Unlike most e-bikes, the Brompton's battery is integrated into a bag that sits on the front of the bike. While that detracts slightly from the classic, streamlined design, it's handy as it can be unclipped for charging or riding as a regular bike – it's probably one of the easiest to ride without battery assistance thanks to its weight. The battery also powers two lights for safe commuting. 
E-bikes mostly use motors and battery options from a few major suppliers: Bosch, Yamaha, Shimano, and Brose. A few other brands exist, but are less reliable or powerful. Some, like the Yamaha system, have more torque and others are quieter. But generally all four make good options. Look for motor output (in watts) which will give you an idea of total power. But watt hours (Wh) is perhaps a better figure to use—it takes into account battery output and life to give a truer reflection of power.
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