Power, gears and hills

Now we are rolling along

With CV joints behaving as they should, the real testing is starting.

After dozens of runs up and down the hill outside our house, my confidence is building in Jane.  Playing it safe, I always start out heading uphill first so that if a problem occurs, gravity will get me home.

At this point the three Lead-Acid batteries easily go 2 miles.  Instrumenting the motor is a going to be key. So far there are two gauges: Motor (or Phase) current and Battery Current.   The relationship between these is dependent on motor speed and load, which has to be estimated based on the speedometer and the current gear.

I have found that there is not a single level road nearby.  This actually creates problems since any incline or decline in the street impacts the power needed to move the car.

So far, the motor current (varies dramatically based on controller settings) peaks around 420 amps (60% of the controller rating of 700A).  The Motor current meter is suspect since is jumps to 20% under the lightest load which is equivalent to 140A (20% of the controller's max amp rating).  It seems like 140A should be delivering quite a bit a power.

Battery current has only peaked at 300A which is great to see given the target battery pack is rated at 600A so this will be well below the limit, extending the overall lifetime of the batteries.

3rd Gear looks to be the best in-city gear.  During yesterday's testing, it cruised comfortably at 20 mph with the 36V battery pack.  The target pack is going to be 84V and speed scales linearly with volts, so 3rd gear should yield >40 mph.

It is a real challenge to understand how much power is actually being used.  The battery volts and amps can be multiplied to compute Watts going into the system.  The motor and controller have a combined efficiency of about 75% (conservative) so that should yield power out of the motor.  Given all of the losses in the drive train and air resistance, the predicted speed based on power should be close.  Here is one example of the power model.   From this at 30 kmh (about 19 mph) the motor should be delivering 2500 W (3.35 HP 745 Watts in a HP).  This should equate to about 3,300 W from the battery (@75% system efficiency).  At 34V, I would expect about 100A of battery current.  This comes close to what the meter reads.

Speed (km/h)	10	20	30	40	50	60
(m/s)	2.8	5.6	8.3	11.1	13.9	16.7
ForceOfAirResistance (N)	2.6	10.6	23.8	42.2	66.0	95.0
ForceOfIncline (N)	127	127	127	127	127	127
ForceOfRollingResistance (N)	115	115	115	115	115	115
TotalDrag (N)	245	253	266	284	308	337
Power to Maintain Speed (W)	756	1560	2462	3511	4755	6244

I would welcome feedback on the calculations and logic.  Next is to figure out total range based on the Watt-Hour capacity of the batteries.  The Kelly Controller has a CAN bus available and will report some useful information such as motor phase current and voltage, motor and controller temperatures, motor RPMs and battery voltage.  The current plan is to program an Arduino controller to read this data along with other system data (battery status, battery current and cell voltage).  The Arduino will act as the car's computer and display for the driver key performance data.  A bit of modern technology for a 40 year old car.

Getting to the bottom on the Thump

The Thumping diagnosed

After a great 2 hour session of trying to debug why Jane would thump loudly, lose power and stop.  It seemed like the controller was cutting power to the motor for some unexplained reason.  We ran it for 4 full minutes on jack stands while applying the brakes to simulate a reasonable load.  The motor was working as hard as it does on the local hills The brakes could apply enough to stall the motor but the thumping never occurred on the stands.  The mystery: what was different on the street versus on the stands.

Well today, first thing, I swapped out the 18V power supply that powers the Kelly KHB72701 controller.  Switched it back to a an old AC adapter with an 400W inverter, very Rube Goldberg.  First 6 inches up the drive and the thumping returned, as bad as ever.

So I opened the hood, had Carly gently apply some gas while I watched the drive shafts.  Sure enough, the driver side inner CV joint was slipping.  I could see the inner shaft rotate from the differential while the outer shaft slipped out about an 1/8" and then jumped back.  This was accompanied by the thumping sound.  Aha.

Here is a picture of what the CV joint looks like. 

When the car is elevated and the wheels drop, a few of the balls are pushed into the hub.  However, on level ground, the balls move out to the edge (like the ones on the left above).  When all six are near the edge, the slightest of force will cause all of them to slip out of their groove, only to jump back into the next slot, ready to thump again.  If the joint is at an angle, 1 or more of the balls will be deep in its groove and unable to slip.

As with anything, once you know what is it, it is easy to find information.  The following video shows another Mini having the exact same problem.

Jane was not thumping quite this bad, was was still violent.

Root Cause

Looking underneath, it became obvious the motor had slipped a bit forward.  Due to the electric, the motor only has one mount, the horizontal stabilizer and a strap holding it to the frame.  The strap allowed the  passenger side of the motor to slide forward about 1 inch, which caused the CV joint to align almost straight from the differential to the wheel hub when off the stands.  Well, it seems like having a constant angle on the CV joint actually prevents all of the balls from being on the edge of the grooves, where the slipping (groove jumping) can occur.  When the joint is perfectly straight, it is quite easy to see how the slipping could occur.  The horizontal angle was straight due to the engine misalignment and  when the car was lowered, the vertical angle would also line up near zero.  While on the stands, there was always a good angle at the CV joint, so slipping was impossible, explaining why the above test revealed nothing.  It is not logical that the controller would know whether the wheels were up and it seems like logic prevails, again.

Solution

So it is time to figure out a better mount for the motor which will prevent the movement.  With the motor sitting in the right position, the drive shafts look symmetric which is a great sign including a permanent angle on the CV joints.  Now, just to engineer a good mount.  With the motor out of position, one shaft was fairly straight (limited angle on the CV) and the other had quite a bit of angle.  Symmetry is a good thing.

