WOT, Motor Efficiency, and speed limiting

[donorcycle]

Hello,

I need to limit my speed a bit to make it to work and back on my 10 AH battery pack. I can limit the speed with a Cycle analyst, or a three position switch. Which would help the motor run more efficiently? I noticed the motor runs smoother with less stuttering at wide open throttle but it is taking to much battery power.

My current top speed is 27-29 MPH. What is the sweet spot in terms of the speed/efficiency trade-off, assuming I'd like to go as fast as possible, but not at the expense of wasteful battery drain?

 

[dogman dan]

Well, its all relative. I'll assume that 20 mph is about the slowest you'd consider. That would be a ton more efficient than Wot at 48v. ( sounds like you run 48v 20 amps on a 9c motor)

There might be a difference in wh/mi between using the switch or using the CA, but it would be pretty small, and barely worth measureing. I say limit speed by whatever method works the most convenient for you. However you do it, just slowing down will make large gains in wh/mi.

The swich makes sense to me, because you could very quickly and easily go back to full speed for than 1/4 mile on the route where it's too nice to have full watts. So climb a hill on full power, then go back to low for the long cruise.

If you ever need to reallllly stretch the range, it's quite amazing how much less power 18 mph takes than 20. About 15 mph is the slowest I've ever been able to go and stand it. I get just about double the range at 15 mph as at 25 mph.

22 mph is not so bad a compromise speed. You get a decent cadence in 48- 14 gear, and quite good efficiency. Yet it's fast enough to not feel completely pokey.

If you have lots of stops, like inner city riding, pedaling up to 10 mph first really makes a huge difference too.

 

[John in CR]

The answer is easy. Get more battery.

 

[dogman dan]

I should have mentioned that. 48v 15 ah is at least 600wh of usable charge even when saving the last bit to extend battery lifepan. It really makes riding a pain to have to sweat your range every ride, and the battery hates 100% discharges.

So much nicer to buy a battery with 20 miles of range even at 27 mph. Till he gets one though, it's slowing down.

Or charging some at work. 8)

 

[ZOMGVTEK]

More capacity is the best bet.

Slower speeds tend to be more efficient. My stock 48V 1000W kit loved 6 MPH, and it would be a drastic drop in efficiency as you moved up from that. Anything over 20 MPH starts to get bad. Keep it 16-18 MPH and its reasonable, try to not go past 20.

A 3 position switch is the best bet for speed limiting. Its easy (as long as your controller supports it), cheap, and it works great.

 

[donorcycle]

Thank you for the responses.

I have 44.4 V, 10AH of Lipo. I only have 5 miles (one way) to work, but there is a bit of stop and go and we get high winds. I wanted stealth, so batteries, controller, and excess cable are all in a triangle pack. There is not currently room in there for another battery. I could move the controller outside, but then I would have a lot of exposed wires. The three position switch sounds convenient, but then I have another (excess) cable to hide somewhere.

 

[voicecoils]

Assuming you don't want to change your battery or slow down:

I second dogman's suggestion - can you charge at work? A second charger is cheaper then a second battery and less weight on your bike.

Then you can use all 440 watt-hours each way. That should be more then enough to make 5 miles at under 30 mph!

A second suggestion is to change your bike's gearing so you can effectively pedal harder at 27-30 mph. The more effort you put in, the less the motor does and the longer it will go. If your system is using 500 watts at a given speed and you add 100 watts pedalling, the motor will drop to 400 watts and you'll extend your battery range by 20%.

 

[John in CR]

Address aerodynamics then. Slower is only more efficient because you give more to the wind at higher speed. Slowing down is definitely not worth the decrease in safety inherent with crossing paths with more cars.

Also, regardless of pack size he should charge at work. Shallower discharges mean greater battery life. Plus his employer can get tax breaks for providing the necessary facility that are worth far more than the amount of electricity used, so it's a win-win for both.

On top of it all is time. Time is the only thing each of us has in this world. Personally, I don't want to waste any of mine. How much is donorcycle's time worth to himself?

Slowing down is wrong no matter how donorcycle slices it.

 

[GrayKard]

If your controller has cruise control, and you have any long sections without speed changes, it's a good way to run more efficiently. Also lets you rest your throttle hand.

Gary

 

[donorcycle]

You guys are the greatest.

This is quality: "Slowing down is wrong no matter how donorcycle slices it."

