The electric airsoft guns were powered primarily by nickel-metal hydride (NiMH) batteries with varying voltages and milliampere-hour ratings. The most common battery is an 8.4 V large battery pack, with a capacity between 2200 and 5000 mAh. Also available are "mini" and "stick" batteries, which generally have 900 ~ 1600 mAh capacities. Voltages for NiMH batteries range from 7.2 V to 12 V. The usual rule of thumb is that the higher the mAh rating, the longer the battery lasts; the higher the voltage, the higher the rate of fire.[4] Recently, however, the more energy-dense lithium polymer (Li-Po) batteries are becoming more popular in the airsoft world, since they last longer, have higher mAh and voltages, and can be charged more frequently without concerns for voltage depression, while at the same time being small and lightweight. Li-Po batteries are usually rated at 7.4 V or 11.1 V, and varying between 500 mAh and 6500 mAh. External modifications, such as metal bodies and reinforced plastics that make AEGs look and feel even more realistic, have become very popular. AEG manufacturers such as Classic Army and Tokyo Marui produce replicas that are visually nearly identical to their real counterparts. Tokyo Marui uses an ABS plastic, whereas Classic Army features full metal bodied guns and stronger furnishings. Most AEGs produced as of late are designed to be as visually realistic as possible.