That's quite an abusive tone and I don't remember you mentioning Nimh previously. I certainly don't accept mid-drive as the most efficient. I did quite a lot of research on ebike motor efficiency using expert sites like Grin and there was huge overlap for efficiency and the most efficient motor I came across was a small geared hub motor. However its a mute point as it really depends on your criteria for efficiency, overall use, up hills etc. As I said previously torque based sensor's help limit motor output. I certainly accept mid-drive as the most powerful capable of the most torque in its most powerful models. If you are looking at ebikes with the smallest capacity battery supplied but with reasonable range it has to be a small geared hub motor. You get entry level geared hub motor ebikes with batteries as low as 150Wh that provide reasonable range like 20 miles but again its about the assistance level. Remember the criticism of direct drive hub motors is their maximum efficiency is at higher speeds but a geared hub motor brings that speed right down because of the internal gearing. Surely efficiency really is where the motor is operating at higher efficiency for more of the time for normal use.
A geared hub motor ebike can be a very light ebike, has far less losses inside the motor as the gearing is less complex and has pretty much no losses when freewheeling and has no motor losses through the drivetrain. It's drivetrain can last much longer so it's drivetrain is less likely to be worn and being less efficient. I totally accept that scaling motor power through the gears has efficiency gains but you have to look at the full picture.
Going back to Halfords Carrera Subway with a 317Wh battery and a torque sensor with geared hub motor they claim up to 40 miles. You get significantly less with a cadence sensor and a similar hub motor so part of the gains is the torque sensor.
If you have a ebike that is significantly lighter as many geared hub motor ebikes then the requirement to use the motor is also reduced.
There is so much going on in mid-drive motors, a large number of nylon cogs typically, sometimes a belt and now they have clutch plates to eliminate drag and the bearings are under huge load due to the riders own efforts going through the motor. They are extremely complex and prone to high wear and failure especially if used as an e-mountain bike. If you factor in failure rate to the calculation of efficiency i.e. the need for replacement parts in the drivetrain and repair of the motor itself on a more regular basis and their shorter life as a solution they look far less efficient anyway. Then factor in many are sold on a highly proprietary basis with no access to spares and restricted part compatibility.
I personally think they are junk solution unless you really, really need that additional torque for hill climbing i.e off-road use.
Definition of efficiency
1
: the quality or degree of being
efficient
2a
: efficient operation
b(1)
: effective operation as measured by a comparison of production with cost (as in energy,
time, and
money)
(2)
: the ratio of the useful energy delivered by a dynamic system to the energy supplied to it
3
: EFFICIENCY APARTMENT