The battery game

[safe]

It can be calculated... but you need to already be at a place where you have mastered all the other stuff. I have my own spreadsheets and simulation programs I've built up over time (and have added the Rewind Calculator to that list) but they have so many little things in them that only I know about that it's probably easier for you to write your own.

There is a lack of intellectual tools available online to be able to do all the "tricks" you want to do.

I suppose eventually someone might create some software product to serve such a need, but it's so specialized that the person will never make a dime doing it. (better to be an in house programmer for such a thing)

Anyway... though "imperfect" as a metaphor it's not that far off to say that everything just scales upwards, so:

If you have "X" amount of heating at 24 volts then you will have "X" * 36/24 amount of heat if you went to 36 volts.

...it's more complicated than that, but probably close enough to make ballpark estimates.

 

[duivendyk]

Fuzzo,trying out following ride to the beach scenario :5km uphill at 6% grade (300m),5 Km at 3.5% (165m),5km at 0 ,then 5km at -3,5 and 5km at -6%.Assume total weight at 130 kg (realistical?) pedaling power 100W (when climbing).No motor load at-3.5 and -6%.Motor eff. 45% at 6%, and 60% at 3.5 %,70% at 0%.Rolling resistance power 25 W when not descending. Controller eff. 95%.I will compute speeds, motor&pedal power&energy use,Any comments?

 

[Fuzzo]

I did a drive through the route on the weekend. I'm discouraged.

Firstly, on my previous ride I had thought I'd made it to Waiatarua. But I was mistaken, that was still about another km, even steeper than what I'd already conquered.

Then the range traversal started to look exceedingly difficult, not to mention dangerous. There are about 8 significant hill ascents in the middle, each of which is about a 50m climb. The final hill at Piha is extremely steep, I would not be able to ride up it at all.

The entire road after Waiatarua is just wide enough for 2 cars to pass each other in opposite directions, with no footpath or verge, and it is 'Open Road', which in NZ means you can go up to 100kmh on it, if conditions allow. It winds along cliff edges and narrow cuttings. Much of it is degraded from heavy traffic, with rippling, flushing, and potholes. It is in the middle of a forest, with no houses until you get to Piha. A lot of teenagers drive along the road to the beach.

Riding a bike along that road in clear fine weather would be dangerous enough. Walking a bike along it would be suicidal. I'd be walking it a lot, with the current power settings.

For some reason, my memory of it was not as bad as the cold reality of driving along it, looking at it from a cyclist's perspective.

Another factor I'd totally forgotten was that all the West Coast beaches have a lot of black sand - which is sand with a high iron content, basically tons and tons of iron filings. These stick to anything magnetic, so I could potentially completely destroy the motor if I rode over a patch of it.

This project is starting to look beyond anything I can achieve whilst retaining any of my current bike.

A lesser project would be more fitting. I still want more range, I just don't have any particular goal for it now.

 

[Fuzzo]

OK, I've settled on a much less ambitious plan. Another bike ride I've wanted to do for some time is the 50km route around Auckland. Last time I tried it, I got lost in South Auckland. Being 12 years old this was a little disconcerting, but I hardened up and found my way home. I think some joker had ripped off one of the signs. This time round I have a map, and I know my way around a lot better. Getting lost is no danger. Getting tired, OTOH....

My bike is good for about 30km at the moment, with moderate pedaling, being used to ride around Auckland. So I need another 20km.

I have purchased another 12V 12AH battery, which fits neatly into my rear carrier. The plan is to overvolt the bike to 36V at 12AH, hopefully adding 50% to my range at the very least. Being SLAs it's possible that the improvement will be more than that, as the extra time should reduce the Peukert effect. The extra 3 kilos only adds about 2% to the weight so I don't anticipate requiring much more power, but overvolting should add quite a lot of power anyway. Theoretically 50%. We shall see if it gets too hot at full throttle, no real way to know yet.

My workshop will be ready this week, so time has come to get hands dirty.

 

[safe]

About that heat...