If the new mount works, battery testing will resume.  A 600A ammeter is on its way.  This will allow for real current monitoring.  The Kelly Controller provides a 0%-100% current scale, but even under minimal load, it jumps to 20% and seem fairly rough at measuring phase current (Motor current).  The new ammeter will report battery current which when multiplied by the voltage will yield instantaneous power, the real indicator of how things are working.  I am intrigued to be working with such large currents, when most of my career has been spent in the milliamp range of the scale.  A 600A fuse is impressive.

First long drive to get fireworks on July-4

I say adventure, they say it's no fun to push a car up a hill.

We took Jane out to go get our July-4 fireworks at the nearest (1 mile away) fireworks stand.  Well it was mostly downhill to the stand, so that went well.  On the way home, things were going well and then we hit the first reasonable incline.  The motor cut-out but then was right back, basically pulsing with a thump-sound.

We pulled to the side, to check things and then started out again, only to have it repeat.  Every time I pushed on the accelerator, it would thump and not go.  Everything under the bonnet looked OK, no error indicators on the controller, batteries seemed to have sufficient power.  No logical explanation.  I had changed a couple of settings in the controller that morning.

So we hopped out and started to push it up the hill.  Luckily a neighbor was driving by and a stranger both joined in for the steepest part of the hill.  We made it home and everyone was a little winded.

Today, I spent with the car up on stands, just testing.  On the stands it performed well, spinning the wheels up to 35 mph in 4th, about 2,100 RPM.  This right inline with the estimates based on 36 volts of batteries with minimal loading.  Reset the settings that were changed yesterday.

So I figured, I could lower it back down and see.  The thumping returned just trying to back-out of the drive.  Back up on the stands.  No thumping.  Back-down and it seems to be running well again.  Up the steep hill with no problems.  Makes no sense.  Recharging the batteries now and then I'll head out for around block testing again.  

So far it's made it 2 miles on a charge with reasonable hills.  Given the quality of the three lead-acid test batteries I've been using, the best I can estimate is with the 60AH of LiFePo4 @ 84V that are planned, the car should be 15-20 miles on a charge. Eventually, it will be easy to double the battery pack to double the range.  Just a cost management situation at this point.  In a small town, 15 miles should be good for a day where nothing is much farther than 3 miles away.  Based on the charger, it should be able to recharge in about 4 hours.  More on that in near future.

This is a GPS plot of the 2 mile journey   Blue Line is speed (15-20 downhill, 7-8 up hill in 2nd gear).  The  gold line is the grade of the road.  You see the hills around the 10% range.  Fairly steep for an electric.  The Greenline is elevation (Green scale on the left).  A 5% grade requires twice the power.

Lots of experimenting

No pictures today, but a few interesting insights from the first few runs around the block.

Living on a fairly steep hill, we have been testing quite a bit going up the hill and then coasting back down.  Not wanting to get stranded at the bottom of the hill entering the neighborhood, it is critical that Jane can make it back home.  So far, it powers right up the 150 yard hill with little hesitation.  Still limited to about 1,200 RPM due to the lead-acid batteries being used for testing.  Three decent used batteries in series is holding  steady at 34-35 volts under load.  The Kelly controller provides a ammeter signal for an analog meter that reads as a percent of "controller maximum motor current".  The car climbs the hill at about 280 amps (40% of 700 amps maximum)  at the motor.  The motor is rates at 125A continuous and 450A maximum for 1 minute, so this seems within the reasonable range for 45 second hill climb.  We did 4 climbs yesterday before the batteries started to dip.  The motor was warm to the touch, but not hot.  The motor's operating max winding temperature is 145C and has a thermistor connected to the controller which should shut things down if it gets close to this temperature.  On a minor uphill grade, the motor is pulling about 140A at maximum RPM.  There are no flat roads in the neighborhood, so until we have enough confidence that Jane can make it home, hill testing is all that is available.

Power Booster

The Kelly controller is recommended to run above 18V, isolated from the main batteries.  The standard starting battery is still in the car to power the electronics (radio, lights, blinkers) but only provides ~12V.  Amazon sells a voltage booster based on the TI LM2577 Link for about $9.  This worked great and now the controller is powered right at 25V.  There are numerous comments that the higher supply voltage enables the controller to operate better, but I am not sure exactly how.

Battery Selection

The search and calculations continue regarding battery choice.  The CALB CA60FI are leading the pack.  They can provide 10C which means a pack will be able to supply 600A for 30-60 seconds.  The motors 1 minute maximum rating is 600A into the controller, so this seems like a good match.  Also, the CALB packs are carried by a number of resellers which adds to their credibility.  The remaining questions revolves around the total target voltage for the battery system.  The default choice is 77V (nominal) which would be 24 packs a 3.2V each.  However, every volt yields about 50 additional RPM so a 84V system will allow the motor to generate additional 350 RPM which in 3rd gear equates to about an additional 5 MPH.  Not a big deal but could mean the different between 35 mph and 40 mph, which even for city driving would be nice.  In 4th gear, it is almost a 7 mph difference.  Now pushing the limits up to 90V (2 more packs) will yield yet another 300 additional rpm and few more mph.  With the lower gearing, it may not be too big of deal.  Putting the full battery system into service at the same time enables the packs to age equally and that should help maintain balance.

A few more experiments this weekend and then it will be time to decide.