On Sun, 23 Jun 2019 14:36:35 +0100, Xeno wrote:

On 23/6/19 9:24 am, Commander Kinsey wrote:
On Sat, 22 Jun 2019 11:41:15 +0100, Xeno wrote:


the *battery* is the *load*.

Not when the battery is fully charges and is being charged
with a bench supply that is delivering 4A to the battery.

Take the case of an alternator charging a battery at ~4 amps. The
battery is the load and it also provides, as part of that function, the
reference *voltage* that the alternator *must have* in order to control
the output.

I believe the reason Rod mentioned load as being seperate to the battery
is this thread is about my car, with a fault that draws current from the
battery, and it's also connected to a bench supply. In this situation,
we refer to "load" as the faulty alarm system. I originally asked how
an alternator could distinguish between the battery charging, and a load
such as your headlamps being on.

In the process of being charged it is using electric current. That
makes it the load.

See above.

What happens to the charger when you disconnect the power with the
battery connected? It should, if designed correctly, shut down since it
no longer sees a load. Otherwise it may destroy itself.

Ooops, got that arse about - disconnect the *battery* with the 240V
power still connected.

How would it destroy itself? Mine in fact just stay at 13.8V open circuit.

Bull****. You're telling me that a bench supply with a battery on the
output and no 240V input will blow up if it doesn't shut down? Wrong.

The battery is the load. And I'm referring to a charger and the leads to
the battery being disconnected, not the 240V input. The same as if you
open circuit the battery on a running, and charging, alternator. It can
be done but there is a high risk of a spike zapping something. The
battery acts to dampen spikes and it's removal from the circuit exposes
the regulator and any other electronic components to spikes.

Yes, but that has nothing to do with having a charger with no battery on it.

Absolutely wrong. The output end of the supply (which probably ends
with smoothing capacitors) is just kept at the normal output voltage by
the battery.

I used the term *charger* for a very good reason.

They are one and the same, constant voltage current limited supplies

Even when it is fully charged it will still take a trickle charge

4A isnt a trickle charge.

That depends entirely on the amp hour rating of the battery.
Also, my bench charger will start off at 4 amps, its maximum capacity.
As the battery becomes charged, that current will drop down to *1 amp*
and, from that point, it will maintain a *trickle charge*.

How ****ing big is that battery?! When I charge a 60Ah lead acid at
13.8V, it drops to about 150mA when full.

When I was working on trucks, it wasn't unusual to see 4 x 12 Volt, 200
AH batteries wired series-parallel, so yes, 1 amp can be a trickle.

Yes with 400Ah I can believe a 1A trickle.

From Wikipedia;
For lead-acid batteries under no load float charging (such as
in SLI batteries), trickle charging happens naturally at the
end-of-charge, when the lead-acid battery internal resistance
to the charging current increases enough to reduce additional
charging current to a trickle, hence the name. In such cases,
the trickle charging equals the energy expended by the
lead-acid battery splitting the water in the electrolyte into
hydrogen and oxygen gases

Trouble is you might want to do a bulk charge first, at 14.4V.

The car alternator regulator is no different. It sees the battery as a
load, determines the voltage reference and pumps up its output.

All it can do is supply a certain voltage. 13.8V is safe. If it uses
14.4V or higher, it has to know when to back off to 13.8. It cannot do
that if there is an external current draw like headlamp, as it won't
know if they're switched on, or if the battery is drawing that current.

The regulator has a Zener diode to *limit* the voltage. An external
current draw will lower the voltage.

Only if the alternator can't provide enough current.

When the
regulator sees the battery voltage at the peak setpoint,

You can't tell a battery is full by voltage. You can only tell by it
drawing less current. The voltage is determined by the charger.

The voltage at the battery will be determined by the battery's *internal
resistance*. As that battery's internal resistance goes higher, as it
will do as it becomes charged, the voltage will go higher and current
will reduce.

********. If I connect a 13.8V regulated supply (or charger, same thing) to a car battery sat on my bench, provided it's not needing a lot of current to charge it from empty, the charger will keep the terminals at precisely 13.8V. The only thing that changes as it becomes completely full is the current, so that's what the charger has to monitor.

it too will
drop the current to a trickle.

Actually it drops the voltage, to 13.8. I used to have a solar battery
regulator which had very detailed instructions saying how it worked. It
took the voltage of the solar cells and altered it up or down a bit to
suit the battery condition.

If you add a load, say by turning
headlights on, that is in *parallel* to the battery and it will drop the
system voltage down a tad. The regulator will see that and pump up the
output current appropriately. The current will apportion itself to the
*two* loads as appropriate to their individual internal resistances.

But it can't tell the difference between current going to the lights and
to the battery.

It doesn't need to. All it needs to know is the aggregate current draw,
the total. That will be indicated by the voltage.

If it's going to switch between fast and slow charge rates, it needs to know where that current is going. There is no way in hell it can tell from it's own internal sensing whether your battery is charging at 10 amps, or you have two 60W headlights switched on.