Does the M939 have a "Vanner" battery charger built in ? Our busses run four 12 volt batteries. There are taps off two of the batteries for 12 volt systems and then they all are connected (series/parallel) for the 24 volt system. The Vanner is connected to each battery charging them accordingly . If the military did this then there would be no problems with the batteries. Also if the 12 volt load is very light it would not be an issue.You're fine, I'm not out to get you. Just happened to notice recent activity regarding a 12v topic.
At the same time, it is important to set the record straight that it has been done this way, per military design intent, on approximately 32,000 vehicles. The batteries in my M939 series were 7 years old when I received it, and all still in great working condition with plenty of reserve capacity left.