Transformer for car battery charger.

[malcscott]

The mains t/x has gone o/c primary in my old battery charger. What voltage secondary do i need to develop the 13.8v? I have a nos RS (207-302) 100VA 15V-0v-15V transformer, would this be ok if i used a bridge rectifier? Malc

[Michael Watterson]

It depends on if there is a "smart regulator" or just a series resistor. The really old ones used the Resistance of the MR.

Pick resistor or voltage or both for desired "trickle" current at 14.2V on battery (modern maintenance free / Calcium car type) or 13.8V (gel type).

[malcscott]

So if i just use one 15v winding feeding a half wave rectifier would this be ok? What current is available from this transformer? (50va per winding)

[Michael Watterson]

Use both windings with two rectifiers and centre tap = 0V. That gives about 18V average and twice the current rating of one winding.

[malcscott]

What max current from this t/x??

[malcscott]

I will go for the bi-phase method rather than fit a metal clad bridge. What would be suitable diodes to use?

[malcscott]

Thanks for the info Jeffrey, dont quite understand using half of the bridge? Any chance of a diag?

[malcscott]

Now i understand, thank you

[Michael Watterson]

Using a bridge you get a 12V charger and you get a 24V charger for free if you put a larger resistor in -ve lead

[malcscott]

Nice one Michael

[crackle]

I thought the maximum current will depend on the state of charge of the battery and the resistances of the rectifer and transformer windings, not just on the voltage.
Mike

[Michael Watterson]

Yes, often in the past when they changed from Metal Rectifiers to Germanium or Silicon they added a resistor. The old MR had enough resistance to limit the maximum current.

"Smart" Chargers and "float in Use" smart PSU/Chargers are rather different.