The battery game

[duivendyk]

But if you knew the relative elevations you'd get some idea of the energy reqd. to get up from either side.I live in a very hilly place and measured some inclines,took a 2ft level put it along a slope keeping it level and measured vertical distance road surface to bottom of it.Slope H/24x 100 .Overvolting is not a good idea unless you can also change the transmission ratio (more reduction to keep motor out of low rpm range).The best solution would be to somehow use a multispeed geared hub like the Shimano Nexus or the SA hubs, but it's difficult to combine motor&pedal input.Look at Safe's latest creation he combines them into a SA multi speed hub with the chain drive,which is pretty neat.
The difficulty is combining two sprockets into the hub input.I don't think it's completely impossible but pretty difficult.There is only limited axial space.I hope you understand that if you have to get up a steep hills and can only do so at low speed since you have limited power,the motor efficiency takes a beating,could go down from say 70% to 50% or even lower,Which is obviously not good news.These motors are basically designed for a particular voltage (speed),if you run them at much reduced speed (voltage) the eff. is reduced esp at high torque (current),so you really need a variable transmission,not as in an IC engine because you have inherently limited torque,but because the eff. goes downhill and the motor might get fried.The result is that they perform poorly in hilly terrain.Walking uphill might not be so bad but if it takes a long time the battery drain could still be substantial .It is possible in principle to use regenerative braking to put charge back into the battery but that would require a special controller.I prefer to get some handle on a situation, if possible, instead of blundering into the unknown,hence my interest in elevations.

 

[safe]

The problem I've had so far with the Sturmey Archer multispeed hub is that they don't actually shift until the power comes off the chain. Since these motors have a lot of flywheel effect it takes time for the motor to slow down enough so that the rear hub can shift.

Compared to a derailler it's not as immediate.

I like the standard deraillers... in fact I'm going to look back into using them again for future projects. My motors have been putting out several horsepower at times and I've gone 6,500 miles on the same low quality derailler without any problems.

But the main thing is figuring out how to connect the motor to the chain. It's not an easy problem.

My present thinking is to use two chains, one chain that connects the pedals to the motor and another that connects the motor to the rear wheel and then use freewheels to make everything independent.

Knowing what your gearing needs to be is a complex task too.

(and matching the motor powerband to the human pedal powerband is important too)

 

[duivendyk]

If you could put a resistive load momentarily across the motor when shifting it would slow down in a hurry.The controller is a one-way proposition.The motor cannot feed power back to the battery,which would have slowed it down.It would have to operate in a 'boost' mode (voltage stepup) mode in order to do this.I think the reason the hub does not appear to shift is that the ratchets transmitting torque remain locked until the drive torque is removed.Is this a freewheel crank?.otherwise back pedaling will do the same thing for you.

 

[Fuzzo]

My motor has its own chain directly connected to the rear wheel (with freewheel). It's completely independent of the human drive system. I can't shift gears.

Overvolting will still give it more power, right? Which might improve efficiency for the hills, not to mention that it gives me more batteries without all the aforementioned complications.

I'll do my best to work out the total elevation of the Waitakere range. There should be data around.

 

[Fuzzo]

Indeed, Wikipedia puts the highest point at 474m. I'm starting from sea level, so this won't be far off my total climb to Waiatarua. Which will help for energy calculations for the two hill climbs, but the amount of climbing done in the range itself is unknown, I'd have to measure it on a hill by hill basis.

 

[duivendyk]

I assume there are 2 major climbs one from the east followed by a undulating section in the center, then at the West end a descent N to the coast,it would be useful to concentrate on those first from an energy perspective (distance& relative elevation ),those are enemies #1&2, and assume a 'flattish' midportion.This will give some idea about the energy/power reqd. situation.Overvolting increases motor speed (also more energy storage for the same Ah capacity) but would really only pay off if you could increase the gear reduction,if that is not in the cards it's probably no better and could even be worse than keeping batt. voltage the same and just increasing cap.It would certainly give you more speed on the flats,but that's not your priority I gather,range is.

 

[safe]

Overvolting can mean overheating too.

Heat increases with:

Heat = Current * Current * Resistance

...and with the strange way these controllers work at below 100% duty cycle (which is the way that they chop up slices of power to allow to be given the motor) the heat gets far worse at lower rpm.

For the typical controller:

MotorCurrentActual * BatteryVoltage * DutyCycle = BatteryCurrent * BatteryVoltage

...so if this formula makes sense it means that the "apparent" voltage goes down as either the throttle is reduced or the current limiter kicks in, but the "apparent" current actually goes UP. It took me a long time to be able to understand this because it seems so counter intuitive. If you wanted the current to match the duty cycle in a linear way then you need to design a special circuit called an "Armature Current Limiting" circuit that actually makes sure that the current doesn't do this non-linear stuff.

The "bottom line" is that without some idea of how to manage the extra heat you will burn up your motor if you overvolt too much.

 

[duivendyk]

That is quite true,your expression basically states power in to controller equals power out to motor,there should have an controller efficiency factor E included also,
Pout= E x Pin .E is duty factor dependent (lower for smaller DF),which makes the situation worse at low output voltage (speed).Overvolting pays off if you want high end performance and your motor can handle the increased rpm.

 

[safe]

Controller specific losses are pretty small compared to motor specific losses. The controllers are rated around 95% efficiency, so you can practically ignore them. Motor efficiency is closely tied to rpm, so as you overvolt it basically scales everything upwards. However, that's not entirely true because in the rewinding thread we talked about how if you keep a constant current limit and raise the voltage it reshapes the efficiency powerband to bring it closer to the ideal.

Most all of these ebikes are set up with current limits that are way too high for their voltage, so generally some overvolting is beneficial as it tends to increase power while making the motor run more "lean" in the process.

But at the same time the lowest rpms tend to get significantly hotter.

So it's a mixed bag... if you can keep the rpms up then the extra voltage is going to be okay. Another potential idea would be to go with a very highly overvolted setup and then just filter out the high heat areas with the "Armature Current Limiting Circuit".

But by the time you've gotten that far you're in way too deep technically for the average person... most people just want to be able to add a battery and not know any more than that.

There are a lot of tricks to this trade... but few know all the tricks and how they work with each other. (some tricks can work against others... it's all about seeking the perfect setup)

 

[Fuzzo]

"Too much" is the question. How much is too much? 12V? 24V ? I don't see any way of working it out other than to try, and to keep an eye on the motor temperature. I'm pretty sure the 200W limit is just to satisfy Australian laws (the bike is originally from there). So the motor might just be limited to 200W, rather than incapable of anything higher. Certainly it never gets the least bit warm in use.