The Electric Bike Saga

Cliff notes version on building a quick and dirty electric bike for $500 that can easily do 28 mph:

  1. Buy a rear 26″ 48V 1000W electric wheel that is brushless and direct drive, for around $180 off of ebay. Rear because if you get one for the front, the torque is known to snap the front fork off while riding.  Makes for a scary and sometimes painful experience when traveling at 28 mph.
  2. Buy enough used or new laptop batteries off of ebay to build the battery that you want.  I built a 20 AH battery out of 130 cells.  ($100)
  3. Buy those 18650 cell holders from china off of ebay. I bought 260 of em to build the battery. ($10)
  4. Buy one of those cheap 48 volt 3 amp dc power supplies for led strips off of ebay.  Mine cost $10, came damaged because one of the coils came off, had to re-solder it on, and then it worked.  It requires some soldering to attach a plug and you have to manually adjust the voltage to 54.6 volts with a screw located up front for charging the assembled battery. Update!  The 3 amps was not enough, I blew up both in a single day.  Ordered the 20 dollar 9amp one and it seems to be working just fine.
  5. Use 60/40 solder, the kind that uses lead (16 oz, $20).  The lead free stuff doesn’t work.  Use a 80 watt soldering iron ($20) if you have one, although a 30 watt sort of works, kind of.  I wrap my batteries in gorilla tape when done.
  6. Use at least 12 gauge wire, any thinner (higher gauge) and the wires will heat up as 26 amps flow through them. I had some lying around, not sure how much it costs…maybe some old house wire from an electrical project?
  7. Go to craigslist, facebook buy and sale, or the garage sale of your choice and buy a cheap mountain bike that has good brakes and a sturdy frame ($150).  Really good brakes.  Did I mention good brakes???
  8. Bam!  Put it all together and you’ll have an electric bike that can go 28 mph easily with no pedaling on the level for around $500.

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The following is the mostly whole sordid story:

I awoke one morning with the itch to build an electric bike.  I had ridden my Schwinn 180 dollar clearance street bike from target to and from work quite often,  but we had recently moved and now a very busy street and some really nasty hills stood in my way.  I had the brilliant idea to convert it to an electric bike.  I got on the local buy and sale on facebook and posted a want ad for a broken electric scooter.  A fellow sold me a broken schwinn s500 24 volt 500 watt brushed electric scooter for 25 dollars.  I created this monstrosity from it:

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I learned a lot about having to tension the chain and using pipe clamps with wood to mount motors.  More importantly I learned that a 24volt 500 watt brushed motor provides enough oomph to maintain speed on the level, which is just not good enough.  I also learned that lead acid batteries are too heavy, die quickly after only 200-300 cycles, and are a pain to charge.  Brushed motors wear out quickly compared to brushless because of the ‘brushes’ and are current hogs compared to brushless motors.

After getting this monstrosity to work I decided to ride it to work, on the way back on the last suicide hill leading to my house the motor sprocket broke. All in all not a bad learning experience for around 35 dollars.  It could maintain speed on the level, but not accelerate very well at all.

 

I am a cheapskate, but I decided after much deliberation and mental struggle I decided to throw some real money at it.  I bought a mountain bike and bike carrier for 150 dollars.  I then bought a direct drive brushless 1000 watt 48 volt electric wheel off eBay for 180 dollars.  It went together really quickly and easily.  The only frustrating thing is that the pedal assist didn’t work.  The neat thing is it can handle hills handily as long as I peddle a bit.

 

I left out the battery discussion on purpose.  Did I mention I was a cheapskate?  A good battery with BMS costs around 500 dollars.  That’s more than the electric wheel and bike together.  They really don’t have used ones either.  To complicate matters, building your own is around $350 dollars, which is still way to expensive.  Instead I got on eBay and bought a lot of 32 used laptop batteries and ripped the cells out of them.  The key is to find the same brand and age so that hopefully I end up with cells that have similar charging and discharging characteristics.  That way I don’t need a BMS.  I was lucky and most of the cells were the same, and only about 20-30 of them were dead.

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Typically it is unwise to solder lithium ion cells because the heat can damage and reduce their capacity, but I was using cells that only cost me about 50 cents a piece so I really didn’t care.  Honestly building the battery took the longest and was the most frustrating.  If I had had the money I would have forked over the five hundred and been done with it.  It took me over three weeks, roughly forty hours to build.  It cost 150 dollars to build, 100 for a lot of 32 batteries, 20 for an eighty watt soldering iron, 20 for 16 oz of 1 mm 60/40 solder, and 10 for 260 pieces of battery holders from ebay, direct from China.  They got here a lot quicker than I expected coming from China and all.

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The old laptop versions only have a discharge rating of about 2C and of course mine were used so only about 1.5C.   That is 1.5 times the capacity (C). Mine were 2600mah when new but being old and discharged at 2 amps gave me a capacity of about 1500 mah.   For the 20 amp hour (AH) battery I wanted that meant 130 cells, in a 13 by ten configuration.  As you know lithium cells need to be maintained between 3.6 and 4.2 volts typically.  13 times 4.2 is 54.6 volts.  And yes that is the fully charged voltage on a typical 48 volt battery.  The typical dishcharged voltage is 3.6 times 13 which is 46.8.   Most of my cells were old and could only hold a charge at around 4.1 volts.  In fact I charged every single one of them to 4.2 volts and if they fell below 4.1 volts after  a week I didn’t use them and classified them as ‘dead’.  I maintain my cells between 3.7 volts and 4.1 volts, especially since the cheap voltmeters I use are only accurate to about plus or minus .05 volts.   This gives me a .05 volt buffer because as you know when a cell exceeds or drops below that band it can die.  And when it dies it is possible for it to be an explosive, fire burning experience.  They will eventually die anyway, but the conservative band has served me well for a year and 250 miles so far.

 

 

 

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