Switched Boost Converter

Switched Boost Converter

Introduction – When driving is worse than biking

I was forced to ask myself a question after spending more than 1 hour driving to a location that was less than 15 miles away.  The question was, could I get here quicker on a bike?  The only thing I miss with my change in locations is the daily bike ride to and from work.  It is counter to have a rewarding day thwarted with a droning commute.  Two weeks ago the ride home took 90 minutes.  That did it for me.  I found the motivation to actively find a way to use the bike and improve my speed and range using what I had.

Purpose – Getting home without getting smoked

My current controller, battery pack, and motor setup allow me to travel over 30 mile at 11.5 mph.  The route I drive can’t be traveled by bike since no path exists on 520.  I would have to travel across I90, which makes the ride 19 miles instead of the 14 on 520.  I figured 45 minutes to ride 19 miles would be the ideal dream, but a bit unrealistic because I would have to average just under 26 mph.  If this was something I was going to do, then I needed to know what I could do with what I had and if upgrades would be viable.

Details – We can rebuild him. We have the technology. We can make him better than he was. Better, stronger, faster.

I’ve got a solid baseline of 11.5 mph with the standard controller, 24V pack, and Bafang motor.  In the past, I’ve used 36V packs and rewired wye/delta Crystalyte motors.  The problem with these was the weight, I had constantly bent rims.  This was the main reason I settled with the 24V pack and geared Bafang motors.  I haven’t bent a rim since making this change.

I revisited the suped-up option but this time using a voltage step up converter, also known as a boost converter.  My initial use was crude, it as all or nothing.  I ran it with it always on boost.  My controller has a limit of 40V due to the low voltage logic regulator limits.  I was able to set it to 39.5V and was impressed with reaching speeds of 19 mph on flats.  But I had a bunch of problems when I took on high amp loads, such as stop and goes or positive grades.  I burnt out fuses, melted solder off the boost converter, and caused components to become loose.  I repaired the boost converters over a dozen times.

If I was going to seriously going to use the boost converter, I had to have the option of switching it on or off when demand changed.  I accomplished this by removing the voltage adjust potentiometer and replacing it with a voltage divider circuit board.  The theory was higher resistance circuit would be the default, until I activated a switch to a lower resistive circuit.  The switch would cause a path of least resistance and would operate at a lower voltage.  The idea worked on the bench.  Now that I could, would it make a difference?  I had to do some initial testing before throwing myself into a doomed project.

I decided to run a baseline route to track how far, how long, how fast, and how much energy I used.  I picked a route that started and stopped at Salmon Bay Park.  I would travel through Ballard, across the bridge by Fisherman’s Terminal, then onto Fremont.  Then I would head out to the APL at the University of Washington and turn around.  My return route went by Hal’s Brewery, then north past 65th before heading west to my starting point.  It is a 13 mile route with over 30% of it having a positive grade.

I would run 4 times along this route.
1st at 24V with no boost
2nd at 24V with boost inline
3rd at 38V with constant boost
4th would be with a switchable boost with either 24V or 39.5V.

Here are the results of the 4 runs in the table below.

Amp Hours Configuration Ave MPH Travel Time Range at 8Ah
3.06 24V Controller Only 11.5 67.83 33.99
3.11 Boost set at 24V 11.5 67.83 33.44
3.75 Switched Boost at 38V 14.4 54.17 27.73
4.76 Boost set at 38V 13.4 58.21 21.85

Some of the observations I had expected, others were a surprise.  I thought I would have used less Ah (ave Amps per hour) across the board.  I’ve been able to get 40 miles out of it, at which it typically goes into LVCO at 8Ah.  The nice surprise was the .05Ah increase while having the boost converter inline with it outputting the same source voltage of 24V.  This meant that switching boost converter was a viable option.  The constant boost setting seemed about right to me.  I expected it to be a hog.  The switched boost really saved on the waste of always having the boost converter engaged.   With the range well below the 19 miles, I concluded that it would be worthwhile to go ahead and continue on with this project.

Relations – What can be expected on untraveled routes

If any route is going to be traveled, the grade will have the determining factor on how much the boost is engaged.  Plotting a route in Topofusion gives me mileage uphill, downhill, and on flat surfaces.  Here I can estimate how long I’ll be running at top speed.  For the commute, my route totals 18.8 miles with 4.32 miles uphill.  Doing the math, my uphill time will be 23 minutes using a formula of dm/s=t (which are distance times minutes per hour divided by speed equals time in minutes).  I used a variance of the formula to determine my horrible average speed home one evening of 9.33 mph, dm/t=s.

Since my average mph in the table above also included my non boost speeds, I decided to use a best guess of 16.5 as my actual speed.  If that is the case, it would take me 52 minutes for the flat and downhill portions of the ride.

The total time to travel the 18.8 mile route with my switched boost converter would take me 1 hour and 15 minutes.  In contrast, this same route would take me 1 hour and 40 minutes without any boost converter.

Getting comparable results on a track is the true world test of any planned route.

Summary – How it went

It was a bit of a plunge to dive in after 2 weeks of having the initial idea.  There is always the thoughts of obstacles that await me, flat tire, heavy foot traffic on trail, road closures, component failure, etc…  The way I get past that is to think of the 1 hour and 30 minutes it took me to drive 14 miles on a clear sunny day in Seattle.  I’ll never get that time back.

So, how did it go?  Not bad, but not as I had hoped for.  The head wind on the ride home was a factor.  It took 15 more minutes due to weather and traffic through downtown and the locks.  I left at 5:25pm, so I was already 25 minutes behind.  It took me 105 minutes total.  I really lucked out though.  The battery pack went into LVCO (Low Voltage Cut Off) at 68th Ave NW, I had one more block to ride up hill..  Needless to say, I didn’t get home at the golden moment.

The morning ride was an interesting contrast.  It took me 95 minutes with 2 rest breaks.  I was pleased to see I had only used 6.5Ah.  This route has 1 mile more uphill than the ride I did home.  You can see how much traffic and weather factor in, especially wind.

I don’t think I can expect better than an average of 1 hour and 40 minutes, or 11.7 mph.  It’s realistic because that’s a long time to ride without a break, so rest is required.  I saved about 10 minutes of travel time by using the boost converter.

To be honest, it may take a 48 volt 40 amp system to make the commute time worthwhile.  My motor is rated for 500 watts.  This means I would have to used an entirely new bike with a 2000 Watt motor.  I would also need a new controller to handle the 48v/40a load.  Here’s the real issue, the battery would have to be a 48v/40ah pack.  The prismatic LiFePO4 cells at 40 Ah weigh 3.3 lbs each.  I would need 16 of them for 48 volts.   That’s over 50 lbs of battery weight at a cost over $1000, that’s crazy stuff!  My current 24 volt 10 Ah pack weight around 6 lbs and costs under $300.  This is why the boost converter was such and appealing option.

I’m thinking the jury is still out.

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