Cycle Analyst

Slow Pre-Purchase Inquiry Becomes Outstanding Customer Service

My pre-purchase inquiry to Grin about using a current transducer went unanswered.  After a week and another inquiry, they said my question would have to be passed up to the engineering staff.  I decided to just order a Cycle Analyst and figure it out for myself.

Three weeks after my initial contact, Grin still has not responded so I sent another email stating that I had purchased a unit and my testing was progressing.  This resulted in an immediate reply from a principal in the company.  Justin was kind enough to offer suggestions as to how I could modify the CA's analog front end to accommodate a current transducer and even included a schematic of that portion of the circuitry (which I had requested).  Although I have gotten this type of information from suppliers of industrial equipment, it's pretty unusual for a consumer-level product.  So, I give my highest recommendation for both the company and this product.

My Cheater Shunt

I loathed the idea of adding a current shunt to the bike.  While waiting for a reply from Grin, I had an idea to try.

The negative battery cable from the Anderson connector to the controller is about 45 cm of 25 mm² cable.  This calculates to around 0.55 milliohms – a perfect resistance for a shunt that would allow the CA to measure +/- 440 amps.

All I needed to do was add two “voltage sense connections” to that battery cable.  Of course, the temperature coefficient of copper wire is much worse than that of the alloy Manganin used for shunts.  Thus, the resistance will change as the cable heats up – about 0.4% per degree C.  But I don't think that will be a problem, at least for the initial testing.

The next photos show both ends of my battery cable/shunt.  The most difficult part of using the battery cable as a current shut is soldering the sense wire to the Anderson pin.  It takes a lot of heat.  My method was to use flux and get in / get out quickly.

The other end of the cable came completely prepared for me.  It's a bolted connection covered by heat-shrink tubing.  I guessed (correctly) what was under the heat shrink and did not even have to uncover it.  The bolted connection has two separate battery negative (“ground”) wires – one for the DC-DC lighting converter and one for the state-of-charge display (both of which I had removed).  One of these ground wires was routed to the CA's blue wire and the other to its black.  The state-of-charge indicator's connector also provided direct access to the positive side of the battery (which goes to the CA's red wire).

June 2023, Cheater Shunt Errata

The resistance I initially calculated above for my cheater shunt is high by a factor of 1.31.  This makes all my previous current readings low by the same factor.

I found the datasheet for the exact wire EM is using for the battery cable (La Triveneta FS17).  This turns out to have a much lower resistance than the generic AliExperss stuff on which I based my original calculation.

The La Triveneta FS17 1 x 25 mm² cable has a maximum resistance of 0.78 ohms per kilometer at 20° C.  Thus, a section 450mm long (measured exactly) equates to a resistance of 0.35 milliohms.

Unfortunately, I no longer have an instrument capable of making resistance measurements that low.  So, to verify the value, I passed a known current through the cheater shunt and measured the voltage developed across it. Its resistance turned out to be 0.407 milliohms at 19.5 ° C. This agrees pretty well with my new computation after accounting for a bit of contact resistance where the connectors are crimped.

In the end, I decided to use 0.423 milliohms because that would be the resistance at 30° C.  This is probably a reasonable value for the ambient temperatures at which I ride, allowing for a bit of temperature rise due to self-heating in the cable.

The bottom line is that the initial testing I reported below is low by a factor of 1.31 regarding current and energy numbers.   I will eventually replace it with more accurate numbers.

Inside the Cycle Analyst

This next photo shows the CA's main PCB.  It measures 69 x 38 mm with components mounted on one side only.  (The LCD is a separate assembly.) 

I'm really impressed with the design.  It would be quite economical to manufacture.  The highest compliment I can give is that it's something I would have been proud to have created.

As you can see, some oval holes provide strain relief for the external wiring.  The open holes indicate places where I removed unnecessary wires to make a cleaner installation for my application.

Notice a pair of black wires near the top leading out of the frame.  The CA can use a thermsitor mounted inside the motor to monitor temperature and reduce power as necessary.  The thermsitor is 10k ohms at 25 degrees C.  It only costs $3 from Grin.  I bought one and soldered it directly inside the display unit so I could measure ambient temperate.  Unfortunately, the display only shows degrees C, so it's a bit inconvenient for an American.  It's also not terribly accurate at temperatures where I will be using it.  It indicated about 19 degrees C when the ambient was more like 13 degrees C.  But there is a simple fix for that - it's possible to use an LM35 (semiconductor) temperature sensor instead of a thermsitor with a bit of circuitry modification.