This is my first build and I finally got running after two months. You guys are right, I tried limiting the speed to 22 MPH on the CA and it was a let down. Not only was it slow, the CA did not handle it well. It seems like it has to go over the target speed by +2 MPH, then throttle beneath the target by -2 MPH then settle on the target if you don't move the throttle. It was HIDEOUS. It was really choppy and didn't save much fuel. I set the CA back up to full speed on the way home and did 28 MPH and peddled along with my 48T/11T. That is living. It's an ideal setup in terms of pedal/power. I didn't go all the way to work because I thought I would run out of batteries. However, I got home and the CA said I used 9.257 AH (of a total of 10AH available) but I still had 2 Volts left on the meter. Maybe I need to calibrate the CA?

I might try the three speed switch. The CA did too much hunting and pecking.

I can't go with a bigger chainring because my frame won't allow it. I have a single 48T on it now with a spacer and the frame Dremeled.

I guess I could crouch for aerodynamics.

I have a manual cruise and it seems like my Infineon controller goes into automatic cruise after maintaining throttle for a period.

I might have to charge at work. I have the expensive Hyperion to charge 12S. I wonder if something cheap would do for work. I have all the time in the world for the charge, so safety is the big concern since the bike will be locked unattended in a stairwell with Lipo. I think I saw an outlet there. The 12S will be a problem because I have the pack set up for simple plug and play with the Hyperion. I don't want to have to disassemble the pack to charge at work. It was hard enough getting everything stuffed in there and it would take too much time and won't stand up to the constant rearranging. I am a little hesitant to bulk charge if that is even possible with 12S2P and no rearrangement.

 

[ZOMGVTEK]

Bulk charge with a small PSU set to a low HVC? Something like a S-350-48 or two S-350-24's in series if you want more power. Set the output to be something low, like 4.00 to 4.10V/cell.

You could have something monitoring the balance port, it would be nice if it had an alarm as well. Ideally you would just want the power supply to kick off when it hits peak voltage, but that might be a bit tricky.

 

[John in CR]

A very conservative (in terms of cutoff voltage and current) bulk charge at work with the extra protection of a timer cutoff sounds like just the ticket.

 

[voicecoils]

A simple cheap bulk charge option would be to get a 300-400W charger from bmsbattery.com or similar set to 48v max (4v per cell) and add wall socket timer set for about 1hr.

That way you are highly unlikely to overcharge any 1 cell (0.2v buffer) and the charger will be automatically shut off by the time, as a further protection.

A pair of cell logs from hobbycity wouldn't hurt either to be able to spot check cell voltages. Smaller and faster then a multimeter.

I disagree that slowing down is always wrong. It depends where you ride and what you aim to accomplish. I like going fast and keeping taxis behind me, but I slow down on bike paths around other cyclists for example.

What's your smallest rear cog? If it's not 11 teeth, you can up the gearing with a new rear cluster from ebikes.ca

Lastly, the CA has an advanced menu that allows tuning of speed or current limits. You can reduce oscillations if you read up on it a bit.

 

[d8veh]

I had the same problem so I added a potentiometer throttle and a switch. It cost about $5 to make. I can set the speed to anything I want with the pot and when I switch to it, I cruise at that speed. It seems to work as a speed control rather than power control, so when I come to a hill, it automatically feeds in more power, so I don't need to adjust it once set. You need at least one brake cut-off switch.There's a wiring diagram on the folder if you want to do it.
http://s451.photobucket.com/albums/qq236/d8veh/Giant%20BPM/?action=view&current=SANY0085.jpg

 

[donorcycle]

very good suggestions. I actually already own a cell log and timer. I don't want to get too elaborate at work, since it will all be in a common stairwell and I don't want to arouse suspicion and have management tell me I can't park there. I have not asked permission and plan on just doing it. However, I don't want it to look like too much of a science project to passers buy.

I found some nice small, metered, adjustable 0-50V, power supplies, that look to be scientific grade. They are slow at 1.5 amps, but that is good to keep the cells in balance. I noticed the faster the charge, the more the cells wander. I figure, since I have nine hours to charge at work, speed does not matter as much as safety. By my estimation it would take about 4.5 hours to charge my pack. I will probably balance charge at home every night since it only takes 25 minutes and I like to be on the safe side.

Tell me if these chargers should work. There are several more left if anyone is interested:
http://www.ebay.com/itm/Power-Designs-5015A-50V-1-5A-DC-Power-Supply-GWO-/280732698810?_trksid=p4340.m263&_trkparms=algo%3DSIC%26its%3DI%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%26otn%3D5%26pmod%3D280739662518%26ps%3D63%26clkid%3D2869695096048421396

Thank you all for your awesomeness

 

[dogman dan]

You in New York? Why so much need for stealth? Mount the controller elsewhere, and get 5 ah more lipo. You shouldn't be carrying a controller all smothered in a bag anyway.

