The anti-idling system is not complicated. The vehicle gets extra lithium-ion batteries, just a little bigger than those used in regular cars. And it gets some extra equipment to automatically flip the engine back on whenever the spare power needs an extra boost. That means the vehicle briefly rumbles back to life every few minutes. In the meantime, soldiers can use their radios and other communication devices, along with air conditioning or heating, without idling the engine nonstop. The Defense Department eventually wants to electrify more and more of its fleet, including all of its noncombat vehicles. The National Defense Authorization Act that was passed in Congress last month for the first time imposes a requirement to do so, by 2035. The Humvee replacement, known as the Joint Light Tactical Vehicle, or JLTV, doesn’t fall under the requirement, but the efforts to improve its fuel efficiency could still have a big impact. The military expects eventually to use more than 60,000 of them, so simple changes that can cut fuel use by a fifth would be significant. The same technology eventually could spread more widely. “If you look at each vehicle, maybe that’s not a lot of fuel, but when you look across the deployed force, that can be really significant,” said Joe Bryan, the chief sustainability officer of the Defense Department.