The proliferation of useful electrical gadgets in military units has led to growing demand for "house current" (110 volts AC) for troops when units are out in the field. The house current is needed to recharge the GPS, communication, computer, flashlight, camera and entertainment equipment troops now take with them when they go to war. Troops have found that rechargeable batteries, and battery chargers, deal with the constant risk of fresh battery supplies not arriving. Army units have lots of generators that produce 110 volt power, but you don't have access to these unless the unit stops and sets up camp. In Iraq, for example, they rarely happened during the fast moving fighting in 2003.
Hummers are often equipped with a power inverter and a third battery to provide some 110 volt power for charging batteries in electronic devices. Battery rechargers are very popular, but you need 110 volt AC (house current), not 24 volt DC (common in all military vehicles) for these. The designs of future military vehicles are starting to include 110 volt AC, but the troops need their house current in the field now. Crews of armored vehicles are rigging inverters for their tanks, infantry vehicles, self-propelled artillery and so on. There is a growing civilian marketplace providing inverter (power conversion) equipment for the troops. These modifications to the armored vehicle electrical systems are illegal, but many unit commanders look the other way, or cooperate in the efforts to keep the batteries charged. Unit commanders are often the heaviest users of electronic devices, and they need their batteries recharged as well.