Trying to figure out the max amps the rectifier puts out

[Hoodatdoi]

Hey guys and gals, recently purchased a 1998 challenger 1800 with twin rotax 787 engines. I’m fairly pleased with the boat and have had it out a couple of times now. I want to be able to use the boat for longer day trips without the sound system killing the battery and leaving me stranded on the water. Looking into the option of adding a second battery and using a Blue Sea Systems Add-a-battery kit. However, the 120amp kit is about double the price of the 65 amp kit. Anybody know what the max amperage the rectifier on these boats would produce? Trying to build this boat on a budget and I haven’t found any specs on the oem rectifier online. Any help would be appreciated!