Iflyj3 told you correctly that there is a bridge rectifier at the input of the Alpha 4. This enables it to run on ac or dc. You can run it on your field battery which may be over 13 volts fully charged.

Whether you input AC or DC, by the time it goes through the bridge rectifier it's definitely DC. Then it goes into a DC/DC converter which can boost the voltage, or reduce the voltage, because it's regulated at the output of the converter circuit. So, by the time it gets to the charging circuits, the voltage is correct as long as the input is within the working range of the convertert circuit, and the current is sufficient as long as the wall wart can supply sufficient current.

You may have noticed that when you charge an 8 cell transmitter pack, the voltage reading will sometimes exceed 14 volts before it peaks, and goes to trickle, even though you aren't using a power source that outputs that much voltage. That's the regulated converter circuit at work.

It's a facinating piece of technology, especially for the era it comes from.

Hope this helps,