An overvolted motor does NOT get hotter in the higher rpms where the efficiency is pretty good assuming that you keep the same controller current limit. Where you will see an increase in heat is in the lower rpms.

So it's possible for you to do something called:

"Throttle Fiddling"

...which is to use less than full throttle in the lower rpms in order to protect the motor from excess heating and actually improve efficiency.

With overvolting your new rules are:

Low Rpm - Use partial throttle despite the strong temptation to use full.

High Rpm - Go ahead and use full throttle because at high rpm the efficiency is great.

...most people cannot resist the temptation of full throttle at low rpms because the "EV Grin" (the strongest feeling power) occurs at the lower rpms while the better power is at the higher rpms. The electric motor is somewhat counter intuitive to use.

With "Throttle Fiddling" the moral of the story is:

"If it feels good don't do it."

 

[duivendyk]

That's too bad,I also found out that the road picture changes quite drasticaly when you get back on a bicycle.Even 'moderate' hills turn into severe obstacles,you need a MC really for that kind of a ride to the beach, I'm afraid.

 

[machiasmort]

Sugestions wanted for a DC motor to put on a chain to my crank pedal, using a freewheel like seen in the Sick bike part kits.

I would like to use a solid on/off switch instead of a controller. The setup I described would allow for speed control with the rear cluster.

 

[Fuzzo]

Low efficiency doesn't particularly concern me. It's high heat I'm worried about. I don't spend a great deal of time at low RPM - don't forget my motor has no gearing - so low RPM means low speed. Unless I'm climbing a hill - then I'll have to get a feel on the first few rides exactly how hot it can get. If it doesn't get hot then I've got the choice - use full throttle and suffer the inefficiency, or throttle back, which will of course mean going even slower. Or pedaling harder to get the RPM up.

I'm over feeling gutted about that beach run. It will sit there as a constant reminder of why we still have ICEs. It can be a future challenge, when I've done some less ambitious fiddling to see what can be accomplished with the current setup. I didn't see any bicycles on the way there, so I'm guessing it's beyond the range of even strong cyclists.

 

[safe]

Motors either produce power or heat. It's the ratio of power verses heat that defines efficiency. So there is no such thing as "not caring" about efficiency and only caring about heat because they are directly related.

The essential problem with overvolting (with no ability to change your gear ratio of motor to rear wheel) is that your "coolest and most efficient" rpm gets increased by whatever amount you overvolt.

So if the "sweet spot" is 10 mph when at 24 volts then the "sweet spot" becomes:

10 * 36 / 24 = 15 mph

...when overvolted to 36 volts.

This means that you need to climb hills FASTER to keep the heat down, but that tends to require more power. When you run the calculation the heat scales proportionally with the slope of the hill.

So you can't win. More volts means more speed, which means more power, which requires more energy that creates more heat.

 

[duivendyk]

True enough, without lower gearing overvolting can be a losers game,it gets you performance that you can't use really when climbing hills.It does get you high speed performance on the flats,which does not help your range much because the power requirement goes up quite fast at speeds above 25-30km/hr.
The fact is that motors produce BOTH power and heat and are at their most efficient at top speed.I went through a math exercise to figure out the efficiency at different speed ratios for a particular efficiency at top speed, based on maintaining current (torque).It is not valid below 25% of max current.So of we have a motor running at 36V and 15A for instance and at say 2400 rpm, with an efficiency of 0.8 (80%),this expression will tell you what for the same current the efficiency will be at lower speeds. If we define the speed ratio maxspeed/actual speed as r ,then E' the new efficiency is:
E'= Emax/(r- (r-1)Emax) ,if r=1 we get E'= Emax which is of course as it should be.
If r=2 (half speed) and half power E'=Emax/ (2-Emax) or E'= 0.8/1.2 E' =0.67 or 67% if Emax had been 0.7 then E' would have become 0.54 at half speed.For 600 rpm (1/4 max speed) we get for 0.80eff. : E' =0.50, and for 0.7 eff,E'= 0.37 only about half the original value.This points up the fact that when you get below 30% of max speed the loss&motor heating really go up.