Since 2012, alternators fitted to a number of vehicles sold in Australia have had to assist the engine comply with European emission standards. This is done by having the alternator detect the state of charge in the cranking battery and reduce the output once the battery is fully charged - this includes charging other batteries - this reduces the load on the engine and thereby reduce the emissions. A BCDC charger can help overcome this problem. Others have said that switching on electrical items usually powered by the cranking engine (eg headlights) signals the alternator to continue charging. As there are a number of alternative charging systems installed in the BT50's it would probably be best to talk to your Mazda dealer to identify what is installed in your ute. A good auto electrician should be able to assist you achieve the charging options you want in your setup.

See www.redarc.com.au/handy-hints/-/new_vehicles_technology_can_affect_dual_battery_systems_charging_performanc/ for some more detail.

Dave