Algernon Goss-Custard wrote:
posted
On Saturday, 2 May 2020 17:52:41 UTC+1, Algernon Goss-Custard wrote:
Fredxx posted
There is very little literature on battery charging, of when a charging
device might switch from constant current to constant voltage at
different initial charging currents.


My car battery charger, bought second-hand forty years ago for 0
just packed up. Can anyone recommend a suitable replacement? Doesn't
have to be portable, just plug into the mains and recharge a flat car or
lawnmower battery. Preferably for the same sort of price I paid for the
last one ...

Yes. Any modern car battery charger will work. All there is to choose
is cheapos versus high MTTF ones, £12 versus £40+.

I might fix the old one.

The metal enclosure is riveted together so I can't open it up. Anyway
IMLE the failure mode for devices based on a transformer is nearly
always shorting or burning out of the windings which can't be fixed.


That's not the only mechanism.

The transformer laminations are dipped in lacquer, then dried.
Then the laminations are fastened together to make the core.

The purpose of the lacquer is to prevent eddy currents from
forming. The assembly is clamped together to make a soft
steel core with the desired magnetization behavior (B-H curve).

If the laminations happen to rust, it can cause the lacquer to be
punctured. Then eddy electrical currents start to flow in a loop, between
laminations. Perhaps a bit of a buzz is heard. The eddy currents heat
up the laminations (which is why the lacquer was there in the first place).

This makes the transformer get hot, even at no load.

I had a RadioShack transformer fail this way. Sitting
in the junk room for 20 years, dry conditions, and
still it managed to have an eddy current problem. You couldn't
leave it plugged in for an hour, as it would get scalding hot.
When it gets hot, *then* there's some room for the other
failure modes to happen. I had to buy another, and today
they're not cheap (the cost spoils the fun of building
your own gadgets).

The selenium rectifiers get pretty hot in those dumb chargers too.
The reason selenium seems to be used, is the relatively high
resistance in the conducting state. They're not efficient
like a semiconductor diode. But the selenium rectifier also
helps "tame" the output behavior, making for a better
battery charger.

Selenium is a kind of impedance protection. Not perfect, but better.
Semiconductor diodes are too "stiff" for this usage (making a
dumb charger with a minimum of components).

If you were to replace the selenium rectifiers with a non-selenium
solution, you would have to make sure the behavior of the output
was modified with additional electronic components. Which kinda
spoils the "I only paid £5 for this".

At 9V, maybe the dumb charger makes 6 amps.
At 12V, maybe 2-3 amps.
At 18V, about 0 amps (end of charging, when knackered battery is present.
A good battery shouldn't rise to 18V.)

If you used semiconductor diodes in place of
the selenium ones, at 9V you might get 20 amps
and a fuse blows or smoke comes out or...
Some additional resistance to current flow
is needed, somehow, but without eroding the
good characteristics as the charge operation
continues and the battery fills up.

You paid £5 40 years ago. If the inflation rate
caused a doubling of the value every 10 years,
your "budget" today would be £80. Since the
new chargers don't use nearly as much copper
wire inside (SMPS, high frequencies, small transformer),
they can do considerably better than £80 a unit.

The device also has to be selected for the task.
The lawnmower battery isn't a lot of amp-hours.
Maybe even the cheap 1 ampere charger would work
(charger is switchable between 6V and 12V battery types).
But to do a good job of charging the car battery,
you would probably enjoy using more current than that.
I have a 3 amp one, which is sufficient for car battery
top-up. It would take a whole day if the battery
was flat (run-down by inattention during COVID).

The Lidl one is probably a bit more current than that.

The car battery type matters to. There are regular batteries,
and there are "start/stop" batteries. Cars that stop the
engine at intersections are the "start/stop" case. Older
cars left the engine running all the time until you got
home and removed the key. The older cars are easier
on their battery, calling for bulk current less often.
The advert for the charger may mention whether it's
compatible with "start/stop". I saw one float charger,
that said it was not intended for start/stop battery types.
That suggests you'd also want to monitor *any* charger,
for aberrant behavior when the main charging sequence is complete.

In the graph here, phase 2 (topping charge) is CV (constant
volts) at 14.4V.

https://batteryuniversity.com/learn/...d_acid_battery

Phase 3 (float charge) is supposed to be CV 13.5V. Some
of the chargers you buy today, don't do phase 3 quite
correctly, and some batteries are "resting" at 15V with
the charger connected. I checked mine, and I did see it
float-charging at 13.5V.

You would not leave the smart charger connected forever,
unless you'd verified (somehow), that it wasn't abusing
the battery (wrong float charge behavior).

Phase 1 CC (bulk charge, 9V rising slowly towards 14.4V)
(if the battery is full, it gets to 14.4V within
the first five minutes or so)
Phase 2 CV (topping charge, 14.4V in a CV mode)
Phase 3 CV (float charge, 13.5V)
Phase 4 (some chargers switch off, leaving battery at 12.8V
if it's in good shape)
(when the battery self-discharges to 11.8V, some
chargers automatically start Phase 1 again, others
need the user to press the button)

The purpose of these user manuals, is to check and see what
ampere-hour size of battery they charge with each. The 5 amp one,
handles a battery up to 120 amp-hours. The 2 amp one is shown
charging a 40 amp-hour battery. You'd check your car battery
rating on the label, to see what ballpark it's in, like 60Ah.
If it was 60Ah, you'd likely want a bit more than a 2 amp charger.

https://no.co/media/nocodownloads/fo...er_guide_1.pdf
https://no.co/media/nocodownloads/fo...er_guide_2.pdf
https://no.co/media/nocodownloads/fo...er_guide_1.pdf

Since for the cheap ones, AldiLidl buys them in "lots" and how
they work could change from lot to lot, you want to find a
discussion thread for the "current" one, to see whether it's
a good deal or a dud in terms of the whole charge sequence. I've
been paying more attention to mine, and notice it's not as "saintly"
as I thought. The "full" light comes on, while it's still
pumping current. It shouldn't do that. I ended up disconnecting
it while it was doing "verification phase". No idea what would have
flashed if it wasn't holding well. Mine could use a LED that
says it's stopped faffing about. But at least so far, I've not
caught it using a wrong voltage.

The dumb chargers, the human operator had to guess at the
fullness, and disconnect the alligator clips before it
was too late. That's why we have smart chargers - in theory,
to do the job well enough, they can be ignored once connected.
You want to be able to walk away for a year, and come back to
a properly charged battery, and no battery damage.

Paul