For the first time ever, BMW has equipped a non-hybrid car with a regenerative braking system. There's a new battery that uses fiberglass mats between the plates to keep the electrolytes put, and a corresponding "intelligent" alternator. The battery technology, known as Absorbtive Glass Mat construction is capable of being charged very quickly without boiling; and they stand up far better to the deep cycling that would be abusive to a traditional lead-acid battery. The ability to receive and deliver high amperages reliably makes the AGM batteries ideal for coupling with the rest of BMWs Efficient Dynamics program. The battery can be fully charged during braking, which reduces the load on the engine by making the alternator work less. The program's intent is to reduce weight and fuel consumption, ultimately keeping CO2 emissions down. BMWs efforts result in a vehicle that has the low-rolling-resistance tires, electrically operated AC and power steering and stop/start systems of a hybrid, without the electric motors.
[Source: Fifth Gear via Jalopnik]