet to see a car that has a sensor in the cable path to the battery. And obviously it's not needed because lead acid car batteries worked for most of a century BEFORE there was any computer to monitor or control anything and the batteries worked fine, lasted just as long.

That is just a connection to the computer dummy and cars had batteries for most of a century BEFORE OBD2.

BS as proven by all the cars I've worked on over the years. Battery, alternator, car loads all tied to one common point. And the VOLTAGE regulator simply monitors that common point for VOLTAGE.
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wrote

It isnt in series with just the battery, stupid.

Don’t need one, you measure the voltage drop across the cable. The resistance of that doesn’t vary enough to matter and so that tells you the current.

Generators do it differently to alternators and now you cant add more water to the battery if it has boiled off some of the water because its been charged at the same current as it was charged at when flat.

In fact they worked worse with generators when you had used up most of the charge when it wouldn’t start when very cold.

That’s wrong too.

That’s how you see the current going to the battery when there isnt an ammeter, fuckwit.

Normally the positive terminal of the battery.

Wrong with computer controlled cars.
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