[torbjorn]

In order to get the battery charged, you must make sure that the charger can supply more than 12 volts. For trickle charging, 13,8 volts is ideal but if you want to be able to fully recharge a battery in reasonable time, up to about 15 volts is needed at the end of the charge period.

As the adaptor is regulated, it will probably deliver almost exactly 12,0 volts when the switch is set to the 12 V position.

Unregulated adapters, on the contrary, usually deliver a much higher voltage (say, from 10 to 50 % above the rated voltage) at no load, but the voltage drops when loading them so that they deliver the rated voltage at rated current. Of course, an unregulated adapter will also be sensitive to varying mains voltage.

[RODALCO]

You can put a 12 volts 10 Watt lamp in series with the charger to limit the current to less than 1 Amp.

It has not much capacity 15 VA at 12 volts is about 1.25 Ampères.
It may be ok as a trickle charger but you need to keep an eye on that the TX does not overheat.

[vtech]

Technically you can, but it is too small(low current) for car battery application & not practical.

1.25 max amperage will take for ever to charge a car battery & more likely will overheat & burn up it's txformer.

[stephen 2]

I have an unused small black Uniross 12 volt adaptor that used to power an electronic piano. Both outlet cables are black. I have figured out using a voltmeter that the one with the white stripe is positive.

Can I cut off the small round metal plug on the end and connect crocodile clips and then use it as a car battery charger?

The adaptor is regulated, not sure what that means? there is a switch to adjust output 3V/4.5V/6V/7.5V/9V/12V.
Output is 1250mA 15VA MAX. Not sure what that means either?

Thank you for any advice/replies.
stephen