Maintaining voltage while dropping amperage

[charliemmann]

My family's gate is controlled by a solar system designed by liftmaster, LA412. Due to the large number of trees we have, my dad did not use the solar panel that came with the system. It came with a 12 volt 10 watt solar panel. Instead, he used a 100 watt 12 volt solar panel, and he used a bigger and different solar use battery, outside of the liftmaster control box. The manual for the liftmaster controller said it can take up to 12 volts 30 watt max. So my dad cut into the 2 wires feeding the power side of the board ( i think ) , and then connected his power wires from his external battery (that is charged by the 100 watt solar panel ) to those 2 wires.
Now the led on the liftmaster control board for 'low battery' stays on, and he thinks the internal 12 volt 33 ah battery is not charging. He thinks he shouldn't need it, since the battery feeding the power side of the control board is full of power.
He has offered to pay me $50 if I can figure out how to limit the amps coming from his external battery ( Concorde 12V 108ah AGM Battery ) , so that it can be hooked up to the solar wires for the control board, as opposed to the battery wires for the control board.
I have been very interested in electronics since I was at least 5. I love taking electronics apart, and drawing pictures of the circuits. There is so much detail. But I can not figure out the best way to limit the amps to under 30 watts?.... I am not sure how watts translate to amps. And then there is the 'open circuit' and 'short circuit' ratings...... Is there such a thing as an amperage regulator, that will maintain the needed 12 volts, but limit the amps to under 30 watts? How would I find an amperage regulator?

[EmmaGriffiths]

Hey charliemman!

I admire your enthusiasm for electronics! What a cool hobby.

This forum is meant to provide help to those working on science fair projects. An inquiry such as yours can be searched online or posted on a site meant to answer science questions.

Keep up your curiosity!

Emma