The Ford Ranger has a 'Smart' battery charging system, which only charges the battery to a preset level to maximise efficiency and extend battery life. Unfortunately this also means that the 'reserve' in the battery is minimal. Ford dealers, for a cost, can reprogram the charging system so that the 'smart system' is turned off and the charging system works like 'traditional' charging system. Extremely useful if you have dual batteries fitted.
A number of owners seem to be having troubles that relate to the so-called Smart charging system. The system is designed to keep the battery charged to a sufficient level to keep the car going, but as you say there is little reserve and that can result in a flat battery. It’s also important when fitting accessories to the vehicle that connected in a way that allows the ECU to identify that there is greater draw on the system and the alternator needs to work harder. Ford can disable the system for you if that’s what you prefer, and lots of owners are doing that.