We've also released our first native 72V battery pack in a conventional downtube battery casing, assembled using the high power Samsung 27100 40T cells so that even with the small 8Ah capacity it is able to deliver 40-50A continuous currents. This is an ideal battery for fast and sporty offroad ebike builds, allowing powerful performance without the weight or complexity of most 72V setups (either two 36V packs in series or a large triangle frame battery).
In New Zealand, the regulations read: "AB (Power-assisted pedal cycle) A pedal cycle to which is attached one or more auxiliary propulsion motors having a combined maximum power output not exceeding 300 watts."[56] This is explained by NZTA as "A power-assisted cycle is a cycle that has a motor of up to 300 watts. The law treats these as ordinary cycles rather than motorcycles. This means that it is not necessary to register or license them.[57] Note that the phrase "maximum power output" that is found in the regulation (but omitted in the explanation) may create confusion because some e-bike motor manufacturers advertise and print on the motor their "maximum input power" because that number is larger (typically motors run at about 80% efficiency [58]) thus give the impression the buyer is getting a more powerful motor. This can cause misunderstandings with law enforcement officers who do not necessarily understand the difference, and when stopping a rider on an e-bike in a traffic stop, look at the number on the motor to determine if the e-bike is legal or not.
But in Great Britain, the European Union definition is applicable, and the tighter restrictions of the Electrically Assisted Pedal Cycle Regulations 1983 (including a 200 kg weight limit on solo bicycles) were removed on 6 April 2015 by the Electrically Assisted Pedal Cycles (Amendment) Regulations 2015.[48][49] Riders must be at least 14 years of age, but no driving licence is required.
I do not recommend hacking a bike into s-pedelec, i.e. make a 25km/h bike go at 45 or faster. If you have an accident you will be in trouble. Just get a legal version. In addition you will have good insurance. Of course, in countries like France or the UK that so far do not know S-Pedelecs, you will have to make a difficult moral choice.... At least, make sure that your rig is safe...

Available products (motors, senors, batteries, control systems. etc.) change fast since the e-bike market shows a strong growth. In principle, get a rear motor if you want a lot of raw power and acceleration (e.g. for a 500W motor). Middle motors are more energy efficient and less subject to problems (e.g. overheating). Middle motors usually are combined with integrated rear hub gears, and rear motors with derailleurs. Both have advantages and disadvantages. E.g. the former require less maintenance, are easy to use in go-and-stop traffic, but require taking off weight on pedals when shifting. Therefore the latter (derailleurs) are better for speedy overland and hills cycling.

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On the other hand, the battery doesn’t lock into the bike, which means you can’t just leave it on your bike when you park at the bike rack. Anyone could walk by and simply remove your battery. That seems like an oversight to me, though perhaps the designers assumed that such a small battery would just be easy to take with you. And it is. My wife could probably lose this battery in her purse.