On Fri, 04 Jan 2019 18:17:29 -0000, Dave Plowman (News) wrote:

In article ,
Commander Kinsey wrote:
Bull****. I've got a very basic 4A battery charger. Just a transformer
and rectifier. If the battery is flat, it'll give out 4A. Once the
battery gets full, it drops to a fraction of an amp. Basically it just
gives out 13.8V maximum - the trickle charge for a lead acid.

The output of a basic battery charger will vary a lot due to the
transformer regulation and so on. A basic one is very unlikely to produce
a constant voltage - unless specifically for SLA gel types.

I presume the transformer and diodes are chosen so it never goes about about 13.5 volts, which means it'll charge at 4A if the battery really needs a boost, limited by the resistance in the transformer coils, then drop off to virtually no current at around 13.5V once it's full. On my 60Ah car battery, it charges at 4A if I've even just used it slightly (for example put the headlights on for a few minutes with the engine off). If I leave it overnight, it's 0.4A in the morning, and the battery is completely full.

The battery itself prevents overcharge by taking far less current once
it's full. Lead acids are very easy to charge without complicated
circuitry. You only need complicated circuitry if you want to charge
them really fast. How do you think a car alternator works? It just
feeds 14.4V to the battery continuously. Probably a bad idea 24/7,
but the car ain't running 24/7.

No alternator I've ever seen stays at 14.4v when the battery is charged.
It would very soon fry the battery. And modern batteries are semi sealed
and don't like gassing.

Maybe modern cars have a regulator to stop that, but older cars (and I mean around the turn of the century ones, not vintage) don't. If you put a meter on the battery when the engine is running, it's always over 14V. I assume batteries don't really mind that as long as it's just for driving time - they'd be upset if you did it 24/7.

I have a very accurate voltmeter fitted to one car, and the volts do go up
to over 14 when the battery is low, but the normal figure is not
surprisingly 13.8v.

You've got me interested now, so I'm going to check my 2002 Renault Scenic.... maybe I've never checked the voltage after the car's been running for a bit.
This website says a continuous charge should be 12.9 to 14.1V for a full battery:
https://www.powerstream.com/SLA.htm
The battery is currently fully charged, I was driving it this afternoon, I'll go start it and take readings every 10 minutes....
14.44V on the battery terminals just after starting, engine idling.
14.22V after 10 minutes (no increase when revving it up).
14.19V after 20 minutes - maybe you're right! Something's making it decide to drop the voltage, although only slightly. I thought a "regulator" on a car alternator was just a set of diodes.

However I then connected my old 4A Bradex car battery charger to it with the engine off, and it quickly rose to 14.4V. Maybe I shouldn't be leaving that on overnight.... although if it's only giving out half an amp, which is 1/120th of it's capacity, that can't do any harm? I have noticed that the green float indicator (which I never trust) is now white and the battery is only a year old. I leave the charger connected all night, since the alarm has a habit of draining the battery overnight for no reason, and I can find no way of disconnecting it without dismantling the entire dashboard - they hide them well from theives!

It can be a little lower with heavy loads at low engine speeds.

What seems to surprise a lot of people is an alternator will rapidly charge a battery at idle engine speed, most people seem to think you have to drive around.

Also, when jumpstarting someone else's car, you don't have to start the donor car. The donor car has a good battery, it's therefore capable of starting the other engine just as easily as its own one (unless you have really ****ty thin jump leads that lose voltage - my jump leads are 13mm thick each - I think they were sold as lorry jump leads, rated at 600 amps or something). I didn't get them to conduct better, but so they didn't fall apart with age like Halfords ones do. I've jumpstarted plenty cars with normal jumpleads without starting the donor car first. On that subject, I occasionally hear weird myths about certain BMW or Mercedes cars exploding (the battery I think) when they're jumpstarted - this can't be true unless they use 6 volts or something?! Or more likely some stupid fool connected the black and red wires the wrong way round?