car battery charger

[Alun]

Shouldn't there be at least something to limit the charging current though the battery?

Even if it's just a small resistor in series with Q1, it still needs current limiting.

You may have got away with out it due to the intrenal resistance of the transformer and the battery.

 

[ante1]

Hi Guys,

I have built and used this circuit and it works fine!

View attachment 37019

 

[audioguru2]

Hi Ante,
I am amazed that it is so simple.
If you try to charge a completely flat battery doesn't the fuse blow or the electrolyte boil?
I was told that a flat battery puts a huge strain on a 50A(?) ordinary alternator.

 

[Platonas]

Certainly current limittation is only achived by the transformer capabilities.
A flat battery would absorb high currents at the begining of its charging until the voltage reaches  a level near the set voltage by the pot.  At this level, conduction is not for the full cycle but for a smaller part than before, hence less heating. 
Anyway, this is not an actual current limitting technic and the charger is intended mainly for keeping the battery at a charged level. 

If someone wants current limittation then a series resistor could be incorporated but the heat losses increase.

A 1ohm series resistor will limit the current of a flat battery (supposed a 10V across it) to about 3-5 Amps depending on transformer voltage and charger settings.  The heat on the resistor would be about 25Watts which means that a resistor of about 50Watt rating must be used (due to high temperatures arising at the resistor)

In this case, the cost of the resistor is almost equal to the cost of the charger 

 

[ante1]

Hi Audioguru,

Fuse?? Why use them they just break all the time! No, but seriously no blown fuses, the short bursts of current combined with the resistance in the battery leads (a few meters of 1 sq/mm) can be the answer. And if you have ever had a completely flat battery you might have experienced the opposite, no reading at all on the A-meter to begin with? After that slowly climbing over a period of time until the chemical process is back to “normal”.

 

[audioguru2]

Hi Ante,
Yeah, I've had a lead-acid battery that was sitting for some time completely discharged, and it took a long time for it to begin charging normally. I think it became "sulphated".
I have also left my car's lights on after a morning drive to work in the dark and getting there at dawn when it is bright. Many hours later the lights were very dim and the battery didn't have enough charge remaining to crank the engine. After wating for about half an hour with the lights off, it started almost normally. After a short drive with the alternator probably pumping many amps into the battery, it acted like normal.
There isn't a fuse. The current is limited (?) by the alternator's windings, rectifiers and wiring. ;D

 

[audioguru2]

I am also sorry, the old LM301A is still being produced but also doesn't work with a supply voltage less than 10V. :