Get a small metal box to contain the controller, and some decent cable sleeve to cover the wires. All the tangle of connectors will be inside the metal box, that can mount on a rear rack. The metal box should conduct enough heat, and can be ventilated as well.

You'll still have excellent stealth, and 12s 15 ah will be a nice battery that can take you further. Two more lipos will be cheaper than a second hyperion and charger too.

 

[icecube57]

A speed switch is nice. It does let you sip and hypermile but in a city like New York if its anything like what Greg in DC is dealing with... probably worse. I would rather have full power on tap at all times to get out of a jam or make the next traffic light. I agree with dogman I would simply get two more packs and make a 15AH pack that way in worst case senario of 1AH a mile (usually for peformance builds probabl .50-.75 for you) you have enough to get to work and back and you didnt run the pack flat in the process. This will increase cyle life of your batteries.

 

[donorcycle]

Thanks for the suggestions.

I want stealth because although the bike will be parked inside, it is in a common public area (stairwell) next to an outside door and there is an after school program for kids in the next room over. Also, I plan on using it in State parks were technically there is a $600 fine for using a motorized vehicle (even though the bike doesn't pollute and can't go faster than a pedal bike, it is motorized). The park rangers can be assholes if they want to be. I also would like to reserve the rear rack for my gear when I go fishing. I guess I could put the controller in a rear rack and my gear on top of that, although that is getting to be a lot of weight in the rear. Or, I could bolt it to the seat tube. The controller doesn't get that hot in the triangle pack, but I haven't tried it on mountain inclines in the dirt yet. I left space around the controller for cooling. I can also open the top of the triangle bag while riding for extra ventallation. Also, part of the reason I went with 44.4 volts was so the components do not overheat.

Another issue is that my batteries are Hyperion. I got three of them for $90 and bought a fourth for $130. I would like to keep all of the batteries the same make and model for safety. So, buying two more would cost $260 which is too much. If I hadn't got such a good deal on the first three Hyperions, I would have bought Nano Techs. In hindsight that's what I probably should have done. Maybe I am too cautious about mismatching batteries, but I plan on driving the bike to work and parking the bike unattended. I don't want a fire. I would be interested in your thoughts about that.

Also, more batteries = more weight.

I think if I bulk charge slowly with an accurate limiting power supply, I could charge at work. I found a supply for $78 that fits the bill and would probably last forever. I could lock everything up: bike, charger, battery pack etc. while parked/charging. You are right, I do not want to buy or bring another expensive Hyperion charger to work with a big honking power supply driving it (that's what I have now at home).

3p would be very nice though. However, everything is cleverly stuffed together now and works, and I don't really want to mess with it fireproofing and all. I am very pleased with the clean stealthy design except for the range.

 

[icecube57]

Well a switch is what tou need then cause you dont leave us may alternatives for other suggestions. With the distance of your commute your capcity might be matched for your ride after all.

 

[donorcycle]

I measured my triangle pack for two more batteries. Two more may not fit. It would be close. Maybe too close to pad them. I have an 18.5" frame with oversized tubing. In hindsight, I would have bought a used chromalloy, thin-tubed frame with a bigger triangle. I think that would fit 6 batteries comfortably. However, with the 18.5" frame with the Thudbuster seatpost causes me to be on my tip-toes when seated on the bike (I'm 6'1") I love the Thudbuster, but it really raised the seat hight making a larger frame a poor fit for me.

I'm going to put the three way switch on and go for a test drive to work right now. I also determined that a longer route, with fewer stops, was actually more battery friendly. Even with pedaling to 10 mph before engaging the motor, stop and go eats batteries.

I also bought the charger for work, so I will have that option.

 

[parabellum]

Be not afraid in mixing batteries, so long you stay in the chemistry it is ok. I mixed puffed (abused in previous life) nanotechs with cheep hard case 20c lipos and have nice bulk charge pack i check for top balance every ~10 charges. 1 string is made of 2s and 3s nanos and other of 4s hard case lipos (1 came with dude cell I removed), then paralleled with balance wires. :wink:

 

[donorcycle]

good to know about mixing battery brands.

The three way switch rocks. It is much smoother than trying to control speed with the CA. Now I have another cable to conceal.

People have commented on how they can not see and feel the bike being electric. I think I will try to work with the four batteries and keep it stealth.

The controller in the pack is getting a little warm, but I am still able to keep my hand on it. That is after full discharge with the pack zipped tight at about 80 degrees Fahrenheit ambient. I am a little concerned about the heat in uphill mountain riding but I will have to check it out. Usually, I go uphill at dawn when it is about 50 degrees Fahrenheit, so it may not be a problem. 10 AH may not get me far though.