Volt meters (multimeters) are cheap and essential for what you are contemplating.  Maybe someone will chime in with a real answer for you, but you could probably run to Walmart, pick up a volt meter, and find out for yourself more quickly, with the added bonus that you'd then have a voltmeter.  But, Aventons come with 48 volt batteries?  How/why would they provide a 12 v lead?  It's possible with more electronics that would consume valuable amps, but again, why would they do that?  A lot of bike lights run on a range of voltages, like 8 to 69 volts.  (I don't know exactly, but something like that.)  Your lights are almost certainly running on 48 volts;  You can very easily check with a volt meter.  
Also, I generally look for accessories with button batteries or that are USB rechargeable.  I want to save my bike battery for getting me home from a long ride.
TT