 Initial Testing

After running the bike on a stand to verify operation, I ran the instrumented EM down the road in front of my house.  I knew the bike was fast on the loop trail, but I hit a surprising 39.7 mph in Green mode!  Although the speed may not be 100% accurate because the loaded rolling tire circumference is slightly smaller than it is static (although I did fudge it down a bit in an effort to compensate).  I think it would be difficult to get the tire circumference perfect as I expect different riding surfaces will change the contact patch (and thus the rolling radius) of such a low-pressure tire.  Electric Motion claims it will do 43.5 mph (and I assume that's in the maximum power mode).  I was initially a bit skeptical of that road speed as it would have the motor spinning well in excess of the motor manufacturer's performance graphs.  Therefore, I think the controller must be performing field weakening to achieve that speed.

Here are some other maximums recorded from that run: 164.7 A driving and -26.1 A at full regen from high speed.  Again, this may not be 100% accurate because of my crude shunt - but certainly is reasonable, and the driving/regen proportion should be correct.

The minimum battery voltage recorded was 45.2 V.  I started at about half-charge (about 53 volts, unloaded).  I'm not sure if the CA records peak power (this may be a setup thing I need to do?) but, 164.7 A * 45.2 V = 7.4 kW.  (I assumed peak power occurred at the maximum current and minimum battery voltage).  This seems reasonable for a motor rated 5 kW continuous / 11 kW peak.

Below is a table of results from various tests.  Some descriptive text follows.

Test Discussion

TEST 1: As mentioned, the first test was a very brief speed run on the asphalt road in front of my house.

All the following tests were conducted at my neighbor's horse ranch.  Although most of the tests were short, they are representative of how I ride there.  The terrain is typical of trials competitions in my area.  Some of the time is spent riding mock sections / practicing obstacles and the remainder would be equivalent to riding a loop trail.  The average speed is quite low.

TEST 2: This test had a fairly high maximum speed (21.7 mph) as I was trying to measure some extremes.  I used the CA's Aux Pot as a regen control.  (This is discussed in another section.)  Mostly, I just kept the Aux Pot's knob at the maximum regen that would not diminish the throttle response.  This caused some “automatic” regen on closed-throttle and provided a reasonable amount of “engine braking.”  My friend who rides in mountainous terrain finds the Aux Pot the perfect adjustment mechanism for energy recovery on long downhills.  I'm not really one to use much engine braking, but Cindy loves it.  Next season, I'll install the Cycle Analyst on her 5.7 for comparison.

I was pleased with the 74 Wh/mile figure.  This makes it seem reasonable that I could actually use the EM as a trail bike (as opposed to its intended purpose as a trials bike).  EM specifies the ePure Race has a range of 26.5 miles.  But in my discharge testing of the battery (rated 1875 Wh) I was only able to extract 1610 Wh.  Using a conservative rate of 74 Wh/mile, this equates to a range of under 22 miles (and that assumes full utilization of the battery). 

Regen versus driving Wh was a disappointing 0.8% average for the whole ride.  I think that speaks more to the terrain than anything else.  Most of the areas for regen are very short.  A few times I spun the knob to full regen on a downhill, and I'd see in excess of 1.4% at the start of the ride (it does not report a peak, just an average).  But -9.5 A versus 125 A is about 7.6%

TEST 3: Very similar to Test 2 but without working to achieve extreme speeds or regeneration.

TEST 4: Testing of my electric bicycle “finger throttle” as a regen control.  Although I initially found using this to be uncomfortable, it is better for me than the rotary potentiometer and I was therefore able to make more use of it.  Unfortunately, I can either have one finger on the clutch lever (trials mode) or a thumb positioned on the regen lever (trail riding mode).  The two don't really mix.  I am far more comfortable in trials mode even riding it as a trail bike.  I do have an idea for integrating variable regen braking into the rear brake pedal that I will be exploring.

TEST 5: Bad Data.  The wheel-speed sensor slipped out of position and did not register speed/distance for some portion of the ride.

TEST 6: Max speed prior to flinging “organic matter” off the tires was about 16 mph.  The 28.7 mph reading is as fast as it will go when the battery is at about 15% remaining capacity – the controller is obviously doing some limiting.

I noticed that the time and mileage do not record until moving at some minimum speed (maybe 2 mph?).  Occasionally, I expect I'm not exceeding the minimum speed while practicing trials.

 LM35 Temperature Sensor

After receiving two batches of counterfeit LM35DZ sensors out of China, I gave up.   When the part was once again available in the US through normal electronics distribution channels, I bought several.

In order to use an LM35 sensor in place of the thermistor, the CA's unOffical documentation (found on Endless-Sphere.com) says to remove R17, a 5k resistor.  This resistor is actually a precision 4.99 k-ohm unit that appears to be marked “R1” because the silk-screened legend is obscured by a via on the “7.”

Furthermore, the unOffical documentation says to put a 470-ohm resistor between Vout and GND.   Although this does quash oscillation in long cable runs, it also alters the temperature reading by about 2° C at room temperature.